记录目前两种实现方式

[DllImport("user32.dll")]
        private static extern bool PrintWindow(IntPtr hwnd, IntPtr hdcBlt, uint nFlags);

        public void WriteBmp(string bmpPath)
        {

            int screenWidth = webbrowser.Document.Body.ScrollRectangle.Width;
            int screenHeight = webbrowser.Document.Body.ScrollRectangle.Height;

            IntPtr myIntptr = webbrowser.Handle;
            int hwndInt = myIntptr.ToInt32();
            IntPtr hwnd = myIntptr;

            // Set hdc to the bitmap

            Bitmap bm = new Bitmap(screenWidth, screenHeight);
            Graphics g = Graphics.FromImage(bm);
            IntPtr hdc = g.GetHdc();

            // Snapshot the WebBrowser

            bool result = PrintWindow(hwnd, hdc, 0);
            g.ReleaseHdc(hdc);
            g.Flush();

            // Save the bitmap, if successful

            if (result == true)
                bm.Save(bmpPath);
        }

第二种

public static class ApiConstants
    {
        public const int SRCCOPY = 13369376;
    }

    public static class Utilities
    {
        public static Image CaptureScreen()
        {
            return CaptureWindow(User32.GetDesktopWindow());
        }

        public static Image CaptureWindow(IntPtr handle)
        {

            IntPtr hdcSrc = User32.GetWindowDC(handle);

            RECT windowRect = new RECT();
            User32.GetWindowRect(handle, ref windowRect);

            int width = windowRect.right - windowRect.left;
            int height = windowRect.bottom - windowRect.top;

            IntPtr hdcDest = Gdi32.CreateCompatibleDC(hdcSrc);
            IntPtr hBitmap = Gdi32.CreateCompatibleBitmap(hdcSrc, width, height);

            IntPtr hOld = Gdi32.SelectObject(hdcDest, hBitmap);
            Gdi32.BitBlt(hdcDest, 0, 0, width, height, hdcSrc, 0, 0, ApiConstants.SRCCOPY);
            Gdi32.SelectObject(hdcDest, hOld);
            Gdi32.DeleteDC(hdcDest);
            User32.ReleaseDC(handle, hdcSrc);

            Image image = Image.FromHbitmap(hBitmap);
            Gdi32.DeleteObject(hBitmap);

            return image;
        }
    }

    public static class User32
    {
        [DllImport("user32.dll")]
        public static extern IntPtr GetDesktopWindow();
        [DllImport("user32.dll")]
        public static extern IntPtr GetWindowDC(IntPtr hWnd);
        [DllImport("user32.dll")]
        public static extern IntPtr GetWindowRect(IntPtr hWnd, ref RECT rect);
        [DllImport("user32.dll")]
        public static extern IntPtr ReleaseDC(IntPtr hWnd, IntPtr hDC);
    }

    public class Gdi32
    {
        [DllImport("gdi32.dll")]
        public static extern bool BitBlt(IntPtr hObject, int nXDest, int nYDest, int nWidth, int nHeight, IntPtr hObjectSource, int nXSrc, int nYSrc, int dwRop);
        [DllImport("gdi32.dll")]
        public static extern IntPtr CreateCompatibleBitmap(IntPtr hDC, int nWidth, int nHeight);
        [DllImport("gdi32.dll")]
        public static extern IntPtr CreateCompatibleDC(IntPtr hDC);
        [DllImport("gdi32.dll")]
        public static extern bool DeleteDC(IntPtr hDC);
        [DllImport("gdi32.dll")]
        public static extern bool DeleteObject(IntPtr hObject);
        [DllImport("gdi32.dll")]
        public static extern IntPtr SelectObject(IntPtr hDC, IntPtr hObject);
    }

    [StructLayout(LayoutKind.Sequential)]
    public struct RECT
    {
        public int left;
        public int top;
        public int right;
        public int bottom;
    }
}

引用https://www.cnblogs.com/jackhuclan/archive/2012/04/23/1909301.html