How to make a screenshot of program window using WinAPI & C#?

I sending WM_PAINT (0x000F) message to window, which I want to screenshot, wParam = HDChandle, but no screenshot in my picturebox. If I send a WM_CLOSE message, all waorking (target window closes). What I do wrong with WM_PAINT? May be HDC is not PictureBox (WinForms) component? P.S. GetLastError() == ""

[DllImport("User32.dll")]
public static extern Int64 SendMessage(IntPtr hWnd, uint Msg, IntPtr wParam, IntPtr lParam);
  .....

SendMessage(targetWindowHandle, 0x000F, pictureBox.Handle, IntPtr.Zero);