Why is CodeContracts recommending a null check in a foreach loop

Question

I have this code:

foreach (UIElement uiElement in list)
{
    uiElement.SetValue(Grid.ColumnProperty, colunmn++);
    uiElement.SetValue(Grid.RowProperty, _uiRoot.RowDefinitions.Count - 1);
    _uiRoot.Children.Add(uiElement);
}

It runs fine, but Code Contracts is giving me a warning: Possibly calling a method on a null reference, uiElement.

How can uiElement ever be null? The list is a List of UIElements so it should iterate through the list without any nulls.

Answer 1

Because you can put nulls in your list, even though you probably won't

you can do

foreach (UIElement uiElement in list.Where(e => e != null))
{
   uiElement.SetValue(Grid.ColumnProperty, colunmn++);
   uiElement.SetValue(Grid.RowProperty, _uiRoot.RowDefinitions.Count -1);
   _uiRoot.Children.Add(uiElement);
}

Answer 2

A list can contain null references. You can insert null into a list. You can also insert a good reference into a list, then set it to null later. For example, if I have a list of People, I can have the list: "Bob", "Fred". Now I grab Bob from the list, do some stuff, and change it to null. A list contains a list of References not items. So it points to the location where the item was. Now when you iterate through the list, location 0 is now null since the reference where Bob was now contains a null.