restrict a form to not open more then one at time

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Windows.Forms;
 
namespace SingletonForm
{
	public class SingletonForm : Form
	{
		#region Singleton pattern specific
		private static SingletonForm SingleInstance;
				
		public static SingletonForm Instance
		{
		    get 
		    {
		    	if(SingleInstance == null)
		    	{
		    		SingleInstance = new SingletonForm();
		    		SingleInstance.FormClosing += new FormClosingEventHandler(Singleton_FormClosing);
		    	}
		    	return SingleInstance;
			}
		}
		
		private static void Singleton_FormClosing(object sender, FormClosingEventArgs e)
		{
		    e.Cancel = true;
		    SingleInstance.Hide();
		}
		
		
		public new void Show()
		{
		    if (base.Visible)
		        base.Select();
		    else
		    {
		        base.Show();
		    }
		}
		
		#endregion
		
		#region Generated
		private System.ComponentModel.IContainer components = null;
		
		public SingletonForm()
		{
			InitializeComponent();
		}	
		
		protected override void Dispose(bool disposing)
		{
			if (disposing) {
				if (components != null) {
					components.Dispose();
				}
			}
			base.Dispose(disposing);
		}
		#endregion		
		
		private void InitializeComponent()
		{
			this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
			this.Text = "Child";
			this.Name = "Child";
		}
	}
	
	public class MainForm : Form
	{
		public MainForm()
		{
			InitializeComponent();
		}
		
		private System.ComponentModel.IContainer components = null;
 
		protected override void Dispose(bool disposing)
		{
			if (disposing) {
				if (components != null) {
					components.Dispose();
				}
			}
			base.Dispose(disposing);
		}
		
		private void InitializeComponent()
		{
			this.button1 = new System.Windows.Forms.Button();
			this.SuspendLayout();
			// 
			// button1
			// 
			this.button1.Location = new System.Drawing.Point(59, 33);
			this.button1.Name = "button1";
			this.button1.Size = new System.Drawing.Size(190, 47);
			this.button1.TabIndex = 0;
			this.button1.Text = "Start the other form";
			this.button1.UseVisualStyleBackColor = true;
			this.button1.Click += new System.EventHandler(this.Button1Click);
			// 
			// MainForm
			// 
			this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
			this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
			this.ClientSize = new System.Drawing.Size(303, 116);
			this.Controls.Add(this.button1);
			this.Name = "MainForm";
			this.Text = "Main Form";
			this.ResumeLayout(false);
		}
		private System.Windows.Forms.Button button1;
		
		void Button1Click(object sender, System.EventArgs e)
		{
			SingletonForm.Instance.Show();
		}
	}
	
	public class Program
	{
		[STAThread]
		private static void Main(string[] args)
		{
			Application.EnableVisualStyles();
			Application.SetCompatibleTextRenderingDefault(false);
			Application.Run(new MainForm());
		}		
	}
}