asp.net menu null exception error

Question

I am playing around with asp.net and a nested master page and I have noticed something very odd. In the code behind for my default page I am trying to access my menu simply by referencing to the menu's id. However each time I do so it appears to be empty throwing me a null exception error, even though I populated it with different menu items in the html file. I was able to access the menu's items this way just fine before using the master page.

Now I was able to circumvent this issue by adding the line page.Master.FindControl("Menu0"); before my for each loop. after this line my menu appears not to be null.

   protected void Page_PreInit()//run when the page is loaded
    {
        Debug.WriteLine("website started");// website started

   Page.Master.FindControl("Menu0");


    foreach (MenuItem item in Menu0.Items)
    {
        string temp = item.Text;
        item.Text = "<span class=\"menuItems\">" + temp + "</span>";//assign correct style

    }

}  

I do not know why progam indicates that my menu is null or why this particular line fix the issue Please I would appreciate help thx

Answer 1

A MasterPage is a naming container. FindControl method does not penetrate through naming containers. What? What does that mean?

If you put a control in a naming container, master page is a naming container, then the names of the controls inside this container are prefixed with the id of the naming container and then the control id is appended. So if the naming control's id is Xxx then the menu within the naming container is 'XxxYyy' where Yyy is the name of the menu.

When you call Page.FindControl it will find Xxx, the master naming container, but it will not search the controls within the naming container, therefore, it returns null.

The same idea applies if you had a GridView and needed to find a control within the GridView.