c#截取后台窗口的图片

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, 0);
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
);