/**//// <summary>
    /// 包含窗口状态变化时的相关数据
    /// </summary>
    public class WindowStateChangedEventArgs : EventArgs
    {
        private readonly FormWindowState oldState;
        private readonly FormWindowState newState;

        public FormWindowState OldState
        {
            get
            {
                return oldState;
            }
        }


        public FormWindowState NewState
        {
            get
            {
                return newState;
            }
        }

        public WindowStateChangedEventArgs(FormWindowState oldState, FormWindowState newState)
        {
            this.oldState = oldState;
            this.newState = newState;
        }


    }

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

namespace MDIHelper
{
    public partial class CustomForm : Form
    {
        //记录状态变化之前的状态
        private FormWindowState preWindowState = FormWindowState.Normal;

        public CustomForm()
        {
            InitializeComponent();
        }

        事件#region 事件

        /**//// <summary>
        /// 窗口状态改变时发生
        /// </summary>
        public event EventHandler<WindowStateChangedEventArgs> WindowStateChanged;

        protected virtual void OnWindowStateChanged(WindowStateChangedEventArgs arg)
        {
            if (this.WindowStateChanged != null)
            {
                this.WindowStateChanged(this, arg);
            }
        }

        #endregion


        重写的方法#region 重写的方法       

        protected override void WndProc(ref Message m)
        {
            switch (m.Msg)
            {
                case 0x0005://change size: WM_SIZE
                    {
                        FormWindowState newState = FormWindowState.Normal;
                        switch (m.WParam.ToInt32())
                        {
                            case 0://SIZE_RESTORED
                                newState = FormWindowState.Normal;
                                break;
                            case 1://SIZE_MINIMIZED
                                newState = FormWindowState.Minimized;
                                break;
                            case 2://SIZE_MAXIMIZED
                                newState = FormWindowState.Maximized;
                                break;
                            default:
                                break;
                        }

                        if (newState != this.preWindowState)
                        {
                            this.OnWindowStateChanged(new WindowStateChangedEventArgs(this.preWindowState, newState));
                            this.preWindowState = newState;
                        }
                    }
                    break;
                default:
                    break;
            }
            base.WndProc(ref m);
        }
        #endregion
    }
}