If you specify Owner, you will get a Modal dialog over the specified WPF window.
To get WinForms compatible Win32 window create a class implements IWin32Window like this
public class OldWindow : System.Windows.Forms.IWin32Window
{
IntPtr _handle;
public OldWindow(IntPtr handle)
{
_handle = handle;
}
#region IWin32Window Members
IntPtr System.Windows.Forms.IWin32Window.Handle
{
get { return _handle; }
}
#endregion
}
And use an instance of this class at your WinForms
IntPtr mainWindowPtr = new WindowInteropHelper(this).Handle; // 'this' means WPF Window
folderBrowserDialog.ShowDialog(new OldWindow(mainWindowPtr));