Transparent window layer that is click-through and always stays on top

Here’s a refined full sample code for making a window topmost – click through – transparent (= alpha blended). The sample makes a rotating color wheel which is rendered with DirectX, or actually with XNA 4.0, because I believe Microsoft has discontinued developing the managed directx and favours XNA today.

using System;
using System.Windows.Forms;
using System.Runtime.InteropServices;
using Microsoft.Xna.Framework.Graphics;

namespace ClickThroughXNA
{
    public partial class Form1 : Form
    {
        // Directx graphics device
        GraphicsDevice dev = null;        
        BasicEffect effect = null;     

        // Wheel vertexes
        VertexPositionColor[] v = new VertexPositionColor[100];

        // Wheel rotation
        float rot = 0;

        public Form1()
        {
            InitializeComponent();

            StartPosition = FormStartPosition.CenterScreen;   
            Size = new System.Drawing.Size(500, 500);
            FormBorderStyle = System.Windows.Forms.FormBorderStyle.None;  // no borders

            TopMost = true;        // make the form always on top                     
            Visible = true;        // Important! if this isn't set, then the form is not shown at all

            // Set the form click-through
            int initialStyle = GetWindowLong(this.Handle, -20);
            SetWindowLong(this.Handle, -20, initialStyle | 0x80000 | 0x20);

            // Create device presentation parameters
            PresentationParameters p = new PresentationParameters();
            p.IsFullScreen = false;
            p.DeviceWindowHandle = this.Handle;
            p.BackBufferFormat = SurfaceFormat.Vector4;
            p.PresentationInterval = PresentInterval.One;

            // Create XNA graphics device
            dev = new GraphicsDevice(GraphicsAdapter.DefaultAdapter, GraphicsProfile.Reach, p);

            // Init basic effect
            effect = new BasicEffect(dev);

            // Extend aero glass style on form init
            OnResize(null);
        }


        protected override void OnResize(EventArgs e)
        {
            int[] margins = new int[] { 0, 0, Width, Height };

            // Extend aero glass style to whole form
            DwmExtendFrameIntoClientArea(this.Handle, ref margins);  
        }


        protected override void OnPaintBackground(PaintEventArgs e)
        {
            // do nothing here to stop window normal background painting
        }


        protected override void OnPaint(PaintEventArgs e)
        {                
            // Clear device with fully transparent black
            dev.Clear(new Microsoft.Xna.Framework.Color(0, 0, 0, 0.0f));

            // Rotate wheel a bit
            rot+=0.1f;

            // Make the wheel vertexes and colors for vertexes
            for (int i = 0; i < v.Length; i++)
            {                    
                if (i % 3 == 1)
                    v[i].Position = new Microsoft.Xna.Framework.Vector3((float)Math.Sin((i + rot) * (Math.PI * 2f / (float)v.Length)), (float)Math.Cos((i + rot) * (Math.PI * 2f / (float)v.Length)), 0);
                else if (i % 3 == 2)
                    v[i].Position = new Microsoft.Xna.Framework.Vector3((float)Math.Sin((i + 2 + rot) * (Math.PI * 2f / (float)v.Length)), (float)Math.Cos((i + 2 + rot) * (Math.PI * 2f / (float)v.Length)), 0);

                v[i].Color = new Microsoft.Xna.Framework.Color(1 - (i / (float)v.Length), i / (float)v.Length, 0, i / (float)v.Length);
            }

            // Enable position colored vertex rendering
            effect.VertexColorEnabled = true;
            foreach (EffectPass pass in effect.CurrentTechnique.Passes) pass.Apply();

            // Draw the primitives (the wheel)
            dev.DrawUserPrimitives(PrimitiveType.TriangleList, v, 0, v.Length / 3, VertexPositionColor.VertexDeclaration);

            // Present the device contents into form
            dev.Present();

            // Redraw immediatily
            Invalidate();            
        }


        [DllImport("user32.dll", SetLastError = true)]
        static extern int GetWindowLong(IntPtr hWnd, int nIndex);

        [DllImport("user32.dll")]
        static extern int SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);

        [DllImport("dwmapi.dll")]
        static extern void DwmExtendFrameIntoClientArea(IntPtr hWnd, ref int[] pMargins);

    }
}