winform 窗体只能实例一个（单例）

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

      

        private void button1_Click(object sender, EventArgs e)
        { //窗口已存在，则获取该窗口焦点
            this.Show(new Form2());
        }
        //判断窗口类型是否存在
        private bool IsExist(string formType)
        {
            bool flag = false;
            foreach (Form item in Application.OpenForms)
            {
                if (formType == item.GetType().ToString())
                    flag = true;
            }
            return flag;
        }
        private void Show(Form form)
        {
            //窗口已存在，则获取该窗口焦点
            if (IsExist(form.GetType().ToString()))
            {
                foreach (Form item in Application.OpenForms)
                {
                    if (item.GetType().ToString() == form.GetType().ToString())
                    {
                        //每次判断需要新建一个实例，在此释放
                        form.Dispose();
                        item.Show();
                        //获取窗口焦点
                        item.Activate();
                    }
                }
            }
            else
            {
                //窗口不存在，打开新窗口
                form.Show();
                form.Activate();
            }
        }
        
    }
}