1、FormTest.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace WindowsFormsTest
{
    public partial class FormTest : Form
    {
        const int BM_CLICK = 0xF5;

        [DllImport("User32.dll", EntryPoint = "SendMessage")]
        private static extern int SendMessage(int hwnd, int wMsg, int wParam, int lParam);

        [DllImport("User32.dll", EntryPoint = "FindWindow")]
        private static extern int FindWindow(string lpClassName, string lpWindowName);

        public FormTest()
        {
            InitializeComponent();
        }

        protected override void OnLoad(EventArgs e)
        {
            FormSub formSub = new FormSub();
            formSub.Show();            
            base.OnLoad(e);
        }

        protected override void OnPaint(PaintEventArgs e)
        {
            base.OnPaint(e);            
        }



        private void SendMessage(string pcMessage, string pcReceiveForm)
        {
            int WINDOW_HANDLER = FindWindow(null, pcReceiveForm);
            if (WINDOW_HANDLER != 0)
            {
                SendMessage(WINDOW_HANDLER, BM_CLICK, 0, 0);
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            SendMessage("娃哈哈", "子窗體");
        }
    }
}
 

2、FormSub.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace WindowsFormsTest
{
    public partial class FormSub : Form
    {
        public FormSub()
        {
            InitializeComponent();
        }

        protected override void OnShown(EventArgs e)
        {
            this.Hide();
        }

        protected override void WndProc(ref Message m)
        {
            const int BM_CLICK = 0xF5;

            if (m.Msg == BM_CLICK)
            {
                MessageBox.Show("子窗體收到事件"); 
            }
            else
            {
                base.WndProc(ref m);
            }
        }
    }
}
 

修改FormSub窗體的標題為“子窗體”，這裡在啟動FormTest窗體的時候啟動FormSub窗體再隱藏，用來後臺做訊息接受的程式。