Visual Basic .NET List-Based Applications: The Music Store

	Lists-Based Applications: The Music Store

Introduction

The DataSet of the .NET Framework is a very impressive class. It allows you to create and manage any type of list, making it a central point for a list-based application, although it is highly used in formal database applications such as those intended for Microsoft SQL Server or Microsoft Access.

To support the creation and management of any type of list, the DataSet class has complete built-in mechanisms for creating tables, their associated columns, and to perform data entry. To demonstrate these functionalities of the DataSet, we are going to create a list-based application that can serve as another introduction to databases. The application simulates a music instrument store that processes orders for customers.

When a customer places an order in the store to purchase a product, the employee can select items by categories, specify the quantity, calculate the total order, and save it.

Practical Learning: Creating the Application

Start Microsoft Visual Studio .NET and create a new Windows Application named MusicStore2
To add a new form, on the main menu, click Project -> Windows Form...
Set the Name to DataCenter and press Enter
To add a new form, on the main menu, click Project -> Windows Form...
Set the Name to Categories and press Enter

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

To add a new form, on the main menu, click Project -> Windows Form...
Set the Name to ItemTypes and press Enter

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

To add a new form, on the main menu, click Project -> Windows Form...
Set the Name to NewStoreItem and press Enter

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

To add a new form, on the main menu, click Project -> Add Windows Form...
Set the Name to PurchaseOrder and press Enter

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

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

In the Class Name combo box, select btnCalculate

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

Display the first form (Form1.cs (Design)) and design it as follows:

Control	Name	Text
Button	btnNewItem	New Store Item
Button	btnNewOrder	New Purchase Order
Button	btnClose	Close

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

In the Class Name combo box, select btnNewOrder

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

In the Class Name combo box, select btnClose
In the Method Name combo box, select Click

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

Save all

Application Data Entry

To perform data entry, we will apply the concepts reviewed for data records.

Practical Learning: Introducing Data Entry

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

Add the following columns to it

(Name)	ColumnName	Modifiers
colItemNumber	ItemNumber	Public
colItemCategory	Category	Public
colTypeOfItem	ItemType	Public
colItemName	ItemName	Public
colUnitPrice	UnitPrice	Public

Click Close twice
Display the Categories form and double-click its Add button

In the top section of the file, type the following:

Imports System.IO
Imports System.Xml
Imports System.Data

Public Class Categories
    Inherits System.Windows.Forms.Form

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
        ' 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

Display the ItemTypes form and double-click its Add button

In the top section of the file, type the following:

Imports System.IO
Imports System.Xml
Imports System.Data

Public Class ItemTypes
    Inherits System.Windows.Forms.Form

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

Save all

Saving the Items and the Orders

Based on the close relationship between the DataSet class and XML, we will save our records as XML.

Practical Learning: Opening an XML File

Display the New Store Item form.
double-click an unoccupied area of its body

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

Implement the 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

In the Class Name combo box, select btnNewCategory
In the Method Name combo box, select Click

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 = 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

In the Class Name combo box, select btnNewType
In the Method Name combo box, select Click

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 = 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

In the Class Name combo box, select txtItemName

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

In the Class Name combo box, select txtUnitPrice

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

In the Class Name combo box, select txtItemNumber

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

In the Class Name combo box, select btnCreate

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

Execute the application
Click the New Store Item button

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

Close the Categories form and open the Item Types form

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

Close the Item Types form

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

Close the forms and return to your programming environment
Display the Purchase Order form and double-click an unoccupied area of its body away from (outside of) any group box

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

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)

            ' 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 If
End Sub

In the Class Name combo box, select lbxCategories

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

In the Class Name combo box, select lbxItemTypes
In the Method Name combo box, select SelectedIndexChanged

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

In the Class Name combo box, select lbxAvailableItems
In the Method Name combo box, select DoubleClick

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

Execute the application and perform a few transactions


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