When would ShowDialog() return null?

Question

WPF's Window.ShowDialog method returns a nullable boolean. So does CommonDialog.ShowDialog.

Now, I understand cases where these would return false (user clicked Cancel or pressed Esc), and when they would return true (code sets Window.DialogResult to true, probably in response to OK being clicked). But null?

My first thought is that clicking the title bar's Close button might return null. But the docs state (and I confirmed by testing) that the title-bar Close button is treated as a Cancel.

So when would Window.ShowDialog or CommonDialog.ShowDialog ever return null?

You have to love MS documentation. "A Nullable)>) value of type Boolean that signifies how a window was closed by the user." Gee, that helps. — Matthew Flaschen, Jun 13 '09 at 06:19
Sounds like the kind of documentation where the writers weren't allowed to talk to the developers and ask questions, but were only allowed access to what they had on the screen... — SqlRyan, Jun 13 '09 at 06:52

score 27 · Accepted Answer · answered Jun 13 '09 at 06:32

27

The method always returns true or false, and this is always equal to the DialogResult property of the window at the time it closes.

But the DialogResult property is null before the window is closed, and another thread could check the property. So it kind of makes sense that the return value is a nullable boolean to match the property, even though it is never actually null.

answered Jun 13 '09 at 06:32

uncleO

7,997
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Not sure where you get the idea that another thread could check DialogResult. If you try, you get an InvalidOperationException ("The calling thread cannot access this object because a different thread owns it.") – Joe White Jun 13 '09 at 06:43
6

Not another thread, but any code running (say) on an event handler on the dialog could retrieve the value of DialogResult before it completes. Personally I think it's a poor design choice. It should have been non-nullable, the getter throwing an exception if accessed before the dialog quits. – Daniel Earwicker Jun 13 '09 at 08:55
I just had this method return `null` and went to this question to look it up, so (since at least now) this method definitely **can** return null. There was no exception whatsoever, the window simply didn't show up for whatever reason and this method simply returned - `null`. I am now left with nothing else than to debug WPF code to find out why. – bokibeg Oct 02 '19 at 11:54

score 2 · Answer 2 · answered Mar 18 '10 at 04:40

2

If I return DialogResult = null in the Click event for a button, the window remains open.

private void OkButton_Click(object sender, RoutedEventArgs e)
{
   Button btn = sender as Button;
   if ( btn != null )
   {
       // forces all control to update...
       btn.Focus();
   }

   // TEST IF OK TO CLOSE
   bool rc = _vm.ProcessOkCommand();
   if (rc)
   {
      DialogResult = true;
   }
   else
   {
      DialogResult = null;
   }
}


<Button Content="OK" Name ="cmdOK" IsDefault="True" Click="OkButton_Click"/>

answered Mar 18 '10 at 04:40

Zamboni

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Right, that's my point -- it looks like ShowDialog won't ever return null. – Joe White Mar 24 '10 at 14:49
It is no better than doing nothing than setting `DialogResult = null;`. Effectively both are same I guess. Correct me if I'm wrong. – Sriram Sakthivel Oct 18 '14 at 15:06

score 2 · Answer 3 · answered Apr 09 '19 at 17:49

I can give you an example I just encountered. Window.ShowDialog() will return null when you perform the following steps:

You first close all of your Application's windows.
All other Window objects that have been instantiated up until now with the new keyword are closed.
You try to instantiate a new Window and try calling Window.ShowDialog() on it. It will return null.

This is because, presumably, you have no existing Window under which your new dialog can bind to in order to behave like a dialog which owns the topmost window state.

When would ShowDialog() return null?

3 Answers3

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