pinvoke.net: Edit: user32.updatelayeredwindow

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Summary: The UpdateLayeredWindow API

!!!!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!

Documentation: UpdateLayeredWindow@msdn on MSDN