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List-Based Applications

 
 

Tables Records

 

Introduction to Records

In our description of tables, we saw that a table was made of one or various columns that represented categories of data. After creating such a table and its columns that represent the categories, you (actually the user) can enter values in the table to make it a valuable list. Filling up a table with values is referred to as data entry.

Data entry is performed by entering a value under the column headers. The group of values that correspond to the same entry or the same line under the columns is called a record. This also means that the records are entered one line, also called a row, at a time. Here is a table filled with various records:

A record on a table is represented as a row (horizontal) of data. To support the various records that belong to a table, the DataTable class is equipped with a property called Rows. The DataTable.Rows property is in fact an object of the DataRowCollection class. The DataRowCollection class provides the necessary properties and methods you can use to create and manage records of a table. A row itself is an object based on the DataRow class.

This lesson is done in two parts: the explanation of database topics and the Practical Learning sections intended to apply the concepts. The explanation sections are as complete as possible so you don't have to follow the Practical Learning exercises. But if you want to perform the assignments in the Practical Learning sections, you must be familiar with the list view control.
 

Practical Learning Practical Learning: Creating the Application

  1. Start Microsoft Visual Studio .NET and create a new Windows Application named MusicStore2
  2. To add a new form, on the main menu, click Project -> Windows Form...
  3. Set the Name to DataCenter and press Enter
  4. To add a new form, on the main menu, click Project -> Windows Form...
  5. Set the Name to Categories and press Enter
  6. Design the form as follows:
     
    Control Name Text Other Properties
    DataGrid     Auto Format: Professional 3
    Label   New Category:  
    TextBox txtNewCategory    
    Button btnAdd Add  
    Button btnClose Close  
    Form     AcceptButton: btnAdd
    CancelButton: btnClose
    FormBorderStyle: FixedDialog
    MaximizeBox: False
    ShowInTaskbar: False
    StartPosition: CenterScreen
  7. To add a new form, on the main menu, click Project -> Windows Form...
  8. Set the Name to ItemTypes and press Enter
  9. Design the form as follows:
     
    Control Name Text Other Properties
    DataGrid     Auto Format: Professional 3
    Label   New Item Type:  
    TextBox txtNewItemType    
    Button btnAdd Add  
    Button btnClose Close  
    Form     AcceptButton: btnAdd
    CancelButton: btnClose
    FormBorderStyle: FixedDialog
    MaximizeBox: False
    ShowInTaskbar: False
    StartPosition: CenterScreen
  10. To add a new form, on the main menu, click Project -> Windows Form...
  11. Set the Name to NewStoreItem and press Enter
  12. Design the form as follows:
     
    Control Name Text Other Properties
    Label   Category:  
    ComboBox cboCategories   DropDownStyle: DropDownList
    Sorted: True
    Button btnNewCategory New  
    Label   Item Type:  
    ComboBox cboItemTypes   DropDownStyle: DropDownList
    Sorted: True
    Button btnNewType New  
    Label   Item Name:  
    TextBox txtItemName    
    Label   Unit Price:  
    TextBox txtUnitPrice 0.00 AlignText: Right
    Label   Item #:  
    TextBox txtItemNumber    
    Button btnCreate Create  
    Button btnClose Close  
    Form     AcceptButton: btnCreate
    CancelButton: btnClose
    MaximizeBox: False
    StartPosition: CenterScreen
  13. Double-click an unoccupied area of the form to access its Load event and implement its as follows:
     
    Private Sub NewStoreItem_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
            ' We will generate a random number for the store item
            Dim tmeNow As DateTime = DateTime.Now
            Dim rndNumber As Random = New Random(tmeNow.Millisecond)
            Dim strNumber As String = CStr(rndNumber.Next(100000, 999999))
    
            ' Display the new number in the Part # text box
            Me.txtItemNumber.Text = strNumber
    
            ' Disable the OK button to indicate that the item is not ready
            Me.btnCreate.Enabled = False
    End Sub
  14. In the Class Name combo box, select btnNewCategory
  15. In the Method Name combo box, select Click and implement the event as follows:
     
    Private Sub btnNewCategory_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnNewCategory.Click
            Dim frmCat As Categories
    
            frmCat = New Categories
            frmCat.ShowDialog()
    End Sub
  16. In the Class Name combo box, select btnNewType
  17. In the Method Name combo box, select Click and implement the event as follows:
     
    Private Sub btnNewType_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnNewType.Click
            Dim frmTypes As ItemTypes
    
            frmTypes = New ItemTypes
            frmTypes.ShowDialog()
    End Sub
  18. To add a new form, on the main menu, click Project -> Add Windows Form...
  19. Set the Name to PurchaseOrder and press Enter
  20. Design the form as follows:
     
    Control Name Text Other Properties
    GroupBox   Selection of Sale Item  
    Label   Category  
    Label   Types  
    Label   Available Items  
    Button btnNewItem New Item  
    ListBox lbxCategories    
    ListBox lbxItemTypes    
    ListBox lbxAvailableItems    
    GroupBox   Items Sold  
    Label   Item #  
    Label   Item Name  
    Label   Unit Price  
    Label   Qty  
    Label   Sub Total  
    TextBox txtItemNumber1    
    TextBox txtItemName1    
    TextBox txtUnitPrice1 0.00 AlignText: Right
    TextBox txtQuantity1 0 AlignText: Right
    TextBox txtSubTotal1 0.00 AlignText: Right
    TextBox txtItemNumber2    
    TextBox txtItemName2    
    TextBox txtUnitPrice2 0.00 AlignText: Right
    TextBox txtQuantity2 0 AlignText: Right
    TextBox txtSubTotal2 0.00 AlignText: Right
    TextBox txtItemNumber3    
    TextBox txtItemName3    
    TextBox txtUnitPrice3 0.00 AlignText: Right
    TextBox txtQuantity3 0 AlignText: Right
    TextBox txtSubTotal3 0.00 AlignText: Right
    TextBox txtItemNumber4    
    TextBox txtItemName4    
    TextBox txtUnitPrice4 0.00 AlignText: Right
    TextBox txtQuantity4 0 AlignText: Right
    TextBox txtSubTotal4 0.00 AlignText: Right
    TextBox txtItemNumber5    
    TextBox txtItemName5    
    TextBox txtUnitPrice5 0.00 AlignText: Right
    TextBox txtQuantity5 0 AlignText: Right
    TextBox txtSubTotal5 0.00 AlignText: Right
    TextBox txtItemNumber6    
    TextBox txtItemName6    
    TextBox txtUnitPrice6 0.00 AlignText: Right
    TextBox txtQuantity6 0 AlignText: Right
    TextBox txtSubTotal6 0.00 AlignText: Right
    GroupBox   Order Processing  
    Button btnCalculate Calculate Order  
    Button btnSave Add to Today's Orders  
    Button btnClose Close  
    Label   Total Order:  
    TextBox txtTotalOrder 0.00 AlignText: Right
    Enabled: False
    Form     CancelButton: btnClose
    MaximizeBox: False
    StartPosition: CenterScreen
  21. Double-click the New Item button and implement its Click event as follows:
     
    Private Sub btnNewItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnNewItem.Click
            Dim frm As NewStoreItem = New NewStoreItem
            frm.ShowDialog()
    End Sub
  22. In the Class Name combo box, select btnCalculate
  23. In the Method Name combo box,, select Click and implement its event as follows:
     
    Private Sub btnCalculate_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnCalculate.Click
            Dim item1UnitPrice As Decimal = 0.0
            Dim item2UnitPrice As Decimal = 0.0
            Dim item3UnitPrice As Decimal = 0.0
            Dim item4UnitPrice As Decimal = 0.0
            Dim item5UnitPrice As Decimal = 0.0
            Dim item6UnitPrice As Decimal = 0.0
            Dim quantity1 As Integer = 0
            Dim quantity2 As Integer = 0
            Dim quantity3 As Integer = 0
            Dim quantity4 As Integer = 0
            Dim quantity5 As Integer = 0
            Dim quantity6 As Integer = 0
            Dim item1SubTotal As Decimal = 0.0
            Dim item2SubTotal As Decimal = 0.0
            Dim item3SubTotal As Decimal = 0.0
            Dim item4SubTotal As Decimal = 0.0
            Dim item5SubTotal As Decimal = 0.0
            Dim item6SubTotal As Decimal = 0.0
            Dim totalOrder As Decimal
    
            Try
                item1UnitPrice = CDec(Me.txtUnitPrice1.Text)
            Catch ex As FormatException
                MsgBox("Invalid Unit Price")
                Me.txtUnitPrice1.Text = "0.00"
                Me.txtUnitPrice1.Focus()
            End Try
    
            Try
                quantity1 = CInt(Me.txtQuantity1.Text)
    
            Catch ex As FormatException
                MsgBox("Invalid Quantity")
                Me.txtQuantity1.Text = "0"
                Me.txtQuantity1.Focus()
            End Try
    
            Try
    
                item2UnitPrice = CDec(Me.txtUnitPrice2.Text)
            Catch ex As FormatException
                MsgBox("Invalid Unit Price")
                Me.txtUnitPrice2.Text = "0.00"
                Me.txtUnitPrice2.Focus()
            End Try
    
            Try
                quantity2 = CInt(Me.txtQuantity2.Text)
            Catch ex As FormatException
                MsgBox("Invalid Quantity")
                Me.txtQuantity2.Text = "0"
                Me.txtQuantity2.Focus()
            End Try
    
            Try
                item3UnitPrice = CDec(Me.txtUnitPrice3.Text)
            Catch ex As FormatException
                MsgBox("Invalid Unit Price")
                Me.txtUnitPrice3.Text = "0.00"
                Me.txtUnitPrice3.Focus()
            End Try
    
            Try
                quantity3 = CInt(Me.txtQuantity3.Text)
            Catch ex As FormatException
                MsgBox("Invalid Quantity")
                Me.txtQuantity3.Text = "0"
                Me.txtQuantity3.Focus()
            End Try
    
            Try
                item4UnitPrice = CDec(Me.txtUnitPrice4.Text)
            Catch ex As FormatException
                MsgBox("Invalid Unit Price")
                Me.txtUnitPrice4.Text = "0.00"
                Me.txtUnitPrice4.Focus()
            End Try
    
            Try
                quantity4 = CInt(Me.txtQuantity4.Text)
            Catch ex As FormatException
                MsgBox("Invalid Quantity")
                Me.txtQuantity4.Text = "0"
                Me.txtQuantity4.Focus()
            End Try
    
            Try
                item5UnitPrice = CDec(Me.txtUnitPrice5.Text)
            Catch ex As FormatException
                MsgBox("Invalid Unit Price")
                Me.txtUnitPrice5.Text = "0.00"
                Me.txtUnitPrice5.Focus()
            End Try
    
            Try
                quantity5 = CInt(Me.txtQuantity5.Text)
            Catch ex As FormatException
                MsgBox("Invalid Quantity")
                Me.txtQuantity5.Text = "0"
                Me.txtQuantity5.Focus()
            End Try
    
            Try
                item6UnitPrice = CDec(Me.txtUnitPrice6.Text)
            Catch ex As FormatException
                MsgBox("Invalid Unit Price")
                Me.txtUnitPrice6.Text = "0.00"
                Me.txtUnitPrice6.Focus()
            End Try
    
            Try
                quantity6 = CInt(Me.txtQuantity6.Text)
            Catch ex As FormatException
                MsgBox("Invalid Quantity")
                Me.txtQuantity6.Text = "0"
                Me.txtQuantity6.Focus()
            End Try
    
            item1SubTotal = item1UnitPrice * quantity1
            item2SubTotal = item2UnitPrice * quantity2
            item3SubTotal = item3UnitPrice * quantity3
            item4SubTotal = item4UnitPrice * quantity4
            item5SubTotal = item5UnitPrice * quantity5
            item6SubTotal = item6UnitPrice * quantity6
    
            totalOrder = item1SubTotal + item2SubTotal + item3SubTotal + _
                            item4SubTotal + item5SubTotal + item6SubTotal
    
            Me.txtSubTotal1.Text = Format(item1SubTotal, "F")
            Me.txtSubTotal2.Text = Format(item2SubTotal, "F")
            Me.txtSubTotal3.Text = Format(item3SubTotal, "F")
            Me.txtSubTotal4.Text = Format(item4SubTotal, "F")
            Me.txtSubTotal5.Text = Format(item5SubTotal, "F")
            Me.txtSubTotal6.Text = Format(item6SubTotal, "F")
            Me.txtTotalOrder.Text = Format(totalOrder, "F")
    End Sub
  24. Display the first form (Form1.vb (Design)) and design it as follows:
     
    Control Name Text
    Button btnNewItem New Store Item
    Button btnNewOrder New Purchase Order
    Button btnClose Close
  25. Double-click the New Store Item button and implement its event as follows:
     
    Private Sub btnNewItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnNewItem.Click
            Dim frm As NewStoreItem = New NewStoreItem
            frm.ShowDialog()
    End Sub
  26. In the Class Name combo box, select btnNewOrder
  27. In the Method Name combo box, select Click and implement its event as follows:
     
    Private Sub btnNewOrder_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnNewOrder.Click
            Dim frm As PurchaseOrder = New PurchaseOrder
            frm.ShowDialog()
    End Sub
  28. In the Class Name combo box, select btnClose
  29. In the Method Name combo box, select Click
  30. Return to the main form. double-click the Close button and implement its Click event as follows:
     
    Private Sub btnClose_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnClose.Click
            End
    End Sub
  31. Save all

Introduction to Data Entry

To allow the user to perform data entry, you must create an appropriate object meant for this task. You have various options. You can use various controls on the same view or provide a data sheet type of view such as the one available from the DataGrid control. In all cases, when the user performs data entry, by default, it is by entering one record at a time. Any time while the user is performing an operation on a record, the record has a status that can be identify by the DataRow.RowState property which is a value based on the DataRowState enumerator.

A record on a table is represented as a row of data. To support the various records that belong to a table, the DataTable class is equipped with the Rows property which is an object of type DataRowCollection with each record an object of type DataRow.

Before adding a new record to a table, you must let the table know. This is done by calling the DataTable.NewRow() method. Its syntax is:

Public Function NewRow() As DataRow

The DataTable.NewRow() method returns a DataRow object. Here is an example:

Private Sub btnCreate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCreate.Click
        dtDirectors = New DataTable
        Dim rowDirector As DataRow = Me.dtDirectors.NewRow()
End Sub
 

Practical Learning Practical Learning: Introducing Data Entry

  1. Display the DataCenter form (DataCenter.vb (Design))
  2. In the Toolbox, click the Data button.
    To create a DataSet, click DataSet and click the DataModule form
     
  3. In the Add Dataset dialog box, click the Untyped Dataset radio button and click OK
  4. While the DataSet control is still selected, in the Properties window, click (Name) and type dsMusicStore
  5. Still in the Properties window, set the DataSetName to MusicStore
  6. Set its Modifiers property to Public so it can be easily accessed from other objects
  7. Click the Tables field to reveal its ellipsis button ellipsis and click the ellipsis button ellipsis
  8. In the Tables Collection Editor, click the Add button
  9. Click (Name) and type tblCategories
  10. Click TableName and type Categories
  11. Set its Modifiers property to Public
  12. Click the Columns field to reveal its ellipsis button ellipsis and click its ellipsis button ellipsis
  13. In the Columns Collection Editor, click the Add button
  14. Change the (Name) to colCategory and change the ColumnName field to Category
  15. Set its Modifiers property to Public
  16. Click Close
  17. In the same way, create a table as
    (Name): tblItemTypes
    TableName: ItemTypes
    Modifiers: Public
  18. Add a column with the following properties:
    (Name): colItemType
    ColumnName: ItemType
    Modifiers: Public

     
  19. In the same way, create a table as
    (Name): tblAvailableItems
    TableName: AvailableItems
    Modifiers: Public
  20. Add the following columns to it
     
    (Name) ColumnName Modifiers
    colItemNumber ItemNumber Public
    colItemCategory Category Public
    colTypeOfItem ItemType Public
    colItemName ItemName Public
    colUnitPrice UnitPrice Public
  21. Click Close twice

Data Entry

 

Adding a Value Based on the Column Index

When you call the DataTable.NewRow() method, the record's status is DataRowState.Detached. After calling the DataTable.NewRow() method, you can specify the value that the column would carry. To do this, you must specify the table's column whose value you want to provide. You can locate a column based on an index as we mentioned already that the columns of a table are stored in the DataTable.Columns property which is based on the DataColumnCollection class. Each column can be identified by its index. Using this index, to assign a new value to the column, you can use the following version of the DataRow.Item property:

Overloads Public Default Property Item(ByVal columnIndex As Integer) As Object

Here is an example:

Private Sub btnCreate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCreate.Click
        dtDirectors = New DataTable
        Dim rowDirector As DataRow = Me.dtDirectors.NewRow()
        colDirector = dtDirectors.Columns.Add()

        rowDirector(0) = "Jonathan Demme"
End Sub

When the record has been added to the table, the record has a status of DataRowState.Added. The above version of the DataRowCollection.Add() method allows you to add a value for one column. To complete a record, you would have to create a value for each column.

 

Practical Learning Practical Learning: Creating a Record From the Column Index

  1. Display the Categories form and double-click its Add button.
    double-click an unoccupied area of its body
  2. In the top section of the file, type the following:
     
    Imports System.IO
    Imports System.Xml
    Imports System.Data
    
    Public Class NewStoreItem
        Inherits System.Windows.Forms.Form
  3. Implement the event as follows:
     
    Private Sub btnAdd_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnAdd.Click
            ' Get a reference to the Data Center because 
            ' that's where the DataSet object resides
            Dim frmData As DataCenter = New DataCenter
    
            ' Create a new record for the Categories table
            Dim rowNewCategory As DataRow = frmData.dsMusicStore.Tables("Categories").NewRow()
            ' Specify only the Category column since the CategoryID is auto-incrementing
            rowNewCategory(0) = Me.txtNewCategory.Text
            ' Add the new record to the Categories table
            frmData.dsMusicStore.Tables("Categories").Rows.Add(rowNewCategory)
    
            ' Display the current records of the Categories table so
            ' the user would know what categories are already in the table
            Me.dataGrid1.DataSource = frmData.dsMusicStore
            Me.dataGrid1.DataMember = "Categories"
    
            ' Reset the text box in case the user wants to add another category
            Me.txtNewCategory.Text = ""
            Me.txtNewCategory.Focus()
    End Sub
  4. Save the form

Adding a Value Based on the Column Variable Name

If you prefer to use the variable name of a column when adding the value, you can use the following version of the property:

Overloads Public Default Property Item(ByVal column As DataColumn) As Object

Here is an example of using this version of the property:

Dim dsVideoCollection As DataSet
    Dim dtDirectors As DataTable
    Dim dtVideoCategories As DataTable
    Dim dtRatings As DataTable
    Dim dtActors As DataTable
    Dim dtFormats As DataTable
    Dim colCategoryID As DataColumn
    Dim colCategory As DataColumn
    Dim colDirector As DataColumn
    Dim colDirectorID As DataColumn
    Dim colVideoTitle As DataColumn
    Dim colActor As DataColumn
    Dim dtVideos As DataTable

    Private Sub btnCreate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCreate.Click
        dtDirectors = New DataTable
        Dim rowDirector As DataRow = Me.dtDirectors.NewRow()
        colDirector = dtDirectors.Columns.Add()

        rowDirector(0) = "Jonathan Demme"
        dtDirectors = New DataTable

        dtVideoCategories = New DataTable("Categories")
        colCategory = New DataColumn("Category")
        dtVideoCategories.Columns.Add(colCategory)

        Dim rowCategory As DataRow = Me.dtVideoCategories.NewRow()
        rowCategory(colCategory) = "Documentary"
End Sub

Practical Learning Practical Learning: Adding a Value Based on the Column Variable Name

  1. Display the ItemTypes form and double-click its Add button
  2. In the top section of the file, type the following:
     
    Imports System.IO
    Imports System.Xml
    Imports System.Data
    
    Public Class NewStoreItem
        Inherits System.Windows.Forms.Form
  3. Implement the event as follows:
     
    Private Sub btnAdd_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnAdd.Click
            Dim frmData As DataCenter = New DataCenter
            Dim strFilename As String = "ItemTypes.xml"
    
            If File.Exists(strFilename) Then
                frmData.dsMusicStore.ReadXml(strFilename)
            End If
    
            Dim rowNewType As DataRow = frmData.dsMusicStore.Tables("ItemTypes").NewRow()
            rowNewType(frmData.colItemType) = Me.txtNewItemType.Text
            frmData.dsMusicStore.Tables("ItemTypes").Rows.Add(rowNewType)
    
            frmData.dsMusicStore.WriteXml(strFilename)
    
            Me.dataGrid1.DataSource = frmData.dsMusicStore
            Me.dataGrid1.DataMember = "ItemTypes"
    
            Me.txtNewItemType.Text = ""
            Me.txtNewItemType.Focus()
    End Sub
  4. Save the form

Adding a Value Based on the Column Object Name

To specify the name of the column, the DataRow class is equipped with an Item property that allows you to identify a column by its object name, by its variable name, or by its index. Based on this, the DataRow property is overloaded with three versions. One of the versions uses the following syntax:

Overloads Public Default Property Item(ByVal columnName As String) As Object

This property expects the object name of the column passed in its square brackets. When calling this property, you can assign it the desired value for the column. Here is an example:

Private Sub btnCreate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCreate.Click
        dtDirectors = New DataTable
        Dim rowDirector As DataRow = Me.dtDirectors.NewRow()
        colDirector = New DataColumn("Director")
        dtDirectors.Columns.Add(colDirector)

        rowDirector(0) = "Jonathan Demme"

        rowDirector = Me.dtDirectors.NewRow()
        rowDirector("Director") = "John Landis"

        dtVideoCategories = New DataTable("Categories")
        colCategory = New DataColumn("Category")
        dtVideoCategories.Columns.Add(colCategory)

        Dim rowCategory As DataRow = Me.dtVideoCategories.NewRow()
        rowCategory(colCategory) = "Documentary"
End Sub

After assigning the desired value to the row, to add the new value to a table, the DataRowCollection class provides the Add() method that is overloaded with two versions. The first version of this method uses the following syntax:

Overloads Public Sub Add(ByVal row As DataRow)

This method simply expects you to pass the DataRow object you previously defined. Here is an example:

Private Sub btnCreate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCreate.Click
        dtDirectors = New DataTable
        Dim rowDirector As DataRow = Me.dtDirectors.NewRow()
        colDirector = New DataColumn("Director")
        dtDirectors.Columns.Add(colDirector)

        rowDirector(0) = "Jonathan Demme"

        rowDirector = Me.dtDirectors.NewRow()
        rowDirector("Director") = "John Landis"

        Me.dtDirectors.Rows.Add(rowDirector)
End Sub

In the same way, you can identify each column of a table by its object name and assign it the appropriate value. Once the record is complete, you can add it to the table. Here is an example:

Private Sub btnCreate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCreate.Click
        dtVideos = New DataTable("ListOfVideos")
        Dim rowVideo As DataRow = Me.dtVideos.NewRow()

        Dim colVideos(7) As DataColumn
        colVideos(0) = New DataColumn("Title")
        colVideos(0).DataType = System.Type.GetType("System.String")
        colVideos(1) = New DataColumn("Director")
        colVideos(1).DataType = System.Type.GetType("System.String")
        colVideos(2) = New DataColumn("YearReleased", System.Type.GetType("System.Int16"))
        colVideos(3) = New DataColumn("Length", System.Type.GetType("System.String"))
        colVideos(4) = New DataColumn("Rating", System.Type.GetType("System.String"))
        colVideos(5) = New DataColumn("Format", System.Type.GetType("System.String"))
        colVideos(6) = New DataColumn("Category", System.Type.GetType("System.String"))

        dtVideos.Columns.AddRange(colVideos)

        rowVideo("Title") = "A Few Good Men"
        rowVideo("Director") = "Rob Reiner"
        rowVideo("YearReleased") = 1993
        rowVideo("Length") = "138 Minute"
        rowVideo("Rating") = "R"
        rowVideo("Format") = "VHS"
        rowVideo("Category") = "Drama"
        Me.dtVideos.Rows.Add(rowVideo)
End Sub

Adding an Array of Records

The above version of the DataRowCollection.Add() method means that you must identify each column before assigning a value to it. If you already know the sequence of columns and don't need to explicitly identify them, you can store all values in an array and simply add the array as a complete record. To do this, you can use the second version of the DataRowCollection.Add() method whose syntax is:

Overloads Public Overridable Function Add(ByVal values() As Object) As DataRow

Here is an example:

Private Sub btnCreate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCreate.Click
        dtVideos = New DataTable("ListOfVideos")
        Dim rowVideo As DataRow = Me.dtVideos.NewRow()

        Dim colVideos(7) As DataColumn
        colVideos(0) = New DataColumn("Title")
        colVideos(0).DataType = System.Type.GetType("System.String")
        colVideos(1) = New DataColumn("Director")
        colVideos(1).DataType = System.Type.GetType("System.String")
        colVideos(2) = New DataColumn("YearReleased", System.Type.GetType("System.Int16"))
        colVideos(3) = New DataColumn("Length", System.Type.GetType("System.String"))
        colVideos(4) = New DataColumn("Rating", System.Type.GetType("System.String"))
        colVideos(5) = New DataColumn("Format", System.Type.GetType("System.String"))
        colVideos(6) = New DataColumn("Category", System.Type.GetType("System.String"))

        dtVideos.Columns.AddRange(colVideos)

        rowVideo("Title") = "A Few Good Men"
        rowVideo("Director") = "Rob Reiner"
        rowVideo("YearReleased") = 1993
        rowVideo("Length") = "138 Minute"
        rowVideo("Rating") = "R"
        rowVideo("Format") = "VHS"
        rowVideo("Category") = "Drama"
        Me.dtVideos.Rows.Add(rowVideo)

        Dim vdoRecord() As String = {"Fatal Attraction", "Adrian Lyne", "1987", _
          "120 Minute", "R", "DVD", "Drama"}

        Me.dtVideos.Rows.Add(vdoRecord)
End Sub
 

There is an alternative to this second version of the DataRowCollection.Add() method. As opposed to passing an array of values to the Add() method, you can first define an array, assign that array to a DataRow variable, then pass that DataRow object to the Add() method. To support this technique, the DataRow class is equipped with an ItemArray property that expects an array.

After creating the records of a table, if a record contains invalid values, the DataRow.HasErrors property can help you identify them.

 

Application Data Storage

 

Introduction

In Lesson 3, we introduced lists as parts of an application and the information they contain. In this lesson, we learned how to fill such lists with information. When the application closes, unfortunately, all the information created while the application was running is lost. While the first goal of an application is to create one or more lists used to organize information, probably the essence of an information-based or a data-based application is to preserve information created when using the application and be able to retrieve that information the next time the application runs, without re-creating it.

There are various ways you can save the information created in an application. As the DataSet class is equipped with all the necessary features used to create and manage one or more lists of an application, it also provides a very high level of saving the information stored in its lists.

Saving a List

Once a new record has been created or when the lists of the data set have been populated with information, you can save the changes and store them to a computer file. By default, the DataSet class is equipped to save its lists as XML. To support this, it is equipped with the WriteXml() method that is overloaded with various versions. One of the versions of this method uses the following syntax:

Overloads Public Sub WriteXml(ByVal fileName As String)

This method takes as argument the name of the new file or its path. When providing this argument, make sure you add the .xml extension to the file name. This method does two things: it checks the existence of the file and it saves. If the file you provided is not found in the path, this method creates it and writes the record(s) to it. If the file exists already, this method opens it, finds its end, and appends the new data at the end. This makes this method very useful and friendly.

If you want to control whether the file should be created from scratch, instead of passing the name of the file to this method, first create a stream using a Stream-derived class such as FileStream. This allows to specify the necessary options using the FileMode, FileAccess, and FileShare properties. Once the stream is ready, pass it to the WriteXml() method because it is also overloaded with the following syntax:

Overloads Public Sub WriteXml(ByVal stream As Stream)

If you want the file to be formatted as text, you can use the following version of the method:

Overloads Public Sub WriteXml(ByVal writer As TextWriter)

If you prefer to use an XmlWriter variable to manage the file, use the following version of the method:

Overloads Public Sub WriteXml(ByVal writer As XmlWriter)

Obviously to use this method, you must first define an XmlWriter type of variable.

Opening a List

To open the data saved from a list, the DataSet class provides the ReadXml() method that is overloaded with various versions. One of the versions of this method uses the following syntax:

Overloads Public Function ReadXml(ByVal fileName As String) As XmlReadMode

This method takes as argument the name of an existing XML file or its path. This method opens the file and provides the XML formatting as it was done when the file was saved. Although this method can read any XML file, if you use it to open a file that was saved by someone else or another application and you want to use it in your application, you must be familiar with the names of its nodes. If it contains names that are not "registered" or recognized by your DataSet object, the lists that compose your application may not be able to read it, not because the list was not formatted right, but because the lists of your application would be holding different names.

If the file was saved using a Stream-based class, you can pass a stream to the method based on the following syntax:

Overloads Public Function ReadXml(ByVal stream As Stream) As XmlReadMode

In the same way, the method provides an equivalent version for the TextWriter and the XmlWriter versions:

Overloads Public Function ReadXml(ByVal reader As TextReader) As XmlReadMode
Overloads Public Function ReadXml(ByVal reader As XmlReader) As XmlReadMode

To use one of these versions, you must first define a TextWriter or an XmlReader type of variable.

When retrieving the content of the XML file, if you want it delivered as text, call the DataSet.GetXml() method. Its syntax is:

Public Function GetXml() As String

As you can see, this method returns a String string.

 

Practical Learning Practical Learning: Opening an XML File

  1. Display the Categories form and double-click its Add button
  2. Change the Click event as follows:
     
    Private Sub btnAdd_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnAdd.Click
            ' Get a reference to the Data Center because 
            ' that's where the DataSet object resides
            Dim frmData As DataCenter = New DataCenter
            ' This is the XML file that will holds the Categories
            Dim strFilename As String = "Categories.xml"
    
            ' If the file exists already, open it
            If File.Exists(strFilename) Then
                frmData.dsMusicStore.ReadXml(strFilename)
            End If
    
            ' Create a new record for the Categories table
            Dim rowNewCategory As DataRow = frmData.dsMusicStore.Tables("Categories").NewRow()
            ' Specify only the Category column since the CategoryID is auto-incrementing
            rowNewCategory(0) = Me.txtNewCategory.Text
            ' Add the new record to the Categories table
            frmData.dsMusicStore.Tables("Categories").Rows.Add(rowNewCategory)
    
            ' Update the XML file
            frmData.dsMusicStore.WriteXml(strFilename)
    
            ' Display the current records of the Categories table so
            ' the user would know what categories are already in the table
            Me.dataGrid1.DataSource = frmData.dsMusicStore
            Me.dataGrid1.DataMember = "Categories"
    
            ' Reset the text box in case the user wants to add another category
            Me.txtNewCategory.Text = ""
            Me.txtNewCategory.Focus()
        End Sub
    End Class
  3. Display the New Store Item form.
    Double-click an unoccupied area of its body and change its Load event as follows:
     
    Private Sub NewStoreItem_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
            Dim frmData As DataCenter = New DataCenter
            Dim rdrMusicStore As XmlTextReader = Nothing
    
            Try
                Dim strFilename As String = "Categories.xml"
    
                If File.Exists(strFilename) Then
                    rdrMusicStore = New XmlTextReader(strFilename)
    
                    ' Scan the XML file
                    While rdrMusicStore.Read()
                        ' every time you find an element, find out what type it is
                        ' If you find a category, put it in the Categories list box
                        If (XmlNodeType.Element <> 0) And (rdrMusicStore.Name = "Category") Then
                            Me.cboCategories.Items.Add(rdrMusicStore.ReadElementString("Category"))
                        End If
                    End While
                End If
    
                strFilename = "ItemTypes.xml"
    
                If File.Exists(strFilename) Then
                    rdrMusicStore = New XmlTextReader(strFilename)
    
                    ' Scan the XML file
                    While rdrMusicStore.Read()
                        ' every time you find an element, find out what type it is
                        ' If you find an ItemType, put it in the Item Types list box
                        If (XmlNodeType.Element <> 0) And (rdrMusicStore.Name = "ItemType") Then
                            Me.cboItemTypes.Items.Add(rdrMusicStore.ReadElementString("ItemType"))
                        End If
                    End While
                End If
            Catch ex As XmlException
                MsgBox("Invalid XML file")
            Finally
                If Not (rdrMusicStore Is Nothing) Then rdrMusicStore.Close()
            End Try
    
            ' We will generate a random number for the store item
            Dim tmeNow As DateTime = DateTime.Now
            Dim rndNumber As Random = New Random(tmeNow.Millisecond)
            Dim strNumber As String = CStr(rndNumber.Next(100000, 999999))
    
            ' Display the new number in the Part # text box
            Me.txtItemNumber.Text = strNumber
    
            ' Disable the Create button to indicate that the item is not ready
            Me.btnCreate.Enabled = False
    End Sub
  4. In the Class Name combo box, select btnNewCategory
  5. In the Method Name combo box, select Click
  6. Change the event as follows:
     
    Private Sub btnNewCategory_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnNewCategory.Click
            Dim frmCat As Categories = New Categories
    
            frmCat.ShowDialog()
    
            Dim rdrMusicStore As XmlTextReader = Nothing
    
            Try
                Dim strFilename As String = "Categories.xml"
    
                If File.Exists(strFilename) Then
                    Me.cboCategories.Items.Clear()
    
                    rdrMusicStore = New XmlTextReader(strFilename)
    
                    ' Scan the XML file
                    While rdrMusicStore.Read()
                        ' every time you find an element, find out what type it is
                        ' If you find a category, put it in the Categories list box
                        If (XmlNodeType.Element <> Nothing) And (rdrMusicStore.Name = "Category") Then
                            Dim strNew As String = rdrMusicStore.ReadElementString("Category")
                            If Not (Me.cboCategories.Items.Contains(strNew)) Then
                                Me.cboCategories.Items.Add(strNew)
                            End If
                        End If
                    End While
                End If
            Catch ex As XmlException
                MsgBox("Invalid XML file")
            Finally
                If Not (rdrMusicStore Is Nothing) Then
                    rdrMusicStore.Close()
                End If
    	 end try
    End Sub
  7. In the Class Name combo box, select btnNewType
  8. In the Method Name combo box, select Click
  9. Change the event as follows:
     
    Private Sub btnNewType_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnNewType.Click
            Dim frmTypes As ItemTypes = New ItemTypes
    
            frmTypes.ShowDialog()
    
            Dim rdrMusicStore As XmlTextReader = Nothing
    
            Try
                Dim strFilename As String = "ItemTypes.xml"
    
                If File.Exists(strFilename) Then
                    rdrMusicStore = New XmlTextReader(strFilename)
    
                    While rdrMusicStore.Read()
                        If (XmlNodeType.Element <> 0) And (rdrMusicStore.Name = "ItemType") Then
                            Dim strNewType As String = rdrMusicStore.ReadElementString("ItemType")
                            If Not Me.cboItemTypes.Items.Contains(strNewType) Then
                                Me.cboItemTypes.Items.Add(strNewType)
                            End If
                        End If
                    End While
                End If
            Catch ex As XmlException
                MsgBox("Invalid XML file")
            Finally
                If Not (rdrMusicStore Is Nothing) Then
                    rdrMusicStore.Close()
                End If
            End try
    End Sub
  10. In the Class Name combo box, select txtItemName
  11. In the Method Name combo box, select TextChanged and implement its event as follows:
     
    Private Sub txtItemName_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) _
    		Handles txtItemName.TextChanged
            If Me.txtItemName.Text = "" Then
                Me.btnCreate.Enabled = False
            Else
                Me.btnCreate.Enabled = True
            End If
    End Sub
  12. In the Class Name combo box, select txtUnitPrice
  13. In the Method Name combo box, select TextChanged and implement its event as follows:
     
    Private Sub txtUnitPrice_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) _
    		Handles txtUnitPrice.TextChanged
            If Me.txtUnitPrice.Text = "" Then
                Me.btnCreate.Enabled = False
            Else
                Me.btnCreate.Enabled = True
            End If
    End Sub
  14. In the Class Name combo box, select txtItemNumber
  15. In the Method Name combo box, select TextChanged and implement its event as follows:
     
    Private Sub txtItemNumber_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) _
    		Handles txtItemNumber.TextChanged
            If Me.txtItemNumber.Text = "" Then
                Me.btnCreate.Enabled = False
            Else
                Me.btnCreate.Enabled = True
            End If
    End Sub
  16. In the Class Name combo box, select btnCreate
  17. In the Method Name combo box, select Click and implement the event as follows:
     
    Private Sub btnCreate_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnCreate.Click
            ' Get a reference to the Data Center because 
            ' that's where the DataSet object resides
            Dim frmData As DataCenter = New DataCenter
    
            ' This is the XML file that will holds the general inventory
            ' of the current items in the store
            Dim strAvailableItems As String = "Inventory.xml"
    
            ' If the file exists already, open it
            If File.Exists(strAvailableItems) Then
                frmData.dsMusicStore.ReadXml(strAvailableItems)
            End If
    
            ' Create a new record for the AvailableItems table
            Dim rowNewItem As DataRow = frmData.dsMusicStore.Tables("AvailableItems").NewRow()
    
            rowNewItem("ItemNumber") = Me.txtItemNumber.Text
            rowNewItem("Category") = Me.cboCategories.Text
            rowNewItem("ItemType") = Me.cboItemTypes.Text
            rowNewItem("ItemName") = Me.txtItemName.Text
            rowNewItem("UnitPrice") = Me.txtUnitPrice.Text
            ' Add the new record to the AvailableItems table
            frmData.dsMusicStore.Tables("AvailableItems").Rows.Add(rowNewItem)
    
            ' Update the XML file
            frmData.dsMusicStore.WriteXml(strAvailableItems)
    
            ' Reset the controls in case the user wants to add another record
            Me.cboCategories.SelectedIndex = -1
            Me.cboItemTypes.SelectedIndex = -1
            Me.txtItemName.Text = ""
            Me.txtUnitPrice.Text = "0.00"
    
            ' We will generate a random number for the store item
            Dim tmeNow As DateTime = DateTime.Now
            Dim rndNumber As Random = New Random(tmeNow.Millisecond)
            Dim strNumber As String = CStr(rndNumber.Next(100000, 999999))
    
            ' Display the new number in the Part # text box
            Me.txtItemNumber.Text = strNumber
    
            ' Disable the OK button to indicate that the item is not ready
            Me.btnCreate.Enabled = False
    End Sub
  18. Execute the application
  19. Click the New Store Item button
  20. Click the top New button to display the Categories form and create a few categories as follows:
     
    Category
    Electric Guitar
    Acoustic Guitar
    Bass
    Keyboard
    Stands
    Cables
    Cases
    DJ Equipment
  21. Close the Categories form and open the Item Types form
  22. Create a few item types as follows:
     
    Item Type
    4-String
    5-String
    6-String
    12-String
    Solid Body
    Hollow Body
    Left-Handed
    Guitar Stand
    DJ CD Players
  23. Close the Item Types form
  24. In the New Store Item form, create the following items (let the computer generate item numbers):
     
    Category Item Type Item Name Unit Price
    Electric Guitar Solid Body Gibson Les Paul Standard Electric Guitar 1950.00
    Bass 6-String Ibanez SR506 6-string Electric Bass 595.50
    Electric Guitar Hollow Body Oscar Schmidt OE40 Hollow Body Electric Guitar 205.95
    Acoustic Guitar Left-Handed Yamaha Left-Handed FG413SL Acoustic Guitar 295.95
    Cables Instrument Cable Monster Cable S-100 Straight Instrument Cable 12.95
    Bass Combo Amps Behringer BX1200 Ultrabass Amplifier (120 Watts, 2-Channel) 150.00
    Keyboard Synthesizers Korg Triton Le 61-Key Workstation Synth 895.95
    Stands Guitar Stand String Swing Metal Guitar Wall Hanger 9.95
    Electric Guitar Solid Body Epiphone LP-100 Electric Guitar 275.95
    Keyboard Digital Piano Yamaha YDP223 88-Key Graded Hammer Piano With Bench 1490.00
    Cables Guitar Cable Spectraflex Guitar Cable 42.25
    Electric Guitar Solid Body Gibson Les Paul Classic Electric Guitar 1625.95
    Stands Guitar Stand Locking Tubular Guitar Stand 4.95
    Electric Guitar Solid Body ESP LTD Viper 400 Electric Guitar 585.50
    Acoustic Guitar 12-String Washburn J28S12DL Cumberland 12-string Guitar 650.75
    Electric Guitar Hollow Body Ibanez Artcore AF75 Electric Guitar 315.95
    Keyboard Synthesizers Korg Triton Extreme 61-Key Synth Workstation 1895.00
    Electric Guitar Solid Body Epiphone Les Paul Standard 525.95
    Stands Guitar Stand Guitar Folding Stand 12.95
    Electric Guitar Hollow Body Gibson ES-335 Reissue 1850.00
    Acoustic Guitar 12-String Martin DM12 850.00
    Stands Guitar Stand Metal Guitar Wall Hanger 9.95
    Acoustic Guitar Left Handed Yamaha FG413SL 295.95
    Electric Guitar Solid Body Gibson Faded SG Special 625.95
    Bass 4-String Ibanez SR500 525.95
    Electric Guitar Hollow Body Oscar Schmidt OE40 225.95
    Electric Guitar Left Handed Schecher C-1 450.95
    Cases Cases Les Paul Hardshell Case 39.95
    Cables Instrument Cable Hosa Dual Instrument Cable 7.25
  25. Close the forms and return to your programming environment

Committing or Rejecting Changes to a List

When a user has created a record, the data set that holds the information is considered to have been modified because, obviously, it doesn't have the same information or the same records it had when the application was launched. You as the programmer have the option of accepting the changes or rejecting them. To accept the changes, call the DataSet.AcceptChanges() method. Its syntax is:

Public Sub AcceptChanges()

If you don't want the changes to take effect, you can reject them by calling the DataSet.RejectChanges() method. Its syntax is:

Public Overridable Sub RejectChanges()

This method can be called to dismiss whatever changes where made on the records of the list(s).

 

Locating Records and Their Values

 

Locating a Record

Before performing any operation on a record, you must be able to locate it. That is, you must be able to identify a record among the various records of a table. To locate a record in the DataTable.Rows collection, the DataRowCollection class provides the Item property that is defined as follows:

Public Default ReadOnly Property Item(ByVal index As Integer) As DataRow

The records of a table are stored in a list (called the DataRowCollection). The first record, which in the example above has the title as "A Few Good Men" and the Director as "Rob Reiner", has an index of 0. The second record has an index of 1, and so on. Here is an example of using it to retrieve the information stored in a record:

Private Sub btnCreate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCreate.Click
        Dim row As DataRow = Me.dtVideos.Rows(4)
End Sub

When you pass an index to this property, the compiler would check whether the record exists. If a record with that index exists, its DataRow value is produced. If you specify an index that is either less than 0 or beyond the number of records in the table, the compiler would throw an IndexOutOfRangeException exception.

To get the number of records that a table contains, access the Count property of its DataRowCollection. The Count property is inherited from the InternalDataCollectionBase class, which is the parent of many collection classes.

When the records of a DataTable object have been created, you can get their list as an array using its List property that is inherited from the InternalDataCollectionBase class. This property returns an ArrayList type of list.

 

Practical Learning Practical Learning: Locating a Record

  1. Display the Purchase Order form and double-click an unoccupied area of its body away from (outside of) any group box
  2. In the top section of the file, type the following:
     
    Imports System.IO
    Imports System.Xml
    Imports System.Data
    
    Public Class PurchaseOrder
        Inherits System.Windows.Forms.Form
  3. Implement the Load event as follows:
     
    Private Sub PurchaseOrder_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) _
    	Handles MyBase.Load
            ' Get a reference to the object that holds the DataSet
            Dim frmData As DataCenter = New DataCenter
            ' Identify the file that holds the categories
            Dim strCategories As String = "Categories.xml"
    
            ' If that file exists, then open it
            If File.Exists(strCategories) Then
                frmData.dsMusicStore.ReadXml(strCategories)
            End If
    
            ' Just in case, empty the Categories list box
            Me.lbxCategories.Items.Clear()
    
            Dim row As DataRow = Nothing
            Dim i As Integer
            ' Scan the whole file to locate each category and retrieve it
            For i = 0 To frmData.tblCategories.Rows.Count - 1 Step 1
                row = frmData.tblCategories.Rows(i)
                Me.lbxCategories.Items.Add(row("Category"))
            Next
    End Sub
  4. Save all

Locating a Value

As mentioned already, a record is in fact one value or a group of values from each of the columns of the table. Consider the following table:

The "A Few Good Men" string is a value of the Title column. In the same way, "VH" is a value of the Format column. In some circumstances, you will need to locate a particular value in order to perform an operation on it. As seen above, you can start by locating the record you need and return its DataRow object. To know the table that the record belongs to, access its DataRow.Table property. This property is declared as follows:

Public ReadOnly Property Table As DataTable

To locate the value that a record holds under a particular column, the DataRow class provides the Item property that is overloaded with three versions (actually six, but we are interested in the first three only). One of the versions of this property uses the following syntax:

Overloads Public Default Property Item(ByVal columnName As String) As Object

To use this property, pass the object name of the column in the square brackets. The following example is based on the above table. It retrieves the title of the third video and displays it in the caption of the form:

Private Sub btnCreate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) _
		Handles btnCreate.Click
        dtVideos = New DataTable("ListOfVideos")
        Dim rowVideo As DataRow = Me.dtVideos.NewRow()

        Dim colVideos(7) As DataColumn
        colVideos(0) = New DataColumn("Title")
        colVideos(0).DataType = System.Type.GetType("System.String")
        colVideos(1) = New DataColumn("Director")
        colVideos(1).DataType = System.Type.GetType("System.String")
        colVideos(2) = New DataColumn("YearReleased", System.Type.GetType("System.Int16"))
        colVideos(3) = New DataColumn("Length", System.Type.GetType("System.String"))
        colVideos(4) = New DataColumn("Rating", System.Type.GetType("System.String"))
        colVideos(5) = New DataColumn("Format", System.Type.GetType("System.String"))
        colVideos(6) = New DataColumn("Category", System.Type.GetType("System.String"))

        dtVideos.Columns.AddRange(colVideos)

        rowVideo("Title") = "A Few Good Men"
        rowVideo("Director") = "Rob Reiner"
        rowVideo("YearReleased") = 1993
        rowVideo("Length") = "138 Minute"
        rowVideo("Rating") = "R"
        rowVideo("Format") = "VHS"
        rowVideo("Category") = "Drama"
        Me.dtVideos.Rows.Add(rowVideo)

        Dim vdoRecord() As String = {"Fatal Attraction", "Adrian Lyne", "1987", _
          "120 Minute", "R", "DVD", "Drama"}

        Me.dtVideos.Rows.Add(vdoRecord)

        Dim row As DataRow = Me.dtVideos.Rows(1)
        Dim strVideoTitle As String = row("Title")

        Text = strVideoTitle
End Sub

Instead of using the index of a column, you can also locate a value using the variable name of its column. To do this, you can use the following syntax of the DataRow indexed property:

Overloads Public Default Property Item(ByVal column As DataColumn) As Object

This property expects the object name of the column passed in its square brackets. The third option you have is to identify the column by its index. To do this, use the following syntax of the DataRow indexed property:

Overloads Public Default Property Item(ByVal columnIndex As Integer) As Object

This property expects the index of the column.

 

Practical Learning Practical Learning: Locating Values

  1. In the Class Name combo box, select lbxCategories
  2. In the Method Name combo box, select SelectedIndexChanged and implement its event as follows:
     
    Private Sub lbxCategories_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) _
    		Handles lbxCategories.SelectedIndexChanged
            ' Get a reference to the object that holds the DataSet
            Dim frmData As DataCenter = New DataCenter
            ' Identify the file that holds the items
            Dim strAvailableItems As String = "Inventory.xml"
            ' Identify the category that the user selected
            Dim strSelectedCategory As String = Me.lbxCategories.Text
    
            ' If that file exists, then open it
            If File.Exists(strAvailableItems) Then
                frmData.dsMusicStore.ReadXml(strAvailableItems)
            End If
    
            ' Empty the Item Types list box
            Me.lbxItemTypes.Items.Clear()
            ' Also empty the Available Items list box
            Me.lbxAvailableItems.Items.Clear()
    
            Dim row As DataRow = Nothing
            Dim i As Integer
            ' Scan the whole table to locate each record
            For i = 0 To frmData.tblAvailableItems.Rows.Count - 1 Step 1
                ' Get a reference to the current record
                row = frmData.tblAvailableItems.Rows(i)
                ' Get the name of the category of the current record
                Dim strCurrentCategory As String = row("Category")
    
                ' If the current category matches the one the user selected...
                If strCurrentCategory = strSelectedCategory Then
                    ' ... then get the corresponding item type
                    Dim strType As String = row("ItemType")
    
                    ' Find out if the Item Types list box already contains the item type
                    ' If it doesn't, then put it in the Item Types list box
                    If Not (Me.lbxItemTypes.Items.Contains(strType)) Then
                        Me.lbxItemTypes.Items.Add(strType)
                    End If
                End If
            Next
    End Sub
  3. In the Class Name combo box, select lbxItemTypes
  4. In the Method Name combo box, select SelectedIndexChanged
  5. Implement the event as follows:
     
    Private Sub lbxItemTypes_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) _
    		Handles lbxItemTypes.SelectedIndexChanged
            ' Get a reference to the object that holds the DataSet
            Dim frmData As DataCenter = New DataCenter
            ' Identify the file that holds the items
            Dim strAvailableItems As String = "Inventory.xml"
            ' Identify the category that the user selected
            Dim strSelectedCategory As String = Me.lbxCategories.Text
            Dim strSelectedType As String = Me.lbxItemTypes.Text
    
            ' If file exists, then open it
            If File.Exists(strAvailableItems) Then
                frmData.dsMusicStore.ReadXml(strAvailableItems)
            End If
            ' Empty the Available Items list box
            Me.lbxAvailableItems.Items.Clear()
    
            Dim row As DataRow = Nothing
            Dim i As Integer
            ' Scan the whole table to locate each record
            For i = 0 To frmData.tblAvailableItems.Rows.Count - 1 Step 1
                ' Get a reference to the current record
                row = frmData.tblAvailableItems.Rows(i)
                ' Get the name of the category of the current record
                Dim strCurrentCategory As String = row("Category")
                ' Get the item type of the current record
                Dim strCurrentType As String = row("ItemType")
    
                ' If the current category matches the one the user selected
                ' and the current item type matches the type the user selected ...
                If (strCurrentCategory = strSelectedCategory) And (strCurrentType = strSelectedType) Then
                    ' ... then get the corresponding item name
                    Me.lbxAvailableItems.Items.Add(row("ItemName"))
                End If
            Next
    End Sub
  6. In the Class Name combo box, select lbxAvailableItems
  7. In the Method Name combo box, select DoubleClick
  8. Implement the DoubleClick event as follows:
     
    Private Sub lbxAvailableItems_DoubleClick(ByVal sender As Object, ByVal e As System.EventArgs) _
    		Handles lbxAvailableItems.DoubleClick
            ' Get a reference to the object that holds the DataSet
            Dim frmData As DataCenter = New DataCenter
            ' Identify the file that holds the items
            Dim strAvailableItems As String = "Inventory.xml"
            ' Identify the item that the user selected
            Dim strSelectedCategory As String = Me.lbxCategories.Text
            Dim strSelectedType As String = Me.lbxItemTypes.Text
            Dim strSelectedName As String = Me.lbxAvailableItems.Text
    
            ' If file exists, then open it
            If File.Exists(strAvailableItems) Then
                frmData.dsMusicStore.ReadXml(strAvailableItems)
    
                Dim row As DataRow = Nothing
                ' Scan the whole table to locate each record
                Dim i As Integer
                For i = 0 To frmData.tblAvailableItems.Rows.Count - 1 Step 1
                    ' Get a reference to the current record
                    row = frmData.tblAvailableItems.Rows(i)
                    ' Get the name of the category of the current record
                    Dim strCurrentCategory As String = row("Category")
                    ' Get the item type of the current record
                    Dim strCurrentType As String = row("ItemType")
                    ' Get the name of the current item
                    Dim strCurrentName As String = row("ItemName")
                    ' Get the item number of the current item
                    Dim strCurrentNumber As String = row("ItemNumber")
                    ' Get the unit price of the current item
                    Dim strCurrentPrice As String = row("UnitPrice")
    
                    ' If the current parts match the ones the user selected ...
                    If (strCurrentCategory = strSelectedCategory) And _
                       (strCurrentType = strSelectedType) And _
                        (strCurrentName = strSelectedName) Then
                        ' ... then consider the corresponding item
                        If Me.txtItemNumber1.Text = "" Then
                            Me.txtItemNumber1.Text = strCurrentNumber
                            Me.txtItemName1.Text = strCurrentName
                            Me.txtUnitPrice1.Text = strCurrentPrice
                            Me.txtQuantity1.Text = "1"
                            Me.txtSubTotal1.Text = strCurrentPrice
                        ElseIf Me.txtItemNumber2.Text = "" Then
                            Me.txtItemNumber2.Text = strCurrentNumber
                            Me.txtItemName2.Text = strCurrentName
                            Me.txtUnitPrice2.Text = strCurrentPrice
                            Me.txtQuantity2.Text = "1"
                            Me.txtSubTotal2.Text = strCurrentPrice
                        ElseIf Me.txtItemNumber3.Text = "" Then
                            Me.txtItemNumber3.Text = strCurrentNumber
                            Me.txtItemName3.Text = strCurrentName
                            Me.txtUnitPrice3.Text = strCurrentPrice
                            Me.txtQuantity3.Text = "1"
                            Me.txtSubTotal3.Text = strCurrentPrice
                        ElseIf Me.txtItemNumber4.Text = "" Then
                            Me.txtItemNumber4.Text = strCurrentNumber
                            Me.txtItemName4.Text = strCurrentName
                            Me.txtUnitPrice4.Text = strCurrentPrice
                            Me.txtQuantity4.Text = "1"
                            Me.txtSubTotal4.Text = strCurrentPrice
                        ElseIf Me.txtItemNumber5.Text = "" Then
                            Me.txtItemNumber5.Text = strCurrentNumber
                            Me.txtItemName5.Text = strCurrentName
                            Me.txtUnitPrice5.Text = strCurrentPrice
                            Me.txtQuantity5.Text = "1"
                            Me.txtSubTotal5.Text = strCurrentPrice
                        Else ' if me.txtItemNumber6.Text= "" then
                            Me.txtItemNumber6.Text = strCurrentNumber
                            Me.txtItemName6.Text = strCurrentName
                            Me.txtUnitPrice6.Text = strCurrentPrice
                            Me.txtQuantity6.Text = "1"
                            Me.txtSubTotal6.Text = strCurrentPrice
                        End If
                    End If
                Next
            End If
    
            Me.btnCalculate_Click(sender, e)
    End Sub
  9. Execute the application and perform a few transactions

Record Maintenance

Once a table has been filled with records, you can perform maintenance operations on it such as changing some records or removing others. To remove a record from a table, you can call the DataRowCollection.Remove() method. Its syntax is:

Public Sub Remove(ByVal row As DataRow)

This method takes as argument a DataRow object and checks whether the table contains it. If that record exists, it gets deleted, including all of its entries for each column. When calling this method, you must pass an exact identification of the record. If you don't have that identification, you can delete a record based on its index. To do this, you would call the DataRowCollection.RemoveAt() method. Its syntax is:

Public Sub RemoveAt(ByVal index As Integer)

This method takes as argument the index of the record you want to delete. If a record with that index exists, it would be deleted.

To delete all records of a table, call the DataRowCollection.Clear() method. Its syntax is:

Public Sub Clear()

This method is used to clear the table of all records.

 

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