How do I work with WIndows Forms in WPF? 
In my WPF program I created a Windows Form class.
In this Form, I placed an OK button and I went into the properties of the button and set the DialogResult to OK.
Now, I am calling this Dialog (Window Form) from the MainWindow.xaml.cs:
|       DialogResult dres;dres = 
form.ShowDialog();
 if (dres != DialogResult.OK) 
return;
 | 
 The compiler is complaining: 
|  Error 3 'System.Nullable<bool>' does not contain a definition 
for 'OK' and no extension method 'OK' accepting a first argument of type 
'System.Nullable<bool>' could be found (are you missing a using directive
 or an assembly reference?)
 |