[DllImport("user32.dll", EntryPoint = "SetWindowPos",CharSet = CharSet.Auto)]

        static extern bool SetWindowPos(

        IntPtr hWnd,

        IntPtr hWndInsertAfter,

        int X,

        int Y,

        int cx,

        int cy,

        uint uFlags

        );

        static readonly IntPtr HWND_TOPMOST = new IntPtr(-1);

        static readonly IntPtr HWND_NOTOPMOST = new IntPtr(-2);

        static readonly IntPtr HWND_TOP = new IntPtr(0);

        const UInt32 SWP_NOSIZE = 0x0001;

        const UInt32 SWP_NOMOVE = 0x0002;

        const UInt32 SWP_NOZORDER = 0x0004;

        const UInt32 SWP_NOREDRAW = 0x0008;

        const UInt32 SWP_NOACTIVATE = 0x0010;

        const UInt32 SWP_FRAMECHANGED = 0x0020;

        const UInt32 SWP_SHOWWINDOW = 0x0040;

        const UInt32 SWP_HIDEWINDOW = 0x0080;

        const UInt32 SWP_NOCOPYBITS = 0x0100;

        const UInt32 SWP_NOOWNERZORDER = 0x0200;

        const UInt32 SWP_NOSENDCHANGING = 0x0400;

        const UInt32 TOPMOST_FLAGS = SWP_NOMOVE | SWP_NOSIZE;

        private void button1_Click(object sender, EventArgs e)

        {

            Process process = Process.Start("rundll32.exe",@"C:\WINDOWS\system32\shimgvw.dll,ImageView_Fullscreen d:\1.jpg");

            IntPtr str = process.Handle;

            SetWindowPos(this.Handle, HWND_NOTOPMOST, 0, 0, 0, 0, 1 | 2); //最后参数也有用1 | 4

        }