Multi-threading and cross-thread function invoke

Introduction

If you have ever worked on a multithreaded application, you probably know that accessing controls of UI thread (like textbox, labels etcc...) directly from another thread is not possible . Acessing UI controls directly from a thread other than UI thread would give you a run-time error .

Here is the simple solution to access the UI controls from a background thread.

Steps

1. Write a function that access the UI controls (here we are displaying error info of the background thread).

Private Sub DisplayError(ByVal errMsg As String)
{
  lblMsg.Text = errMsg
}

2. Write another function that decides if it is being called from the UI thread or from another thread- If its called from its own thread, call the DisplayError() function directly or else invoke it through a delegate.

private Sub DisplayErrorOnUI(ByVal errMsg As String)
{
    If (Me.InvokeRequired) Then
        Dim invokeDelegate As _invokeUIControlDelegate = _
                 New _invokeUIControlDelegate(AddressOf DisplayError)
        Me.Invoke(_invokeUIControlDelegate, errMsg)
    Else
        DisplayError(errMsg)  
}

3. Declare a delegate in the class (or form in this case) to the above function (i.e., isplayErrorOnUI()).

Private Delegate Sub _invokeUIControlDelegate(ByVal errMsg As String)

4. In the background thread call the InvokeUIControl() function as required.

Try
  // do something here that gives error
Catch ex As Exception
  DisplayErrorOnUI(ex.Message)
End Try

This will definitely help... Thanks.