有关C# 窗体消息

我建了2个窗体，实现2个窗体之间发送消息和接收消息的功能，但发送消息的时候总是找不到接收消息的窗体
//发送窗体
    //类SendMessageForm  窗体name:SendMessageForm
    public partial class SendMessageForm : Form
    {
        [DllImport("User32.dll", EntryPoint = "SendMessage")]
        private static extern int SendMessage(
        int hWnd, // handle to destination window 
        int Msg, // message 
        int wParam, // first message parameter 
        int lParam // second message parameter 
        );        [DllImport("User32.dll", EntryPoint = "FindWindow")]
        private static extern int FindWindow(string lpClassName, string
        lpWindowName);
        //定义消息常数 
        public const int USER = 0x500;
        public const int TEST = USER + 1; 
        
        
        
        public SendMessageForm()
        {
            InitializeComponent();
        }
        private void SendMsgToMainForm(int MSG) 
        {
            int WINDOW_HANDLER = FindWindow("AccMessage", @"AccMessageform"); 
            if(WINDOW_HANDLER == 0) 
            { 
                throw new Exception("Could not find Main window!"); //报错误的地方
            } 
          SendMessage(WINDOW_HANDLER,MSG,100,200); 
         
        }        private void button1_Click(object sender, EventArgs e)
        {
            SendMsgToMainForm(1111);
        } 
   } //接收窗体  这个窗体是MDI窗体
    //类：AccMessageform  窗体名称：AccMessageform    
    public partial class AccMessageform : Form
    {
        public AccMessageform()
        {
            InitializeComponent();
        }        private void button1_Click(object sender, EventArgs e)
        {
            SendMessageForm sm = new SendMessageForm();
            sm.MdiParent = this;
            sm.Show();
        }        /// 重写窗体的消息处理函数 
        protected override void DefWndProc(ref System.Windows.Forms.Message m)
        {
            switch (m.Msg)
            {
                //接收自定义消息 USER，并显示其参数 
                case SendMessageForm.USER:
                    string message = string.Format("Received message! parameters are :{0},{1}", m.WParam,m.LParam);
                    MessageBox.Show(message);
                    break;
                default:
                    base.DefWndProc(ref m);
                    break;
            }
            //Console.WriteLine(m.LParam); 
        } 
    }