Properties and Object Oriented Database Development with VB.NET 2005 - Accessing the object properties from the form

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Modifying the class as in the previous section would cause errors to be thrown in your form code.  You need to modify the form code as follows to work with properties:

Private Sub btnSearch_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSearch.Click
        Me.lblErrMsg.Text = ""
        Try
            Dim ep As New Emp
            Dim objReturned As Emp
            objReturned = ep.getEmployee(Me.txtEmpno.Text)
            Me.txtDeptno.Text = objReturned.deptno
            Me.txtEname.Text = objReturned.ename
            Me.txtSal.Text = objReturned.sal
        Catch ex As Exception
            Me.lblErrMsg.Text = ex.Message
        End Try
    End Sub

    Private Sub btnAdd_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnAdd.Click
        Me.lblErrMsg.Text = ""
        Try
            Dim objParam As New Emp
            With objParam
                .deptno = Me.txtDeptno.Text
                .empno = Me.txtEmpno.Text
                .ename = Me.txtEname.Text
                .sal = Me.txtSal.Text
            End With
            Dim ep As New Emp
            ep.add(objParam)
        Catch ex As Exception
            Me.lblErrMsg.Text = ex.Message
        End Try
    End Sub

Developing read-only/write-only properties

There will be times when you need only read-only/write-only properties.  The best example would be something like the following statement:

a = Me.ListBox1.Items.Count

If I try to write the same statement shown above as something like the following, it would give an error:

Me.ListBox1.Items.Count = a

That means we can only read from the property “Count,” but not assign any value to it, which is what is called a “read-only” property. 

Let us now try to determine a read-only property called “EmployeeCount,” which returns the number of employees existing in the table.  The code would be something like the following:

Public ReadOnly Property EmployeeCount() As Integer
        Get
            Dim cn As New SqlConnection("Data Source=.sql2k5;initial catalog=sample;user id=sa;password=eXpress2005")
            Dim cmd As New SqlCommand("select count(*) from sample.dbo.emp", cn)
            Try
                Dim n As Integer
                cmd.Connection.Open()
                n = cmd.ExecuteScalar
                Return n
            Catch ex As Exception
                Throw New Exception(ex.Message)
            Finally
                If cmd.Connection.State = ConnectionState.Open Then
                    cmd.Connection.Close()
                End If
                cmd.Dispose()
                cn.Dispose()
            End Try
        End Get
    End Property

You can observe that the above property doesn’t have any “set” part and the property declaration is given with “readonly.” To use this property, I added a simple label called “lblNoOfEmployees” to the form and added the following code:

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Try
            Dim ep As New Emp
            Me.lblNoOfEmployees.Text = "No. of employees: " & ep.EmployeeCount
        Catch ex As Exception
            Me.lblErrMsg.Text = ex.Message
        End Try
    End Sub

Very similar to the above read-only property, we also have a write-only property.  The write-only property is used only to assign a value, but not for retrieving a value.  In the case of the write-only property, we will have only a “set” part without a “get” part and the declaration is given with “writeonly.” 

Next: Using read-only properties to deal with error messages >>
 

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