Captures the specified region of the a window to a System.Drawing.Bitmap.

ScreenCapture.cs

using System;
using System.Drawing;
using System.Runtime.InteropServices;

public static class ScreenCapture
{
    class NativeMethods
    {
        [DllImport("user32.dll")]
        internal static extern IntPtr GetWindowDC(IntPtr hWnd);

        [DllImport("user32.dll")]
        internal static extern IntPtr ReleaseDC(IntPtr hWnd, IntPtr hDC);

        [DllImport("gdi32.dll")]
        internal static extern IntPtr CreateCompatibleDC(IntPtr hDC);

        [DllImport("gdi32.dll")]
        internal static extern IntPtr CreateCompatibleBitmap(IntPtr hDC, int nWidth, int nHeight);

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

        [DllImport("gdi32.dll", SetLastError = true)]
        internal static extern bool BitBlt(IntPtr hObject, int nXDest, int nYDest, int nWidth, int nHeight, IntPtr hObjectSource, int nXSrc, int nYSrc, int dwRop);

        internal const int SRCCOPY = 0x00CC0020;

        [DllImport("gdi32.dll")]
        internal static extern bool DeleteDC(IntPtr hDC);

        [DllImport("gdi32.dll")]
        internal static extern bool DeleteObject(IntPtr hObject);
    }

    public static Bitmap CaptureWindow(IntPtr handle, Rectangle area)
    {
        var hdcSrc = NativeMethods.GetWindowDC(handle);

        if (hdcSrc == default(IntPtr))
        {
            throw new Exception("Failed to get source window device context. Does it exist?");
        }

        try
        {
            var hdcDest = NativeMethods.CreateCompatibleDC(hdcSrc);

            if (hdcDest == default(IntPtr))
            {
                throw new Exception("Failed to get destination device context.");
            }

            try
            {
                var hBitmap = NativeMethods.CreateCompatibleBitmap(hdcSrc, area.Width, area.Height);

                if (hBitmap == default(IntPtr))
                {
                    throw new Exception("Failed to create get bitmap.");
                }

                try
                {
                    var hOld = NativeMethods.SelectObject(hdcDest, hBitmap);

                    try
                    {
                        if (!NativeMethods.BitBlt(hdcDest, 0, 0, area.Width, area.Height, hdcSrc, area.Left, area.Top, NativeMethods.SRCCOPY))
                        {
                            throw new Exception("BitBlt failed");
                        }

                        return Bitmap.FromHbitmap(hBitmap);
                    }
                    finally
                    {
                        NativeMethods.SelectObject(hdcDest, hOld);
                    }
                }
                finally
                {
                    NativeMethods.DeleteObject(hBitmap);
                }
            }
            finally
            {
                NativeMethods.DeleteDC(hdcDest);
            }
        }
        finally
        {
            NativeMethods.ReleaseDC(handle, hdcSrc);
        }
    }
}