程序代码：

using System;
using System.Windows.Forms;

namespace Test
{
    class Program
    {
        class MyEventArgs : EventArgs
        {
            public string Message {get;set;}
        }

        static event EventHandler<MyEventArgs> form1SentMessage;
        static event EventHandler<MyEventArgs> form2SentMessage;

        class MyForm1 : Form
        {
            public MyForm1()
            {
                Button button = new Button { Text = "Button1" };
                button.Click += (s, a) =>
                {
                    form1SentMessage(null, new MyEventArgs { Message = "Hello from form1!" });
                };

                Controls.Add(button);

                form2SentMessage += (s, a) =>
                {
                    MessageBox.Show("Hey, the form2 has sent a message : '" + a.Message + "'", "form1", MessageBoxButtons.OK);
                };
            }
        }

        class MyForm2 : Form
        {
            public MyForm2()
            {
                Button button = new Button { Text = "Button2" };
                button.Click += (s, a) =>
                {
                    form2SentMessage(null, new MyEventArgs { Message = "Hello from form2!" });
                };

                Controls.Add(button);

                form1SentMessage += (s, a) =>
                    {
                        MessageBox.Show("Hey, the form1 has sent a message : '" + a.Message + "'", "form2", MessageBoxButtons.OK);
                    };
            }
        }

        class MainForm : Form
        {
            public MainForm()
            {
                MyForm1 form1 = new MyForm1();
                form1.Text = "Form1";
                form1.Show();

                MyForm2 form2 = new MyForm2();
                form2.Text = "Form2";
                form2.Show();
            }
        }

        [STAThread]
        static void Main(string[] args)
        {
            Application.Run(new MainForm());
        }
    }
}