已打开的客户端程序直接打开而不另开程序

上一篇写了关于如何写入注册表通过外部链接来调客户端，客户端打开后如果再打开链接就会又开启一个程序，想着直接在已打开的程序里继续操作，下面简单介绍一下，写的不是很详细，也是参考别人的文章，链接没有保存，以后如果再看到了再补充。
废话到此结束直接上代码：

		//API 常數定義
        private const int SW_HIDE = 0;

        private const int SW_NORMAL = 1;

        private const int SW_MAXIMIZE = 3;

        private const int SW_SHOWNOACTIVATE = 4;

        private const int SW_SHOW = 5;

        private const int SW_MINIMIZE = 6;

        private const int SW_RESTORE = 9;

        private const int SW_SHOWDEFAULT = 10;


        [DllImport("user32.dll", EntryPoint = "FindWindow")]
        private extern static IntPtr FindWindow(string lpClassName, string lpWindowName);

        [DllImport("user32.dll")]
        private static extern bool ShowWindowAsync(IntPtr hWnd, int nCmdShow);

        [DllImport("User32.dll")]
        private static extern bool SetForegroundWindow(IntPtr hWnd);

        [DllImport("User32.dll", EntryPoint = "SendMessage")]
        private static extern int SendMessage(int hWnd, int Msg, int wParam, ref COPYDATASTRUCT lParam);

        /// 

        /// 消息代码 经测试必须是这个 不知道为啥
        /// 
        const int WM_COPYDATA = 0x004A;
        
	string processName = Process.GetCurrentProcess().ProcessName;
    Process[] processes = Process.GetProcessesByName(processName);
    //如果该数组长度大于1，说明多次运行
    if (processes.Length > 1)
    {
        //激活窗口
        IntPtr handle = FindWindow(null, "你的应用程序名称");
        ShowWindowAsync(handle, SW_NORMAL);
        SetForegroundWindow(handle);

        //向MainForm发送消息 将数据传递过去 COPYDATASTRUCT是自己定义的一个结构体
        COPYDATASTRUCT cds = new COPYDATASTRUCT();
        string message = param;
        byte[] sarr = System.Text.Encoding.Default.GetBytes(message);
        int len = sarr.Length;
        cds.dwData = (IntPtr)Convert.ToInt16(1);//可以是任意值 
        cds.cbData = len + 1;//指定lpData内存区域的字节数 
        cds.lpData = message;//发送给目标窗口所在进程的数据 
        SendMessage((int)handle, WM_COPYDATA, 0, ref cds);
    }
    /// 

    /// 传递消息携带传递数据的对象
    /// 
    public struct COPYDATASTRUCT
    {
        public IntPtr dwData;

        public int cbData;

        [MarshalAs(UnmanagedType.LPStr)]
        public string lpData;
    }

发送的消息如何接受呢，请看：

        /// 

        /// 接收消息
        /// 
        /// 
        protected override void DefWndProc(ref Message m)
        {
            switch (m.Msg)
            {
                case WM_COPYDATA:
                    COPYDATASTRUCT cds = new COPYDATASTRUCT();
                    Type t = cds.GetType();
                    cds = (COPYDATASTRUCT)m.GetLParam(t);
                    string strResult = cds.lpData;
                    //string strType = cds.dwData.ToString();
                    Param = strResult;
                    this.Init();
                    break;
                default:
                    base.DefWndProc(ref m);
                    break;
            }
        }