Gtk# 3 semi-transparent window

Gtk3TransparentWindow.cs

using System;
using Gtk;
using Cairo;

namespace TransTest
{
	class MainClass
	{
		public static void Main (string[] args)
		{
			Application.Init ();
			new TransparentWindow ();
			Application.Run ();
		}
	}
	class TransparentWindow : Gtk.Window
	{
		Box box1 = new Box (Orientation.Vertical, 0);
		double alpha = 0.5;
		Scale SC = new Scale (Orientation.Horizontal, 0, 1, 0.05);
		public TransparentWindow () : base(Gtk.WindowType.Toplevel)
		{
			// This allows to pain directly on window
			this.AppPaintable = true;
			// This makes window transparent
			this.Visual = Gdk.Screen.Default.RgbaVisual;
			this.SetSizeRequest (200, 200);
			this.Add (box1);
			SC.Value = alpha;
			SC.ChangeValue += delegate {
				alpha = SC.Value;
				QueueDraw ();
			};
			box1.Add (SC);
			box1.Add (new Label("Test Label"));
			ShowAll ();
		}
		protected override bool OnDrawn (Context cr)
		{
			cr.SetSourceRGB (0, 0, 0);
			cr.PaintWithAlpha (alpha);
			return base.OnDrawn (cr);
		}
	}
}