ShowDialog return type: Form vs. Window

Question

In WPF / .Net, both System.Windows.Window and System.Windows.Forms.Form have the .ShowDialog method.

The difference is that the Windows.Window version returns a nullable bool (bool? or Nullable<bool>), while the Forms.Form version returns a straight bool.

I haven't yet seen the nullable bool ever be null, and cannot find a case where it would be in the MSDN documentation.

Can anyone explain why Windows.Window.ShowDialog returns a nullable? Should I check for it? What does it mean if I see null?

Answer 1

see here: Window.DialogResult Property

DialogResult is null when the dialog box is shown but neither accepted nor canceled.