模态窗体隐藏执行两次FormClosing

Question

简单的测试工程
主窗体：

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;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Form2 fr = new Form2();
            fr.ShowDialog();
        }
    }
}
弹出模态窗体：
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;

namespace WindowsFormsApplication1
{
    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }

        private void Form2_FormClosing(object sender, FormClosingEventArgs e)
        {
            e.Cancel = true;
            Visible = false;
        }
    }
}
为什么Form2的FormClosing执行两次？