The following example source code shows how to capture the screen and save it to an image. Hope you find it useful.

/* Author: Perry Lee
* Submission: Capture Screen (Add Screenshot Capability to Programs)
* Date of Submission: 12/29/03
*/
using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.Runtime.InteropServices;
// If you have any questions regarding functions (methods) imported from
// GDI32.dll and User32.dll refer to 'msdn.microsoft.com'
class GDI32
{
[DllImport("GDI32.dll")]
public static extern bool BitBlt(int hdcDest,int nXDest,int nYDest,int nWidth,int nHeight,int hdcSrc,int nXSrc,int nYSrc,int dwRop);
[DllImport("GDI32.dll")]
public static extern int CreateCompatibleBitmap(int hdc,int nWidth, int nHeight);[DllImport("GDI32.dll")]
public static extern int CreateCompatibleDC(int hdc);
[DllImport("GDI32.dll")]
public static extern bool DeleteDC(int hdc);
[DllImport("GDI32.dll")]
public static extern bool DeleteObject(int hObject);
[DllImport("GDI32.dll")]
public static extern int GetDeviceCaps(int hdc,int nIndex);
[DllImport("GDI32.dll")]
public static extern int SelectObject(int hdc,int hgdiobj);
class User32
{
[DllImport("User32.dll")]
public static extern int GetDesktopWindow();
[DllImport("User32.dll")]
public static extern int GetWindowDC(int hWnd);
[DllImport("User32.dll")]
public static extern int ReleaseDC(int hWnd,int hDC);
}
class Example
{
public void CaptureScreen(string fileName,ImageFormat imageFormat)
{
int hdcSrc = User32.GetWindowDC(User32.GetDesktopWindow()),
hdcDest = GDI32.CreateCompatibleDC(hdcSrc),
hBitmap = GDI32.CreateCompatibleBitmap(hdcSrc,
GDI32.GetDeviceCaps(hdcSrc,8),GDI32.GetDeviceCaps(hdcSrc,10)); GDI32.SelectObject(hdcDest,hBitmap);
GDI32.BitBlt(hdcDest,0,0,GDI32.GetDeviceCaps(hdcSrc,8),
GDI32.GetDeviceCaps(hdcSrc,10),hdcSrc,0,0,0x00CC0020);
SaveImageAs(hBitmap,fileName,imageFormat);
Cleanup(hBitmap,hdcSrc,hdcDest);
}
private void Cleanup(int hBitmap,int hdcSrc,int hdcDest)
{
User32.ReleaseDC(User32.GetDesktopWindow(),hdcSrc);
GDI32.DeleteDC(hdcDest);
GDI32.DeleteObject(hBitmap);
}
private void SaveImageAs(int hBitmap,string fileName,ImageFormat imageFormat)
{
Bitmap image =
new Bitmap(Image.FromHbitmap(new IntPtr(hBitmap)),
Image.FromHbitmap(new IntPtr(hBitmap)).Width,
Image.FromHbitmap(new IntPtr(hBitmap)).Height);
image.Save(fileName,imageFormat);
}
}

Explanation of methods:

public void CaptureScreen(string fileName,ImageFormat imageFormat)
{
int hdcSrc = User32.GetWindowDC(User32.GetDesktopWindow()), // Get a handle to the desktop window
hdcDest = GDI32.CreateCompatibleDC(hdcSrc), // Create a memory device context
hBitmap = GDI32.CreateCompatibleBitmap(hdcSrc, // Create a bitmap and place it in the memory DC
GDI32.GetDeviceCaps(hdcSrc,8),GDI32.GetDeviceCaps(hdcSrc,10));
// GDI32.GetDeviceCaps(hdcSrc,8) returns the width of the desktop window
// GDI32.GetDeviceCaps(hdcSrc,10) returns the height of the desktop window
GDI32.SelectObject(hdcDest,hBitmap); // Required to create a color bitmap
GDI32.BitBlt(hdcDest,0,0,GDI32.GetDeviceCaps(hdcSrc,8), // Copy the on-screen image into the memory DC
GDI32.GetDeviceCaps(hdcSrc,10),hdcSrc,0,0,0x00CC0020);
SaveImageAs(hBitmap,fileName,imageFormat); // Save the screen-capture to the specified file using the designated image format
Cleanup(hBitmap,hdcSrc,hdcDest); // Free system resources
}
private void Cleanup(int hBitmap,int hdcSrc,int hdcDest)
{
// Release the device context resources back to the system
User32.ReleaseDC(User32.GetDesktopWindow(),hdcSrc);
GDI32.DeleteDC(hdcDest);
GDI32.DeleteObject(hBitmap);
}
private void SaveImageAs(int hBitmap,string fileName,ImageFormat imageFormat)
{
// Create a bitmap from the Windows handle
Bitmap image = new Bitmap(Image.FromHbitmap(new IntPtr(hBitmap)),
Image.FromHbitmap(new IntPtr(hBitmap)).Width,
Image.FromHbitmap(new IntPtr(hBitmap)).Height);
image.Save(fileName,imageFormat);
}