private void WpfUsercontrol_OnLoaded(object sender, RoutedEventArgs e)
{
    Window window = Window.GetWindow(this);
    if (window != null)
        window.Closing += window_closing;

    Form form = this.Parent as Form;
    //Error: Cannot convert from System.Windows.DependencyObject to System.Windows.Forms.Form
}

var hwndSource = (HwndSource)PresentationSource.FromDependencyObject(this);
var host = (ElementHost)Control.FromChildHandle(hwndSource.Handle);
Form form = (Form)host.TopLevelControl;