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Example Application:
Solas Property Rental

 

Introduction

To support arrays in an application, the .NET Framework provides the array class. The primary characteristic of the array class is that it follows the features, including the strengths and weaknesses, of an array. One of the strengths of an array is that each item can be located by a specific index. One of the weaknesses of an array is that, unlike linked list, the collection of items can neither shrink nor grow.

Because array is a class, it is equipped with properties and methods. Besides the regular features of a class, array is serializable. This means that you can use either binary or SOAP serialization to save the items of an array.

To serialize our list, we will use binary serialization.

 

Practical Learning Practical Learning: Introducing Array Serialization

  1. If you want to follow this exercise, start Microsoft Visual Basic
  2. To start a new application, on the main menu, click File -> New Project...
  3. In the Templates list, click Windows Application
  4. Set the Name to SolasPropertyRental1 and click OK
  5. In the Solution Explorer, right-click Form1.vb and click Rename
  6. Type RentalProperties.vb and press Enter
  7. From the Toolbox, click ListView and click the form
  8. While the list view is still selected on the form, in the Properties window, click Columns and click its ellipsis button
  9. Create the following columns:
     
    (Name) Text TextAlign Width
    colPropertyIndex # Right 30
    colPropertyNumber Property #   70
    colPropertyType Property Type   100
    colPropertyCondition Condition   80
    colBedrooms Bedrooms Right  
    colBathrooms Bathrooms Right 70
    colMonthlyRent Monthly Rent Right 80
  10. Click OK
  11. Complete the design of the form as follows:
     
    Control Name Text Other Properties
    ListView lvwProperties   FullRowSelect: True
    GridLines: True
    View: Details
    Anchor: Top, Bottom, Left, Right
    Button btnNewProperty New Property Anchor: Bottom, Left
    Button btnClose Close Anchor: Bottom, Right
  12. To create a new form, on the main menu, click Project -> Add New Item...
  13. In the Templates list, click Windows Form
  14. Set the Name to NewProperty and click Add
  15. Design the form as follows:
     
    Control Name Text Other Properties
    Label   Property Type:  
    Button btnOK OK  
    Combo Box cboPropertyTypes Unknown Items:
    Apartment
    Single Family
    Townhouse
    Unknown
    Button btnOK OK DialogResult: OK
    Label   Property Condition:  
    Combo Box cboPropertyConditions Unknown Items:
    Excellent
    Good
    Bad Shape
    Unknown
    Button btnCancel Cancel DialogResult: Cancel
    Label   Bedrooms:  
    TextBox txtBedrooms 0 TextAlign: Right
    Label   Bathrooms:  
    TextBox txtBathrooms 0.00 TextAlign: Right
    Label   Monthly Rent:  
    TextBox txtMonthlyRent 0.00  
    Form     FormBorderStyle: FixedDialog
    AcceptButton: btnOK
    CancelButton: btnCancel
    StartPosition: CenterScreen
    MaximumBox: False
    MinimumBox: False
    ShowInTaskbar: False
  16. Save all

Using an Indexed Property

The application we are creating is used by a property rental company to review its available products. To manage the houses and apartments to rent, we will create a class that holds a property. Because our application is list-based, we will create an indexed property in a class so we can access each property with an index applied to an instance of the that class.

One of the problems with an array is that its list of items must be established in the beginning, but in our application we want the user to be able to add an item to the array. In the next section, we will see that, when the application starts, we create a default list of items but we will still give the user the ability to create or add items. To solve the problem of monitoring the items in the list, we will add a flag to each property. When a default item has not been set, this flag will indicate it. Eventually, when a user adds an item, we will change this flag to indicate that the property has been changed from its default values. We could use a Boolean value for this flag to hold a true or a false result but it is sometimes a little complex to serialize some Boolean values of a collection. For this reason, we will make the flag a numeric value. Before a property has been set, this flag will hold a 0 value. When a property has been set, this flag will become 1.

Starting the List

One of the problems with an array is that, when creating it, you must specify the number of items in the list. This is because, from the beginning, the list uses a fixed number of items. This also implies that you cannot add an item that increases the volume of the collection. Because the number of items is fixed, after the list has been created, you can only replace an existing item.

In our application, we need to overcome or at least go around these limitations of arrays. There are various actions we can take. To start, when the application is launched, we will create a list of items but we will fill out each item with insignificant default values. This simply allows us to create a complete array.

Practical Learning Practical Learning: Creating an Indexed Property

  1. To create a new class, on the main menu, click Project -> Add Class...
  2. In the Templates list, make sure Class is selected.
    Set the Name to RentalProperty and click Add
  3. Change the header file as follows:
    Imports System
    
    <Serializable()> Public Class RentalProperty
        ' These are the characteristics of a property
        Dim propCode As Long
        Dim type As String
        Dim cond As String
        Dim beds As Short
        Dim baths As Double
        Dim val As Double
    
        ' This flag is used to specify whether a property 
        ' still holds the default values or it has 
        ' previously been updated
        Dim CreationFlag As Integer
    
        ' This constructor is used to create 
        ' default values for a property
        Public Sub New()
            Dim rnd As Random = New Random
            propCode = rnd.Next(100000, 999999)
            type = "Unknown"
            cond = "Unknown"
            beds = 0
            baths = 0.0
            val = 0.0
            CreationFlag = 0
        End Sub
    
        Public Property PropertyCode() As Long
            Get
                Return propCode
            End Get
            Set(ByVal value As Long)
                propCode = value
            End Set
        End Property
    
        Public Property PropertyType() As String
            Get
                Return type
            End Get
            Set(ByVal value As String)
                type = value
            End Set
        End Property
    
        Public Property PropertyCondition() As String
            Get
                Return cond
            End Get
            Set(ByVal value As String)
                cond = value
            End Set
        End Property
    
        Public Property Bedrooms() As Short
            Get
                Return beds
            End Get
            Set(ByVal value As Short)
                beds = value
            End Set
        End Property
    
        Public Property Bathrooms() As Double
            Get
                If baths <= 0 Then
                    Return 0.0
                Else
                    Return baths
                End If
            End Get
            Set(ByVal value As Double)
                baths = value
            End Set
        End Property
    
        Public Property MonthlyRent() As Double
            Get
                If val <= 0 Then
                    Return 0.0
                Else
                    Return val
                End If
            End Get
            Set(ByVal value As Double)
                val = value
            End Set
        End Property
    
        Public Overrides Function ToString() As String
            Return "Property #:   " & PropertyCode & 
                   vbCrLf & "Type:         " & PropertyType & 
                   vbCrLf & "Condition:    " & PropertyCondition & 
                   vbCrLf & "Bedrooms:     " & Bedrooms & 
                   vbCrLf & "Bathrooms:    " & Bathrooms & 
                   vbCrLf & "Monthly Rent: " & MonthlyRent.ToString("C")
        End Function
    
    End Class
  4. To create a new class, on the main menu, click Project -> Add Class...
  5. In the Templates list, make sure Class is selected.
    Set the Name to PropertyListing and click Add
  6. Change the file as follows:
    Imports System.IO
    Imports System.Runtime.Serialization
    Imports System.Runtime.Serialization.Formatters.Binary
    
    Public Class PropertyListing
        Public Dim props(100) As RentalProperty ' = New RentalProperty()
    
        Public Sub New()
    
            ' Check if the default list of properties has never been created.
            ' If there is no default list of properties,
            ' Then create it and save the file
            If Not File.Exists("Properties.prp") Then
                CreateDefaultListing()
            End If
    
            ' Since we have a file that holds the list of properties
            ' open it and store the properties in our array
            OpenProperties()
        End Sub
    
        Public Property RentalProperty(ByVal index As Integer) As RentalProperty
            Get
                If (index >= 0) And (index < 100) Then
                    Return props(index)
                Else
                    Return Nothing
                End If
            End Get
            Set(ByVal value As RentalProperty)
                props(index) = value
            End Set
        End Property
    
        Public Sub SaveProperties()
            Dim strFilename As String = "Properties.prp"
            Dim fstProperties As FileStream = New FileStream(strFilename, 
            FileMode.Create)
            Dim bfmProperties As BinaryFormatter = New BinaryFormatter
    
            Try
                bfmProperties.Serialize(fstProperties, props)
            Catch ex As ArgumentNullException
                MsgBox("The properties listing is not available")
            Catch exc As SerializationException
                MsgBox("The listing could not be saved")
            Finally
                fstProperties.Close()
            End Try
        End Sub
    
        Public Sub CreateDefaultListing()
            Dim rndNumber As Random = New Random
            Dim i As Integer
            Dim rental As RentalProperty
    
            For i = 0 To 100
                rental = New RentalProperty
                rental.PropertyCode = rndNumber.Next(100000, 999999)
                rental.PropertyType = "Unknown"
                rental.PropertyCondition = "Unknown"
                rental.Bedrooms = 0
                rental.Bathrooms = 0
                rental.MonthlyRent = 0.0
                rental.CreationFlag = 0
                props(i) = rental
            Next
    
            SaveProperties()
        End Sub
    
        Public Sub OpenProperties()
            Dim strFilename As String = "Properties.prp"
            Dim fstProperties As FileStream = Nothing
            Dim bfmProperties As BinaryFormatter = Nothing
    
            ' If the list of properties had already been created
            ' then open it
            If File.Exists(strFilename) Then
    
                Try
                    fstProperties = New FileStream(strFilename, 
              FileMode.Open)
                    bfmProperties = New BinaryFormatter
                    props = bfmProperties.Deserialize(fstProperties)
                Catch ex As ArgumentNullException
                    MsgBox("The properties listing is not available")
                Catch exc As SerializationException
                    MsgBox("The listing could not be opened")
                Finally
                    fstProperties.Close()
                End Try
        else
                Return
            End If
        End Sub
    End Class
  7. Save all

Adding or Updating an Item

To give the user the ability to add a property to the list, we will look for an item whose creation flag is set to 0. When we find a flag with that value, we will consider that its corresponding property has not been set. Once we find that property, we will replace it with the values of the new property.

Displaying the List

As discussed so far, our array will contain both default and appropriate properties. Default properties are those that contain unrecognizable values. To give a good impression of a list of properties that can be added to, there is no reason to display the properties that have never been updated by the user. We will show only properties that the user know have good values. To identify these properties, once again the flag we created in the class of properties will be helpful. When we want to display the properties, we will check the status or value of this property. If it is set to 0, we will not display the property.

Practical Learning Practical Learning: Displaying the Properties

  1. Display the first form and double-click an empty area of its body
  2. On top of the Load event, create a new method and implement the Load event as follows:
    Public Class RentalProperties
        Public Sub ShowProperties()
            Dim properties As PropertyListing = New PropertyListing
    
            properties.OpenProperties()
            lvwProperties.Items.Clear()
            Dim i As Integer
    
            For i = 0 To 100
                Dim iFlag As Integer = properties.props(i).CreationFlag
                If iFlag <> 0 Then
                    Dim lvi As ListViewItem = 
    			lvwProperties.Items.Add((i + 1).ToString())
    
                    lvi.SubItems.Add(properties.props(i).PropertyCode.ToString())
                    lvi.SubItems.Add(properties.props(i).PropertyType)
                    lvi.SubItems.Add(properties.props(i).PropertyCondition)
                    lvi.SubItems.Add(properties.props(i).Bedrooms.ToString())
                    lvi.SubItems.Add(properties.props(i).Bathrooms.ToString())
                  lvi.SubItems.Add(properties.props(i).MonthlyRent.ToString("C"))
                End If
            Next
        End Sub
        
        Private Sub Form1Load(ByVal sender As System.Object, 
    		ByVal e As System.EventArgs) Handles MyBase.Load
            ShowProperties()
        End Sub
    End Class
  3. In the Class Name combo box, select btnNewProperty
  4. In the Method Name combo box. select Click
  5. Implement the event as follows:
    Public Class RentalProperties
        Public Sub ShowProperties()
            Dim properties As PropertyListing = New PropertyListing
    
            properties.OpenProperties()
            lvwProperties.Items.Clear()
            Dim i As Integer
    
            For i = 0 To 100
                Dim iFlag As Integer = properties.props(i).CreationFlag
                If iFlag <> 0 Then
                    Dim lvi As ListViewItem = 
    			lvwProperties.Items.Add((i + 1).ToString())
    
                  lvi.SubItems.Add(properties.props(i).PropertyCode.ToString())
                    lvi.SubItems.Add(properties.props(i).PropertyType)
                    lvi.SubItems.Add(properties.props(i).PropertyCondition)
                    lvi.SubItems.Add(properties.props(i).Bedrooms.ToString())
                    lvi.SubItems.Add(properties.props(i).Bathrooms.ToString())
                 lvi.SubItems.Add(properties.props(i).MonthlyRent.ToString("C"))
                End If
            Next
        End Sub
        Private Sub Form1Load(ByVal sender As System.Object, 
    		ByVal e As System.EventArgs) Handles MyBase.Load
            ShowProperties()
        End Sub
    
        Private Sub btnNewPropertyClick(ByVal sender As Object, 
            ByVal e As System.EventArgs) Handles btnNewProperty.Click
            Dim dlgProperty As NewProperty = New NewProperty
            Dim i As Integer
    
            If dlgProperty.ShowDialog() = 
    		System.Windows.Forms.DialogResult.OK Then
                Dim listing As PropertyListing = New PropertyListing
    
                For i = 0 To 100
                    If listing.props(i).CreationFlag = 0 Then
                        Try
                            listing.props(i).PropertyType = 
                            dlgProperty.cboPropertyTypes.Text()
                        Catch ex As FormatException
                            MsgBox("Invalid Property Type")
                        End Try
    
                        Try
                            listing.props(i).PropertyCondition = 
                            dlgProperty.cboPropertyConditions.Text()
                        Catch exc As FormatException
                            MsgBox("Invalid Property Condition")
                        End Try
    
                        Try
                            listing.props(i).Bedrooms = 
                            Short.Parse(dlgProperty.txtBedrooms.Text)
                        Catch exc As FormatException
                            MsgBox("Invalid Bedroom Value")
                        End Try
    
                        Try
                            listing.props(i).Bathrooms = 
                                      CDbl(dlgProperty.txtBathrooms.Text)
                        Catch excp As FormatException
                            MsgBox("Invalid Bathroom Value")
                        End Try
    
                        Try
                            listing.props(i).MonthlyRent = 
                                       CDbl(dlgProperty.txtMonthlyRent.Text)
                        Catch excpt As FormatException
                            MsgBox("Unrecognizable Monthly Rent Value")
                        End Try
    
                        listing.props(i).CreationFlag = 1
                        listing.SaveProperties()
                        ShowProperties()
    
                        Return
                    End If
                Next
    
                MsgBox("You cannot create a new property." & 
                   "You can only modify or replace an existing one.")
            End If
    
            ShowProperties()
        End Sub
    End Class
  6. In the Class Name combo box, select btnClose
  7. In the Method Name combo box. select Click
  8. Implement it as follows:
    Private Sub btnCloseClick(ByVal sender As Object, 
            ByVal e As System.EventArgs) Handles btnClose.Click
            End
    End Sub
  9. Execute the application to see the result
  10. Create a few properties
     
    Solas Property Rental
  11. Close the application and execute it again
 

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