What's wrong with my cross-thread call in Windows Forms?

Question

I encounter a problem with a Windows Forms application.

A form must be displayed from another thread. So in the form class, I have the following code:

private delegate void DisplayDialogCallback();

public void DisplayDialog()
{
    if (this.InvokeRequired)
    {
        this.Invoke(new DisplayDialogCallback(DisplayDialog));
    }
    else
    {
        this.ShowDialog();
    }
}

Now, every time I run this, an InvalidOperationException is thrown on the line this.ShowDialog();:

"Cross-thread operation not valid: Control 'SampleForm' accessed from a thread other than the thread it was created on."

What's wrong with this piece of code? Isn't it a valid way to make cross-thread calls? Is there something special with ShowDialog()?

Answer 1

Try this one:

private delegate void DisplayDialogCallback();

public void DisplayDialog()
{
    if (this.InvokeRequired)
    {
        this.Invoke(new DisplayDialogCallback(DisplayDialog));
    }
    else
    {
        if (this.Handle != (IntPtr)0) // you can also use: this.IsHandleCreated
        {
            this.ShowDialog();

            if (this.CanFocus)
            {
                this.Focus();
            }
        }
        else
        {
            // Handle the error
        }
    }
}

Please note that InvokeRequired returns

true if the control's Handle was created on a different thread than the calling thread (indicating that you must make calls to the control through an invoke method); otherwise, false.

and therefore, if the control has not been created, the return value will be false!