FIX: You receive a "System.NullReferenceException" exception when you merge parent and child menus in an MDI form-based application in the .NET Framework 1.1 (890328)

MORE INFORMATION

Steps to reproduce the problem

1. Start Microsoft Visual Studio .NET 2003.
2. On the File menu, click New, and then click Project.
3. Click Visual C# Projects, click Windows Application, and then click OK.
4. In Solution Explorer, right-click Form1.cs, click View Code, and then replace all the code that is in Form1.cs with the following code.

   using System;
   using System.Drawing;
   using System.Collections;
   using System.ComponentModel;
   using System.Windows.Forms;
   using System.Data;
   
   namespace MDIMenuTest
   {
   	/// <summary>
   	/// Summary description for Form1.
   	/// </summary>
   	public class Form1 : System.Windows.Forms.Form
   	{
   		private System.Windows.Forms.MainMenu mainMenu1;
   		private System.Windows.Forms.MenuItem menuMain;
   		private System.Windows.Forms.MenuItem MenuFromMain;
   		/// <summary>
   		/// Required designer variable.
   		/// </summary>
   		private System.ComponentModel.Container components = null;
   
   		public Form1()
   		{
   			//
   			// Required for Windows Form Designer support
   			//
   			InitializeComponent();
   
   			// create new child
   			Child formChild = new Child();
   			formChild.MdiParent = this;   // set parent
   			formChild.Show();          // display child
   		}
   
   		/// <summary>
   		/// Clean up any resources being used.
   		/// </summary>
   		protected override void Dispose( bool disposing )
   		{
   			if( disposing )
   			{
   				if (components != null) 
   				{
   					components.Dispose();
   				}
   			}
   			base.Dispose( disposing );
   		}
   
   		#region Windows Form Designer generated code
   		/// <summary>
   		/// Required method for Designer support - do not modify
   		/// the contents of this method with the code editor.
   		/// </summary>
   		private void InitializeComponent()
   		{
   			this.mainMenu1 = new System.Windows.Forms.MainMenu();
   			this.menuMain = new System.Windows.Forms.MenuItem();
   			this.MenuFromMain = new System.Windows.Forms.MenuItem();
   			// 
   			// mainMenu1
   			// 
   			this.mainMenu1.MenuItems.AddRange(new System.Windows.Forms.MenuItem[] {
   																					  this.menuMain});
   			// 
   			// menuMain
   			// 
   			this.menuMain.Index = 0;
   			this.menuMain.MenuItems.AddRange(new System.Windows.Forms.MenuItem[] {
   																					 this.MenuFromMain});
   			this.menuMain.MergeType = System.Windows.Forms.MenuMerge.MergeItems;
   			this.menuMain.Text = "Menu";
   			// 
   			// MenuFromMain
   			// 
   			this.MenuFromMain.Index = 0;
   			this.MenuFromMain.Text = "MenuFromMain";
   			// 
   			// Form1
   			// 
   			this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
   			this.ClientSize = new System.Drawing.Size(292, 266);
   			this.IsMdiContainer = true;
   			this.Menu = this.mainMenu1;
   			this.Name = "Form1";
   			this.Text = "Form1";
   			this.Load += new System.EventHandler(this.Form1_Load);
   
   		}
   		#endregion
   
   		/// <summary>
   		/// The main entry point for the application.
   		/// </summary>
   		[STAThread]
   		static void Main() 
   		{
   			Application.Run(new Form1());
   		}
   
   		
   	}
   }
   

5. On the Project menu, click Add Windows Form, click Windows Form under Templates, name the form Child.cs, and then click Open.
6. In Solution Explorer, right-click Child.cs, click View Code, and then replace all the code that is in Child.cs with the following code.

   using System;
   
   using System.Drawing;
   
   using System.Collections;
   
   using System.ComponentModel;
   
   using System.Windows.Forms;
   
   using System.Diagnostics;
   
    
   
   namespace MDIMenuTest
   
   {
   
   	/// <summary>
   
   	/// Summary description for Child.
   
   	/// </summary>
   
   	public class Child : System.Windows.Forms.Form
   
   	{
   
   		private System.Windows.Forms.MainMenu mainMenu1;
   
   		private System.Windows.Forms.MenuItem menuMain;
   
   		private System.Windows.Forms.MenuItem MenuFromChild;
   
   		private System.Windows.Forms.MenuItem menuItem1;
   
   		/// <summary>
   
   		/// Required designer variable.
   
   		/// </summary>
   
   		private System.ComponentModel.Container components = null;
   
    
   
   		public Child()
   
   		{
   
   			//
   
   			// Required for Windows Form Designer support
   
   			//
   
   			InitializeComponent();
   
    
   
   			//
   
   			// TODO: Add any constructor code after InitializeComponent call
   
   			//
   
   		}
   
    
   
   		/// <summary>
   
   		/// Clean up any resources being used.
   
   		/// </summary>
   
   		protected override void Dispose( bool disposing )
   
   		{
   
   			if( disposing )
   
   			{
   
   				if(components != null)
   
   				{
   
   					components.Dispose();
   
   				}
   
   			}
   
   			base.Dispose( disposing );
   
   		}
   
    
   
   		#region Windows Form Designer generated code
   
   		/// <summary>
   
   		/// Required method for Designer support - do not modify
   
   		/// the contents of this method with the code editor.
   
   		/// </summary>
   
   		private void InitializeComponent()
   
   		{
   
   			this.mainMenu1 = new System.Windows.Forms.MainMenu();
   
   			this.menuMain = new System.Windows.Forms.MenuItem();
   
   			this.MenuFromChild = new System.Windows.Forms.MenuItem();
   
   			// 
   
   			// mainMenu1
   
   			// 
   
   			this.mainMenu1.MenuItems.AddRange(new System.Windows.Forms.MenuItem[] {
   
   																					  this.menuMain});
   
   			// 
   
   			// menuMain
   
   			// 
   
   			this.menuMain.Index = 0;
   
   			this.menuMain.MenuItems.AddRange(new System.Windows.Forms.MenuItem[] {
   
   																					 this.MenuFromChild,
   
   			});
   
   			this.menuMain.MergeType = System.Windows.Forms.MenuMerge.MergeItems;
   
   			this.menuMain.Text = "Menu";
   
   			this.menuMain.Popup += new System.EventHandler(this.menuMain_Popup);
   
   			// 
   
   			// MenuFromChild
   
   			// 
   
   			this.MenuFromChild.Index = 0;
   
   			this.MenuFromChild.Text = "MenuFromChild";
   
    
   
   			// 
   
   			// Child
   
   			// 
   
   			this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
   
   			this.ClientSize = new System.Drawing.Size(292, 266);
   
   			this.Menu = this.mainMenu1;
   
   			this.Name = "Child";
   
   			this.Text = "Child";
   
    
   
   		}
   
   		#endregion
   
    
   
   		private void menuMain_Popup(object sender, System.EventArgs e)
   
   		{
   
   			// The goal is to dynamically change the child menu items during run time.
   
   			// Then, the new child menu items will be merged into the MDI main menu automatically by the .NET Framework.
   
   			// However, the program crashed after trying to add the following menu item to the MDI menu.
   
   			MenuItem menuItem = new MenuItem("New Menu Item From Child");
   
   			this.menuMain.MenuItems.Add(menuItem);
   
   		}
   
   	}
   
   }

7. On the Debug menu, click Start to compile and then run the project.
8. In the new application, click any menu choice. You receive the following error message:
   Oject reference not set to an instance of an object