如何在Winforms上使用C#向应用程序发送消息？

using System;
using System.Runtime.InteropServices;
using System.Windows.Forms;

public partial class MainForm : Form
{
    // 引入SendMessage函数
    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    public static extern IntPtr SendMessage(IntPtr hWnd, int msg, IntPtr wParam, IntPtr lParam);

    // 定义消息常量
    private const int WM_USER = 0x0400;
    private const int WM_MY_MESSAGE = WM_USER + 1;

    public MainForm()
    {
        InitializeComponent();
    }

    private void btnSendMessage_Click(object sender, EventArgs e)
    {
        // 向应用程序发送消息
        SendMessage(this.Handle, WM_MY_MESSAGE, IntPtr.Zero, IntPtr.Zero);
    }

    protected override void WndProc(ref Message m)
    {
        // 处理接收到的消息
        if (m.Msg == WM_MY_MESSAGE)
        {
            // 在这里编写处理消息的逻辑
            MessageBox.Show("接收到消息！");
        }

        base.WndProc(ref m);
    }
}