C# type casting

Hi,
I am new to C# and have not done much object oriented programming. Nevertheless, I have to create a 3 level dynamic and scalable menu in .NET 1.2, C# and ASP. C# is used to build and draw the menu.
Basically I have to create two classes "Menu" and "MenuItem".
"Menu" is a class that will have "Name", "URL",  "Children" variables. "MenuItem" class will have "Name" and "URL" variables. "Children"  is a collection of "MenuItem" objects and/or "Menu" objects. The idea is to use polymorphism so that if menu has a child that has a child we are using "Menu".
Each aspx page has several variables that are read by the menu script to know which section to highlight in the menu.

I am getting an error

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidCastException: Specified cast is not valid.

Source Error:


Line 90:                   
Line 91:                   foreach(object m in this.ChildMenu )
Line 92:                   {      Menu menu = (Menu)m; //cast it back to Menu object
Line 93:                         //error handeling
Line 94:                         if(menu._childMenu != null)
 

Source File: C:\Inetpub\wwwroot\DIR006\dev\root\ontario\business\test2.aspx    Line: 92


here is the code


<%@ Page Language="c#" %>
<%@ Import namespace="System"%>
<%@ Import namespace="System.Collections"%>
<script language="c#" runat="server">

 
public class Menu
{
      
      //region private fields      
      private string _menuName;      
      private string _linkUrl;
      private int _menuLevel;      
      private ArrayList _childMenu;
      
      //private string _childMenu;
      //endregion

      public Menu(){
            this._menuName = "";
            this._linkUrl = "";
            this._menuLevel = 0;
            this._childMenu = new ArrayList();
      }//end Nav
      
      public Menu(string mName, string mUrl, int mLevel)
      {
            this._menuName = mName;
            this._linkUrl = mUrl;
            this._menuLevel = 0;
            this._childMenu = new ArrayList();
            
      
      }//end Nav
      
      public Menu(string mName, string mUrl, ArrayList chMenu, int mLevel )
      {
            this._menuName = mName;
            this._linkUrl = mUrl;
            this._menuLevel = mLevel;
            this._childMenu = chMenu;
      }//end Nav

      public string MenuName
      {
            get { return _menuName; }
            set { _menuName = value; }
      }

      
      public string LinkUrl
      {
            get { return _linkUrl; }
            set { _linkUrl = value; }
      }
      
      public int MenuLevel
      {
            get { return _menuLevel; }
            set { _menuLevel = value; }
      }
      
      public ArrayList ChildMenu
      {
            get { return _childMenu; }
            set { _childMenu = value; }
      }
      
            
      public string printMenu()
      {      
            string menuNav = "";
            
            
            //build menu with no subsection
            if(this._childMenu == null)
            {
                  if(this._childMenu[0] is MenuItem)
                  {            
                        //loop through the menuItems. create menues without subsections            
                        foreach(MenuItem item in this.ChildMenu )
                        {      
                              menuNav += "<div id=\"menu_level" + item.MenuLevel + "\">" + item.printMenuItem().ToString() + "</div>";                                          
                        }
                        
                  }
            }
            else   /// menu with subsections
            {
                  
                  foreach(object m in this.ChildMenu )
                  {      Menu menu = (Menu)m; //cast it back to Menu object
                        //error handeling
                        if(menu._childMenu != null)
                        {                        
                              foreach(object sM in menu.ChildMenu )
                              {
                                    Menu subMenu = (Menu)sM; //cast it back to Menu object
                                    if(subMenu._childMenu != null)
                                    {                        
                                          foreach(object sCh in subMenu.ChildMenu)
                                          {
                                                Menu subChild = (Menu)sCh; //cast it back to Menu object
                                                menuNav += "<div id=\"menu_level" + subChild.MenuLevel + "\">" + subChild.printMenu().ToString() + "</div>";
                                          }                              
                                    }
                              }      
                        }
                        else
                        {
                              menuNav += "this menu should be using MenuItem object";
                        }
                  }
                        
            }
            
            return menuNav;      
            
      }
      
      //pint menues that have subsections
      public string drawNav()
      {                  
            return this.printMenu();            
                  
      }
      
      
      
      ///
      
      
      
}

///////////////////////////MenuItem//////////////////////////////////////////////////////
public class MenuItem
{
      private string _menuName;      
      private string _linkUrl;      
      private int _menuLevel;
      private ArrayList _childMenu;
      
      //endregion

      
      public MenuItem(){
            this._menuName = "";
            this._linkUrl = "";
            this._menuLevel = 0;
      }//end Nav
      
      public MenuItem(string mName, string mUrl, int mLevel){
            this._menuName = mName;
            this._linkUrl = mUrl;
            this._menuLevel = mLevel;
            
      }//end Nav
      
      //get and set
      public string MenuName
      {
            get { return _menuName; }
            set { _menuName = value; }
      }

      
      public string LinkUrl
      {
            get { return _linkUrl; }
            set { _linkUrl = value; }
      }
      
      public int MenuLevel
      {
            get { return _menuLevel; }
            set { _menuLevel = value; }
      }
      
      ///print items
      public string printMenuItem()
      {      
            string menuNav = "";            
            
            //menuNav += "<div id=\"menu_level2\">";
            menuNav += "<a href=\"" + this._linkUrl + "?WT.svl=nav\">" + this._menuName + "</a>";            
            //menuNav +=  + "</div>";
                        
            return menuNav;
      }      
      
      
}



</script>

<body>
<div id="main">
<%


MenuItem oMenuItem = new MenuItem();
Menu oMenu = new Menu();

//direct Energy Business srvices section
oMenuItem = new MenuItem("Business Services","/ontario/business/default.aspx",1);
oMenu.ChildMenu.Add(oMenuItem);


//Doing Business with Direct Energy section
oMenuItem = new MenuItem("Consultative Process","/ontario/business/business_with_direct/consultive_process.aspx",2);
oMenu = new Menu("Doing Business","/ontario/business/business_with_direct/default.aspx",1);
oMenu.ChildMenu.Add(oMenuItem);

            
Response.Write("<div style=\"width:146px;\">");
Response.Write("" + oMenu.drawNav());
Response.Write("</div>");

%>
</div>
</body>