c#下使用WM_COPYDATA

        [DllImport("User32.dll")]
        public static extern int SendMessage(IntPtr hwnd, int msg, int wParam, ref COPYDATASTRUCT lParam);

        public const int WM_COPYDATA = 0x004A;

        /// <summary>
        /// WM_COPYDATA消息，进程间传输信息专用结构
        /// </summary>
        public struct COPYDATASTRUCT
        {
            public IntPtr dwData;//类型必须IntPtr，用来自动适应32位 64位
            public int cbData;

            public Intptr lpData;

        }

发送消息：

                string msg = "asdf";
                byte[] sarr = Encoding.Default.GetBytes(msg);
                COPYDATASTRUCT cds;
                cds.dwData = IntPtr.Zero;                                   //可以是任意值
                cds.cbData = sarr.Length;                               //指定lpData内存区域的字节数
                cds.lpData = Marshal.UnsafeAddrOfPinnedArrayElement(sarr,0);  //发送给目标窗口所在进程的数据

                SendMessage(hdl, WM_COPYDATA, 0, ref cds);