主窗口后台代码：

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 WindowsFormsApplication2{    public partial class Form1 : Form    {        public Form1()        {            InitializeComponent();        }        //建立集合来记录        List
     
       AllForm = new List();        private void button1_Click(object sender, EventArgs e)        {            Form2 f2 = new Form2(this, textBox1.Text);//实例化类            f2.Name = "f2";//重新给其name赋值，以免发生冲突            OnlyOne(f2);        }                //保证弹出窗口唯一的方法，私有        private void OnlyOne(Form FF)        {            bool hasForm = false;            foreach (Form f in AllForm)            {                if (f.Name == FF.Name)                {                    hasForm = true;                    f.WindowState = FormWindowState.Normal;                    f.Focus();                }            }            if (hasForm)            {                FF.Close();            }            else            {                AllForm.Add(FF);                FF.Show();            }        }        private void button2_Click(object sender, EventArgs e)        {            Form3 f3 = new Form3();            f3.Name = "f3";            OnlyOne(f3);        }        //弹出窗口关闭后，在集合里将其移除，避免误关或关闭后无法再次弹出        public void RemoveForm(Form ff)        {            AllForm.Remove(ff);        }    }}
     

弹出窗口的后台代码：

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 WindowsFormsApplication2{    public partial class Form2 : Form    {        Form1 F1 = null;        public Form2(Form1 f1, string uname)        {            InitializeComponent();            F1 = f1;            label1.Text = uname + "，欢迎回来！";        }        private void Form2_FormClosing(object sender, FormClosingEventArgs e)        {            F1.RemoveForm(this);        }    }}