Why does "as T" get an error but casting with (T) not get an error?

Question

Why can I do this:

public T GetMainContentItem<T>(string moduleKey, string itemKey)
{
    return (T)GetMainContentItem(moduleKey, itemKey);
}

but not this:

public T GetMainContentItem<T>(string moduleKey, string itemKey)
{
    return GetMainContentItem(moduleKey, itemKey) as T;
}

It complains that I haven't restricted the generic type enough, but then I would think that rule would apply to casting with "(T)" as well.

Answer 1

Because 'T' could be a value-type and 'as T' makes no sense for value-types. You can do this:

public T GetMainContentItem<T>(string moduleKey, string itemKey)
    where T : class
{
    return GetMainContentItem(moduleKey, itemKey) as T;
}

Answer 2

If T is a value type this is an exception, you need to make sure T is either Nullable or a class.

Answer 3

Is T a value type? If so, if the as operator fails, it will return null, which cannot be stored in a value type.