Delegates in VB.NET

Delegates in .NET Framework are equivalent to Function pointers in C and C++ language. Delegates helps in multithreaded application where one thread is accessing a UI element created by another thread.

For e.g. the follwoing code will always return an error of type "Cross-thread operation not valid: Control 'TextBox1' accessed from a thread other than the thread it was created on."

Imports System.Threading

Imports System.Reflection

Public Class Form1
    Private trd As Thread
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        trd = New Thread(AddressOf DoTask)

        trd.IsBackground = True
        trd.Start()

    End Sub

    Private Sub DoTask()
        Dim i As Integer
        i = 1
        While i <= 1000
            System.Console.WriteLine("the value of i is:" + i.ToString())
            i = i + 1
            Thread.Sleep(1000)
            TextBox1.Text = i.ToString()
        End While
    End Sub

In the above example the thread is invoking the textbox1 which is running under MainForm Thread. That's why we receive this error. This can be removed with the help of Delegates. The following code shows how to remove the above error and run the code:

Imports System.Threading

Imports System.Reflection

Public Class Form1
    Private trd As Thread

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        trd = New Thread(AddressOf DoTask)
        trd.IsBackground = True
        trd.Start()
    End Sub

    Private Sub DoTask()

        Dim i As Integer
        i = 1
        While i <= 1000
            System.Console.WriteLine("the value of i is:" + i.ToString())
            i = i + 1
            Thread.Sleep(1000)
            SetControlPropertyValue(TextBox1, "Text", i)
       End While
    End Sub

    Private Delegate Sub SetControlValueCallback(ByVal oControl As Control, ByVal propName As String, ByVal propValue As Object)

    Private Sub SetControlPropertyValue(ByVal oControl As Control, ByVal propName As String, ByVal propValue As Object)

        If oControl.InvokeRequired Then
            Dim d As New SetControlValueCallback(AddressOf SetControlPropertyValue)
            oControl.Invoke(d, New Object() {oControl, propName, propValue})
        Else
            Dim t As Type = oControl.[GetType]()
            Dim props As PropertyInfo() = t.GetProperties()
            For Each p As PropertyInfo In props
                If p.Name.ToUpper() = propName.ToUpper() Then
                    p.SetValue(oControl, propValue.ToString(), Nothing)
                End If
            Next
        End If
    End Sub

End Class