c#截取后台窗口的图片，自测可用，据说性能很一般，用用吧

     struct RECT  
     {  
         public int Left; // x position of upper-left corner  
         public int Top; // y position of upper-left corner  
         public int Right; // x position of lower-right corner  
         public int Bottom; // y position of lower-right corner  
     }  
     public static Bitmap GetWindow(IntPtr hWnd)  
     {  
         IntPtr hscrdc = GetWindowDC(hWnd);  
         RECT rect = new RECT();  
         GetWindowRect(hWnd, out rect);  
         IntPtr hbitmap = CreateCompatibleBitmap(hscrdc, rect.Right - rect.Left, rect.Bottom - rect.Top);  
         IntPtr hmemdc = CreateCompatibleDC(hscrdc);  
         SelectObject(hmemdc, hbitmap);  
         PrintWindow(hWnd, hmemdc, );  
         Bitmap bmp = Bitmap.FromHbitmap(hbitmap);  
         DeleteDC(hscrdc);  
         DeleteDC(hmemdc);  
         return bmp;  
     }

     //API声明

     \[DllImport("user32.dll")\]  
     private static extern bool GetWindowRect(IntPtr hWnd, out RECT lpRect);

     \[DllImport("gdi32.dll")\]  
     public static extern IntPtr CreateDC(  
                 string lpszDriver,        // driver name驱动名  
                 string lpszDevice,        // device name设备名  
                 string lpszOutput,        // not used; should be NULL  
                 IntPtr lpInitData  // optional printer data  
      );

     \[DllImport("gdi32.dll")\]  
     public static extern int BitBlt(  
      IntPtr hdcDest, // handle to destination DC目标设备的句柄  
      int nXDest,  // x-coord of destination upper-left corner目标对象的左上角的X坐标  
      int nYDest,  // y-coord of destination upper-left corner目标对象的左上角的Y坐标  
      int nWidth,  // width of destination rectangle目标对象的矩形宽度  
      int nHeight, // height of destination rectangle目标对象的矩形长度  
      IntPtr hdcSrc,  // handle to source DC源设备的句柄  
      int nXSrc,   // x-coordinate of source upper-left corner源对象的左上角的X坐标  
      int nYSrc,   // y-coordinate of source upper-left corner源对象的左上角的Y坐标  
      UInt32 dwRop  // raster operation code光栅的操作值  
      );

     \[DllImport("gdi32.dll")\]  
     public static extern IntPtr CreateCompatibleDC(  
      IntPtr hdc // handle to DC  
      );

     \[DllImport("gdi32.dll")\]  
     public static extern IntPtr CreateCompatibleBitmap(  
      IntPtr hdc,        // handle to DC  
      int nWidth,     // width of bitmap, in pixels  
      int nHeight     // height of bitmap, in pixels  
      );

     \[DllImport("gdi32.dll")\]  
     public static extern IntPtr SelectObject(  
      IntPtr hdc,          // handle to DC  
      IntPtr hgdiobj   // handle to object  
      );

     \[DllImport("gdi32.dll")\]  
     public static extern int DeleteDC(  
      IntPtr hdc          // handle to DC  
      );

     \[DllImport("user32.dll")\]  
     public static extern bool PrintWindow(  
      IntPtr hwnd,               // Window to copy,Handle to the window that will be copied.  
      IntPtr hdcBlt,             // HDC to print into,Handle to the device context.  
      UInt32 nFlags              // Optional flags,Specifies the drawing options. It can be one of the following values.  
      );

     \[DllImport("user32.dll")\]  
     public static extern IntPtr GetWindowDC(  
      IntPtr hwnd  
      );