方法一(get/set pixel)

核心语句：

resultBitmap.SetPixel(x, y, sourceBitmap.GetPixel(offsetX + x, offsetY+y))

        /// <summary>
        /// get a certain rectangle part of a known graphic
        /// </summary>
        /// <param name="bitmapPathAndName">path and name of the source graphic</param>
        /// <param name="width">width of the part graphic</param>
        /// <param name="height">height of the part graphic</param>
        /// <param name="offsetX">the width offset in the source graphic</param>
        /// <param name="offsetY">the height offset in the source graphic</param>
        /// <returns>wanted graphic</returns>
        public Bitmap GetPartOfImage(string bitmapPathAndName, int width, int height,int offsetX,int offsetY)
        {
            Bitmap sourceBitmap = new Bitmap(bitmapPathAndName);
            Bitmap resultBitmap = new Bitmap(width, height);
            for (int x = 0; x < width; x++)
            {
                for (int y = 0; y < height; y++)
                {
                    if (offsetX + x < sourceBitmap.Size.Width & offsetY + y < sourceBitmap.Size.Height)
                    {
                        resultBitmap.SetPixel(x, y, sourceBitmap.GetPixel(offsetX + x, offsetY+y));
                    }
                }
            }
            return resultBitmap;
        }

该方法速度较慢

方法二(graphics.drawimage)

核心代码：

Graphics g = Graphics.FromImage(resultBitmap)

g.DrawImage(sourceBitmap, resultRectangle, sourceRectangle, GraphicsUnit.Pixel)

/// <summary>
        /// get a certain rectangle part of a known graphic
        /// </summary>
        /// <param name="bitmapPathAndName">path and name of the source graphic</param>
        /// <param name="width">width of the part graphic</param>
        /// <param name="height">height of the part graphic</param>
        /// <param name="offsetX">the width offset in the source graphic</param>
        /// <param name="offsetY">the height offset in the source graphic</param>
        /// <returns>wanted graphic</returns>
        public Bitmap GetPartOfImage(string bitmapPathAndName, int width, int height, int offsetX, int offsetY)
        {
            Bitmap sourceBitmap = new Bitmap(bitmapPathAndName);
            Bitmap resultBitmap = new Bitmap(width, height);
            using (Graphics g = Graphics.FromImage(resultBitmap))
            {
                Rectangle resultRectangle = new Rectangle(0, 0, Width, height);
                Rectangle sourceRectangle = new Rectangle(0+offsetX, 0+offsetY, Width, height);
                g.DrawImage(sourceBitmap, resultRectangle, sourceRectangle, GraphicsUnit.Pixel);
            }
            return resultBitmap;
        }

速度较快，可完全鄙视掉方法一

using (Graphics g = this.CreateGraphics())
{
g.CopyFromScreen(0, 0, 100, 100, new Size(100, 100));
}