BitmapUtils.Snapshot(null, @"\test.bmp");

Class to call from
using System;
using System.Windows.Forms;
using System.Runtime.InteropServices;
using System.IO;
namespace Snapshot
{

public class BitmapUtils
{
const int XPelsPerMeter = 0xb12; // 72 ppi, 96 would work well too
const int YPelsPerMeter = 0xb12;

public static void Snapshot(Control ctl, string FileName)
{
IntPtr hDC;
if ( ctl != null )
{
ctl.Focus();
IntPtr hReal = GetFocus();
hDC = GetDC(hReal);
}
else
{
hDC = GetDC(IntPtr.Zero); //Full screen
}
IntPtr hMemDC = CreateCompatibleDC(hDC);
BITMAPINFOHEADER bi = new BITMAPINFOHEADER();
bi.biSize = (uint)Marshal.SizeOf(bi);
bi.biBitCount = 24; // Creating RGB bitmap. The following three members don't matter
bi.biClrUsed = 0;
bi.biClrImportant = 0;
bi.biCompression = 0;
bi.biHeight = ctl != null? ctl.Height: Screen.PrimaryScreen.Bounds.Height;
bi.biWidth = ctl != null? ctl.Width: Screen.PrimaryScreen.Bounds.Width;
bi.biPlanes = 1;
int cb = (int)(bi.biHeight * bi.biWidth * bi.biBitCount / 8); //8 is bits per byte
bi.biSizeImage = (uint)cb;
bi.biXPelsPerMeter = XPelsPerMeter;
bi.biYPelsPerMeter = YPelsPerMeter;
IntPtr pBits = IntPtr.Zero;
//Allocate memory for bitmap bits
IntPtr pBI = LocalAlloc(GPTR, bi.biSize);
// Not sure if this needed - simply trying to keep marshaller happy
Marshal.StructureToPtr(bi, pBI, false);
//This will return IntPtr to actual DIB bits in pBits
IntPtr hBmp = CreateDIBSection(hDC, pBI, 0, ref pBits, IntPtr.Zero, 0);
//Marshall back - now we have BITMAPINFOHEADER correctly filled in
//Marshal.PtrToStructure(pBI, bi);
BITMAPINFOHEADER biNew = (BITMAPINFOHEADER)Marshal.PtrToStructure(pBI, typeof( BITMAPINFOHEADER ));
//Usual stuff
IntPtr hOldBitmap = SelectObject(hMemDC, hBmp);
//Grab bitmap
int nRet = BitBlt(hMemDC, 0, 0, bi.biWidth, bi.biHeight, hDC, 0, 0, SRCCOPY);
// Allocate memory for a copy of bitmap bits
byte[] RealBits = new byte[cb];
// And grab bits from DIBSestion data
Marshal.Copy(pBits, RealBits, 0, cb);
// This simply creates valid bitmap file header, so it can be saved to disk
BITMAPFILEHEADER bfh = new BITMAPFILEHEADER();
bfh.bfSize = (uint)cb + 0x36; // Size of header + size of BITMAPINFOHEADER size of bitmap bits
bfh.bfType = 0x4d42; //BM
bfh.bfOffBits = 0x36; //
int HdrSize = 14;
byte[] header = new byte[HdrSize];
BitConverter.GetBytes(bfh.bfType).CopyTo(header, 0);
BitConverter.GetBytes(bfh.bfSize).CopyTo(header, 2);
BitConverter.GetBytes(bfh.bfOffBits).CopyTo(header , 10);
//Allocate enough memory for complete bitmap file
byte[] data = new byte[cb + bfh.bfOffBits];
//BITMAPFILEHEADER
header.CopyTo(data, 0);
//BITMAPINFOHEADER
header = new byte[Marshal.SizeOf(bi)];
IntPtr pHeader = LocalAlloc(GPTR, (uint)Marshal.SizeOf(bi));
Marshal.StructureToPtr(biNew, pHeader, false);
Marshal.Copy(pHeader, header, 0, Marshal.SizeOf(bi));
LocalFree(pHeader);
header.CopyTo(data, HdrSize);

//Bitmap bits
RealBits.CopyTo(data, (int)bfh.bfOffBits);
FileStream fs = new FileStream(FileName, FileMode.Create);
fs.Write(data, 0, data.Length);
fs.Flush();
fs.Close();
data = null;

DeleteObject(SelectObject(hMemDC, hOldBitmap));
DeleteDC(hMemDC);
ReleaseDC(hDC);
}
struct BITMAP
{
public int bmType;
public int bmWidth;
public int bmHeight;
public int bmWidthBytes;
public ushort bmPlanes;
public ushort bmBitsPixel;
public byte[] bmBits;
}

struct BITMAPINFO
{
public BITMAPINFOHEADER bmiHeader;
public RGBQUAD[] bmiColors;
}
struct RGBQUAD
{
public byte rgbBlue;
public byte rgbGreen;
public byte rgbRed;
public byte rgbReserved;
}
struct BITMAPINFOHEADER
{
public uint biSize;
public int biWidth;
public int biHeight;
public ushort biPlanes;
public ushort biBitCount;
public uint biCompression;
public uint biSizeImage;
public int biXPelsPerMeter;
public int biYPelsPerMeter;
public uint biClrUsed;
public uint biClrImportant;
}

struct BITMAPFILEHEADER
{
public ushort bfType;
public uint bfSize;
public ushort bfReserved1;
public ushort bfReserved2;
public uint bfOffBits;
}
const int GPTR = 0x40;
[DllImport("coredll.dll")]
internal static extern IntPtr SHLoadDIBitmap(
string szFileName );
[DllImport("aygshell.dll", EntryPoint="#75")]
internal static extern IntPtr SHLoadImageFile(
string szFileName );
[DllImport("coredll.dll")]
private static extern IntPtr GetFocus();
[DllImport("coredll.dll")]
private static extern IntPtr LocalAlloc(uint flags, uint cb);
[DllImport("coredll.dll")]
private static extern IntPtr LocalFree(IntPtr hMem);
[DllImport("coredll.dll")]
private static extern IntPtr CreateDIBSection(IntPtr hdc, BITMAPINFOHEADER hdr, uint colors, ref IntPtr pBits, IntPtr hFile, uint offset);
[DllImport("coredll.dll")]
private static extern IntPtr CreateDIBSection(IntPtr hdc, IntPtr hdr, uint colors, ref IntPtr pBits, IntPtr hFile, uint offset);
[DllImport("coredll.dll")]
private static extern IntPtr GetStockObject(int obj);
[DllImport("coredll.dll")]
private static extern int GetObject(IntPtr hObj, int cb, [In, Out]byte[] pb);
[DllImport("coredll.dll")]
private static extern int GetObject(IntPtr hObj, int cb, [In, Out]int[] pb);
[DllImport("coredll.dll")]
private static extern int GetObject(IntPtr hObj, int cb, IntPtr obj);
[DllImport("coredll.dll")]
private static extern int GetObject(IntPtr hObj, int cb, ref BITMAP obj);
[DllImport ("coredll.dll")]
public static extern IntPtr GetDC( IntPtr hWnd );
[DllImport ("coredll.dll")]
public static extern void ReleaseDC( IntPtr hDC );
[DllImport ("coredll.dll")]
public static extern void DeleteDC( IntPtr hDC );
[DllImport ("coredll.dll")]
public static extern int BitBlt(IntPtr hdcDest, int nXDest, int nYDest, int nWidth, int nHeight, IntPtr hdcSrc, int nXSrc, int nYSrc, uint dwRop);
[DllImport ("coredll.dll")]
public static extern IntPtr CreateCompatibleBitmap(IntPtr hdc, int nWidth, int nHeight);
[DllImport ("coredll.dll")]
public static extern IntPtr CreateCompatibleDC(IntPtr hdc);
[DllImport ("coredll.dll")]
public static extern IntPtr SelectObject(IntPtr hdc, IntPtr hObj);
[DllImport ("coredll.dll")]
public static extern void DeleteObject( IntPtr hObj );

const int SRCCOPY = 0x00CC0020;
}
}