How to get the bitmap/image from a Graphics object in C#?

Question

I want to know what the intermediate state of the buffer where the Graphics object is drawing some stuff. How do I get hold of the bitmap or the image that it is drawing on?

Answer 1

You can get his hdc that's a pointer to the surface buffer, and eventually copy his content to another hdc with bitblt function. This way you can create a copy of the drawing surface on a bitmap.

enum TernaryRasterOperations : uint
{
    /// 
dest = source
    SRCCOPY = 0x00CC0020,
    /// 
dest = source OR dest
    SRCPAINT = 0x00EE0086,
    /// 
dest = source AND dest
    SRCAND = 0x008800C6,
    /// 
dest = source XOR dest
    SRCINVERT = 0x00660046,
    /// 
dest = source AND (NOT dest)
    SRCERASE = 0x00440328,
    /// 
dest = (NOT source)
    NOTSRCCOPY = 0x00330008,
    /// 
dest = (NOT src) AND (NOT dest)
    NOTSRCERASE = 0x001100A6,
    /// 
dest = (source AND pattern)
    MERGECOPY = 0x00C000CA,
    /// 
dest = (NOT source) OR dest
    MERGEPAINT = 0x00BB0226,
    /// 
dest = pattern
    PATCOPY = 0x00F00021,
    /// 
dest = DPSnoo
    PATPAINT = 0x00FB0A09,
    /// 
dest = pattern XOR dest
    PATINVERT = 0x005A0049,
    /// 
dest = (NOT dest)
    DSTINVERT = 0x00550009,
    /// 
dest = BLACK
    BLACKNESS = 0x00000042,
    /// 
dest = WHITE
    WHITENESS = 0x00FF0062,
    /// 

    /// Capture window as seen on screen.  This includes layered windows 
    /// such as WPF windows with AllowsTransparency="true"
    /// 
    CAPTUREBLT = 0x40000000
}

[DllImport("gdi32.dll", EntryPoint = "BitBlt", SetLastError = true)]
[return: MarshalAs(UnmanagedType.Bool)]
static extern bool BitBlt([In] IntPtr hdc, int nXDest, int nYDest, int nWidth, int nHeight, [In] IntPtr hdcSrc, int nXSrc, int nYSrc, TernaryRasterOperations dwRop);

public static Bitmap CopyGraphicsContent(Graphics source, Rectangle rect)
{
    Bitmap bmp = new Bitmap(rect.Width, rect.Height);

    using (Graphics dest = Graphics.FromImage(bmp))
    {
        IntPtr hdcSource = source.GetHdc();
        IntPtr hdcDest = dest.GetHdc();

        BitBlt(hdcDest, 0, 0, rect.Width, rect.Height, hdcSource, rect.X, rect.Y, TernaryRasterOperations.SRCCOPY);

        source.ReleaseHdc(hdcSource);
        dest.ReleaseHdc(hdcDest);
    }

    return bmp;
}