In System.Windows.Forms.Button there is a property DialogResult, where is this property in the System.Windows.Controls.Button (WPF)?
Just make sure that you've shown the form using ShowDialog rather than Show. If you do the latter you'll get the following exception raised:
ShowDialog
Show
InvalidOperationException was unhandled DialogResult can be set only after Window is created and shown as dialog.
InvalidOperationException was unhandled
DialogResult can be set only after Window is created and shown as dialog.