C# Signature:

[DllImport("user32.dll", ExactSpelling = true, SetLastError = true)]
static extern bool UpdateLayeredWindow(IntPtr hwnd, IntPtr hdcDst,
   ref Point pptDst, ref Size psize, IntPtr hdcSrc, ref Point pptSrc, uint crKey,
   [In] ref BLENDFUNCTION pblend, uint dwFlags);

User-Defined Types:

BLENDFUNCTION

Notes:

None.

Tips & Tricks:

Ecco qua il modo per utilizzare un PNG semi trasparente come sfondo per i vostri Form. Riporto qua la procedura completa:

Traslation in english. This is the way to use PNG semitransparent background for yours form:

    public void SetBackground(Control control, Bitmap bitmap)
    {
        /*Parametri accettati dalla procedura:
        control = il form o controllo da renderizzare
        bitmap = la PNG da utilizzare come sfondo*/

        // Imposta le dimensioni del controllo come quelle della bitmap
        control.Width = bitmap.Width;
        control.Height = bitmap.Height;
        if (bitmap.PixelFormat != System.Drawing.Imaging.PixelFormat.Format32bppArgb)
        {
            //Se l'immagine è in formato errato...
            throw new ApplicationException("L'immagine deve essere in formato a 32 bpp con canale alpha");
        }
        IntPtr hBitmap = IntPtr.Zero;
        IntPtr oldBitmap = IntPtr.Zero;
        IntPtr screenDc = Win32.GetDC(IntPtr.Zero);
        IntPtr memDc = Win32.CreateCompatibleDC(screenDc);
        try
        {
            hBitmap = bitmap.GetHbitmap(Color.FromArgb(0));
            oldBitmap = Win32.SelectObject(memDc, hBitmap);
            Win32.Size size = new Win32.Size(bitmap.Width, bitmap.Height);
            Win32.Point pointSource = new Win32.Point(control.Left, control.Top);
            Win32.Point topPos = new Win32.Point(0, 0);
            Win32.BLENDFUNCTION blend = new Win32.BLENDFUNCTION();
            blend.BlendOp = 0;
            blend.BlendFlags = 0;
            blend.SourceConstantAlpha = byte.MaxValue;
            blend.AlphaFormat = 1;
            Win32.UpdateLayeredWindow(control.Handle, screenDc, ref topPos, ref size, memDc, ref pointSource, 0, ref blend, 2);
        }
        catch (Exception ex)
        {
            throw ex;
        }
        finally
        {
            Win32.ReleaseDC(IntPtr.Zero, screenDc);            
            if (hBitmap != IntPtr.Zero)
            {
                Win32.SelectObject(memDc, oldBitmap);
                Win32.DeleteObject(hBitmap);
            }
            Win32.DeleteDC(memDc);
        }
    }    

    public class Win32
    {
        //Classe che contiene le dichiarazioni API e i tipi
        public enum Bool: int
        {
            @False = 0,

            @True = 1

        }

        public struct Point
        {
            public int x;

            public int y;


            public Point(int x, int y)
            {
                this.x = x;
                this.y = y;
            }
        }

        public struct Size
        {
            public int cx;

            public int cy;


            public Size(int cx, int cy)
            {
                this.cx = cx;
                this.cy = cy;
            }
        }

        public struct BLENDFUNCTION
        {
            public byte BlendOp;

            public byte BlendFlags;

            public byte SourceConstantAlpha;

            public byte AlphaFormat;

        }

        public const int ULW_ALPHA = 2;

        public const byte AC_SRC_OVER = 0;

        public const byte AC_SRC_ALPHA = 1;

        [DllImportAttribute("user32.dll")]
        public extern static Bool UpdateLayeredWindow(IntPtr handle, IntPtr hdcDst, ref Point pptDst, ref Size psize, IntPtr hdcSrc, ref Point pprSrc, int crKey, ref BLENDFUNCTION pblend, int dwFlags);

        [DllImportAttribute("user32.dll")]
        public extern static IntPtr GetDC(IntPtr handle);

        [DllImportAttribute("user32.dll", ExactSpelling=true)]
        public extern static int ReleaseDC(IntPtr handle, IntPtr hDC);

        [DllImportAttribute("gdi32.dll")]
        public extern static IntPtr CreateCompatibleDC(IntPtr hDC);

        [DllImportAttribute("gdi32.dll")]
        public extern static Bool DeleteDC(IntPtr hdc);

        [DllImportAttribute("gdi32.dll")]
        public extern static IntPtr SelectObject(IntPtr hDC, IntPtr hObject);

        [DllImportAttribute("gdi32.dll")]
        public extern static Bool DeleteObject(IntPtr hObject);
    }

Sample Code:

This blog entry by Mike Swanson includes a downloadable sample called AlphaWindow that calls UpdateLayeredWindow from Windows Forms for a cool alpha blended effect.

http://blogs.msdn.com/mswanson/archive/2005/07/07/436618.aspx

Alternative Managed API:

Do you know one? Please contribute it!

Please edit this page!

Do you have...

• helpful tips or sample code to share for using this API in managed code?
• corrections to the existing content?
• variations of the signature you want to share?
• additional languages you want to include?

Select "Edit This Page" on the right hand toolbar and edit it! Or add new pages containing supporting types needed for this API (structures, delegates, and more).