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I have a main UI thread which runs the application and creates the main window form (let's call it W). I have also a secondary thread that I spin up and which creates a dialog box (let's call it B).

I want to set the owner of the dialog B to be the main window W. The setting of Bs owner happens on the thread that created B. Basically:

b.Owner = w;

but this throws a cross-thread exception telling me that I am tryng to access the W object from the wrong thread.

So I tried to execute the code on the main UI thread, by using a Control.Invoke on W. But then, I get the same error telling me that I am trying to access B from the wrong thread:

System.InvalidOperationException was unhandled by user code
  Message=Cross-thread operation not valid: Control 'B' accessed from a
  thread other than the thread it was created on.
  Source=System.Windows.Forms

How am I supposed to do it right?

Pierre Arnaud
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  • Don't use forms on multiple threads. It will give you more trouble than it's worth. – SLaks Mar 11 '11 at 13:51
  • Did you read this SO post: http://stackoverflow.com/questions/3046245/whats-wrong-with-my-cross-thread-call-in-windows-forms ? If this does not help you, you should post the code which gives you the error. – Doc Brown Mar 11 '11 at 13:51
  • Perhaps you can tell us which feature(s) of ownership you were hoping to take advantage of (hopefully not all of them), and we could suggest ways to achieve similar effects? – Damien_The_Unbeliever Mar 11 '11 at 14:05
  • Thanks to all who posted; I know that I am asking for trouble. I need to run some legacy code on the main UI thread, and I want to have a progress dialog showing while this is happening; and I can't let the main UI thread handle the progress dialog. So I went for a progress dialog managed 100% by another thread. – Pierre Arnaud Mar 12 '11 at 21:42

3 Answers3

8

It's a bit of a bug in Winforms, Windows actually supports making the owner a window that was created on another thread. There's a way to disable that check, something you should never do. Except when you have to I suppose:

    private void button1_Click(object sender, EventArgs e) {
        var t = new Thread(() => {
            Control.CheckForIllegalCrossThreadCalls = false;
            var frm = new Form2();
            frm.Show(this);
            Control.CheckForIllegalCrossThreadCalls = true;
            Application.Run(frm);
        });
        t.SetApartmentState(ApartmentState.STA);
        t.Start();
    }

I do not know if this is 100% safe, there could be a Winforms interaction that screws things up. You are in untested waters here, infested with threading sharks.

Hans Passant
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  • Your suggestion is interesting. In my case, I can make sure that the code which shows the dialog won't interfere with the main UI thread. I'll give it a try. – Pierre Arnaud Mar 12 '11 at 21:45
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    Have you found out anything further about the safety of the above approach? I would like to do this (have one form own a form which is bound to a different thread) but see two concerns: (1) Showing a form which is owned by another will call `Owner.AddOwnedForm` which is probably not thread-safe; better to do the action on the owner form's thread; (2) If two forms bound to different threads try to show themselves simultaneously, one might set `CheckForIllegalCrossThreadCalls=true` between the time the other sets it false and the time it performs the operation that requires it to be false. – supercat Jun 19 '13 at 18:25
  • Apparently another solution exists, where you pass the parent's `.Handle` to the thread's procedure as a parameter, and inside the procedure [create a `NativeWindow`](https://stackoverflow.com/a/10957261/11683) from that handle and pass it to `.Show()`. Then it is not required to switch off the `CheckForIllegalCrossThreadCalls`. – GSerg Dec 17 '19 at 17:02
3

B needs to be created on the UI thread.

You can still interact with B from the secondary thread by using Control.Invoke.

ChrisF
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  • In my case, it won't work for what I intend to do: my `B` window won't update if the main UI thread is blocked doing its work. – Pierre Arnaud Mar 12 '11 at 21:43
2

If you're actually running two message loops on different threads, then there's no way to do what you're after. If you want W to own B, you're going to have to create B on the main thread and Invoke all of your interaction with B from the second thread.

Adam Robinson
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