I have a windows application in VB.NET. The main form has a listview with approx 15 items and five subitems in each item. The list view acts as a display grid for data. The application performs many operations at once so naturally I have made it threaded to keep the main form responsive. The threads are running in their own classes. To give you a general idea see below.
CODE
Public Class frmMain
…
Dim loginThread As New Thread(AddressOf modBotControler. dosomething)
loginThread.start
…
End class
Public Class modBotControler
…
Sub dosomething()
…
End sub
End class
Now I need to update the listview from the thread in dosomething(). At first I just passed a variable as a property into the modBotControler class. This worked fine except the program would crash for no good reason sparatically. I moved to Visual Studio 2005 and found the error of my way (at least I hope so). Apparently you can not change the value of a windows control from a different thread than it was created on. Research showed that MSDN offered a solution. ms-help://MS.VSExpressCC.v80/MS.NETFramework.v20.en/dv_fxmclictl/html/138f38b6-1099-4fd5-910c-390b41cbad35.htm
The solution uses a delegate to move the operation to the thread that the main form is on.
CODE
' This event handler creates a thread that calls a
' Windows Forms control in a thread-safe way.
Private Sub setTextSafeBtn_Click( _
ByVal sender As Object, _
ByVal e As EventArgs) Handles setTextSafeBtn.Click
Me.demoThread = New Thread( _
New ThreadStart(AddressOf Me.ThreadProcSafe))
Me.demoThread.Start()
End Sub
' This method is executed on the worker thread and makes
' a thread-safe call on the TextBox control.
Private Sub ThreadProcSafe()
Me.SetText("This text was set safely.")
End Sub
' This method demonstrates a pattern for making thread-safe
' calls on a Windows Forms control.
'
' If the calling thread is different from the thread that
' created the TextBox control, this method creates a
' SetTextCallback and calls itself asynchronously using the
' Invoke method.
'
' If the calling thread is the same as the thread that created
' the TextBox control, the Text property is set directly.
Private Sub SetText(ByVal [text] As String)
' InvokeRequired required compares the thread ID of the
' calling thread to the thread ID of the creating thread.
' If these threads are different, it returns true.
If Me.textBox1.InvokeRequired Then
Dim d As New SetTextCallback(AddressOf SetText)
Me.Invoke(d, New Object() {[text]})
Else
Me.textBox1.Text = [text]
End If
End Sub
Now I am sure that this works wonderful if you have everything thing in one class but I do not. One requirement is that the SetText function be private. You will also notice that when SetText is called, Me.SetText("This text was set safely."), that they used the Me operator which I can not do outside the current class. I have tried passing a reference to the main form and calling it that way, didn’t work. I have tried passing a reference to the SetTextDelegate, didn’t work. I have also tried about 100 other different things, didn’t work. If anyone has ever been in this sisutiton, please let me know what you did to fix it. BTW, the example uses a textbox and I am using a listview. Does that make a difference?
