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

namespace 打开唯一窗体
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        List<Form> flist = new List<Form>();
        private void button1_Click(object sender, EventArgs e)
        {
            bool has = false;
            Form2 f2 = new Form2(this);
            foreach(Form f in flist)
            {
                if(f is Form2)
                {
                    has = true;
                    f2 = f as Form2;
                }
            }
            if (has)
            {
                f2.WindowState = FormWindowState.Normal;
                //焦点进入
                f2.Focus();
            }
            else 
            {
                flist.Add(f2);
                f2.Show();
            }
        }
        public void deleteform(Form f) 
        {
            flist.Remove(f);
        }
    }
}

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

namespace 打开唯一窗体
{
    public partial class Form2 : Form
    {
        Form1 F1 = null;
        public Form2(Form1 f1)
        {
            InitializeComponent();
            F1 = f1;
        }

        private void Form2_FormClosing(object sender, FormClosingEventArgs e)
        {
            F1.deleteform(this);
        }
    }
}