C#: how to take a screenshot of a portion of screen

like

TakeScreenshot(new Rectangle(0,0,100,100), "output.jpg");

Use the following:

Rectangle rect = new Rectangle(0, 0, 100, 100);
Bitmap bmp = new Bitmap(rect.Width, rect.Height, PixelFormat.Format32bppArgb);
Graphics g = Graphics.FromImage(bmp);
g.CopyFromScreen(rect.Left, rect.Top, 0, 0, bmp.Size, CopyPixelOperation.SourceCopy);
bmp.Save(fileName, ImageFormat.Jpeg);

Here is the code to capture the screen. Change the values to the size you need.

 Bitmap printscreen = new Bitmap(Screen.PrimaryScreen.Bounds.Width, Screen.PrimaryScreen.Bounds.Height);  

 Graphics graphics = Graphics.FromImage(printscreen as Image);  

 graphics.CopyFromScreen(0, 0, 0, 0, printscreen.Size);  

 printscreen.Save(@"C:\printscreen.jpg", ImageFormat.Jpeg);

Or make method which will return you captured image like this :

Image CaptureScreen(int sourceX, int sourceY, int destX, int destY, 
            Size regionSize)
{
    Bitmap bmp = new Bitmap(regionSize.Width, regionSize.Height);
    Graphics g = Graphics.FromImage(bmp);
    g.CopyFromScreen(sourceX, sourceY, destX, destY, regionSize);
    return bmp;
}
 ......
 // call 
 Image image = CaptureScreen(sourceX, sourceY,  destX,  destY, regionSize);
 image.Save(@"C:\Somewhere\screen.jpg);

Use the Graphics.CopyFromScreen method. Google turns up this tutorial.