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 winform唯一弹窗{    public partial class Form1 : Form    {        public Form1()        {            InitializeComponent();        }        List<Form> allform = new List<Form>();        //跳转到form2界面，把值传过去        private void button1_Click(object sender, EventArgs e)        {            Form2 f2 = new Form2(this,textBox1.Text);//实例化，把值传到form2            f2.Name = "f2";//重新给form2的name赋值，免除发生冲突            onlyone(f2);        }        //私有，保证弹出窗口是唯一的方法        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();            }        }        //跳转到form3界面，把值传过去        private void button2_Click(object sender, EventArgs e)        {            Form3 f3 = new Form3();            f3.Name = "f3";            onlyone(f3);        }        //弹窗关闭后，要在集合中移除，避免关闭后无法再次弹出        public void removeform(Form ff)        {            allform.Remove(ff);        }    }}

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 winform唯一弹窗{    public partial class Form2 : Form    {        Form1 F1 = null;        public Form2(Form1 f1,string uname)        {            InitializeComponent();            F1 = f1;            //显示用户名            label1.Text = uname;        }        private void Form2_FormClosing(object sender, FormClosingEventArgs e)        {            F1.removeform(this);        }    }}