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.

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

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.

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

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.

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.

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
     
    
11. Close the application and execute it again