Sunday, June 15, 2008

Creating Form on different Thread (UI Threading)

Level: Advanced

Knowledge Required:

Recently I was doing some research on Threading, I found that we can create Form in other Thread, but it requires to Execute Message Loop using Application.Run.

I have created 2 forms (Form1 and Form2) and put a Button on Form1 which creates the Form2 in different thread and shows it.
Form1 Code:
Public Class Form1
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Debug.Print("Form1.Button1_Click: Current Thread=" & Threading.Thread.CurrentThread.ManagedThreadId)
Dim t As Threading.Thread
t = New Threading.Thread(AddressOf ShowForm2)
End Sub
Private Sub ShowForm2()
Debug.Print("Form1.ShowForm2: Current Thread=" & Threading.Thread.CurrentThread.ManagedThreadId)
Dim frmNew As Form2
frmNew = New Form2
'here frmNew.Show() will NOT work properly
'because we are in other thread then main thread

End Sub
End Class

And in Form2 I just put code in Load Event Handler to check the Current Thread in which Form2 is.
Form2 Code:
Public Class Form2
Private Sub Form2_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Debug.Print("Form2_Load: Current Thread=" & Threading.Thread.CurrentThread.ManagedThreadId)
End Sub
End Class

I have put the Debug statements so that we can see which thread is performing that task, so the output of program,
Form1.Button1_Click: Current Thread=11
Form1.ShowForm2: Current Thread=12
Form2_Load: Current Thread=12

As you can see the output, when Button1 was clicked that was Thread=11 and when ShowForm2 method on Form1 was executed then it was Thread=12, therefore when Form2_Load() event fired it was also Thread=12.

So the new thread that was created will remain alive until the Form2 is NOT closed. I am NOT sure but I think this type of behaviour is called UI Threading.

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