How do I work with WIndows Forms in WPF?

Question

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?)