Question

An extension method expects an open generic IEnumerable.

Should I check inside the method wether the list is null.

I have the opinion that a list must never be null but have count == 0.

How do you handle such a case?

UPDATE:

I forgot to mention that the method is a recursive method where the list is recursively called/passed of course.

Answer 1

Yes you should check that. It’s also fairly common pattern to throw in such case, especially in LINQ:

public static void MyExtension<T>(this IEnumerable<T> source)
{
    if (source == null)
    {
        throw new ArgumentNullException("source");
    }

    // ...
}

Handling empty list is much easier than handling list instance that is null. Think about it in this way; list with no items is simply empty collection – fairly common case. List that is null… represents what? Exceptional situation, and should be handled as such.

Update:

I looked around on what Microsoft has to say about my speculations that throwing ANE is common pattern (as opposed to letting CLR throw NRE – which at points might be too vague) and it seems to be correct. We can find at ANE documentation page that:

ArgumentNullException (…) is provided so that application code can differentiate between exceptions caused by null arguments and exceptions caused by arguments that are not null.

And later, in framework design guidelines’ Exception Throwing section:

Do report execution failures by throwing exceptions. If a member cannot successfully do what it is designed to do, that should be considered an execution failure and an exception should be thrown.

In your case, since you mentioned your method must never accept null list argument, it is simple execution failure situation.