为什么我的活动空无一人？

您正在将事件处理程序添加到另一个ChildForm实例.将MainForm的button1_click更改为如下所示:

  private void button1_Click(object sender, EventArgs e)
  {
          ChildFrm.ShowDialog()
  }

你的应用程序应该工作正常.

这是有效的MainForm.cs:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        ChildFrm.MyEvent += new EventHandler(HandleTheEvent);
    }

    ChildForm ChildFrm = new ChildForm();

    private void button1_Click(object sender, EventArgs e)
    {
        ChildFrm.ShowDialog();
    }

    public void HandleTheEvent(object sender, EventArgs e)
    {
        textBox1.Text = "event is fired";
    }
}