下面的程式碼使用API函式FindWindow查詢名稱為窗體a的窗體，並呼叫API函式SendMessage向目標窗體傳送訊息關閉目標窗體。

    public const int WM_CLOSE = 0x10;
    [DllImport("user32.dll", EntryPoint = "SendMessageA")]
    public static extern int SendMessage(IntPtr hwnd, int wMsg, int wParam, int lParam);
    [DllImport("User32.dll", EntryPoint = "FindWindow"
 
)]
    private static extern IntPtr FindWindow(string lpClassName, string lpWindowName);

    IntPtr hwnd_win;
    hwnd_win = FindWindow(null, "窗體a");
    SendMessage(hwnd_win, WM_CLOSE, 0, 0);