優化基本的單例模式

Form1

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 WindowsFormsApplication4
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        
        private void button1_Click(object sender, EventArgs e)
        {
  
            Form2 frm2 = Form2.Form2Single();//呼叫Form2中建立窗體的方法          
            frm2.Show();
 
        }
 
    }
}
 

Form2

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 WindowsFormsApplication4
{
    public partial class Form2 : Form
    {
        private Form2()
        {
            InitializeComponent();
        }
        private static Form2 form2 = null;
        public static Form2 Form2Single()
        {
            if(form2==null)
            {
                form2 = new Form2();
            }
            return form2;
        }
    }
}
 

在Form2中如果採用

if(form2==null)   {      form2 = new Form2();   }

會出現在關閉窗體form2之後，再次點選按鈕出現錯誤，frm2.Show();出現錯誤。關閉之後form2！=null，所以會出現錯誤；採用以下的判斷就不會出現錯誤

if ((f22 == null) || (f22.IsDisposed))  {                 f22 = new Form2();  }