c# - Shouldn't calling Enumerable.AsEnumerable() on null variable throw an exception?

Question

I had a piece of code that used this expression - someCollection.AsEnumerable(). Later on I realized that, in some cases the someCollection was set to null. However, I never got any exception from this code. I even ran it through the debugger. If I remember correctly, the expression someCollection.AsEnumerable() evaluated to null if someCollection was set to null.

Shouldn't calling someCollection.AsEnumerable() throw an exception if someCollection was set to null? Am I missing something?

标签： c#linq

Answer 1

AsEnumerable is an extension method. It's defined like this:

public static class Enumerable
{
    public static IEnumerable<TSource> AsEnumerable<TSource>(this IEnumerable<TSource> source)
    {
        return source;
    }
}

It's valid to call extension methods on null -- writing someCollection.AsEnumerable() is just syntactic sugar for writing Enumerable.AsEnumerable(someCollection). You'd be surprised if you wrote Enumerable.AsEnumerable(null) and got a NullReferenceException!

As you can see, there's no test for null inside AsEnumerable - if it's passed null, it will just return null.

---

AsEnumerable could have been written to contain an explicit test for null -- if (source == null) throw new ArgumentNullException(nameof(source)); (which would throw an ArgumentNullException, rather than a NullReferenceException).

But, from MSDN:

The AsEnumerable<TSource>(IEnumerable<TSource>) method has no effect other than to change the compile-time type of source from a type that implements IEnumerable<T> to IEnumerable<T> itself.

If you have a null typed as an IQueryable<T>, I think it's perfectly valid for this method to turn it into a null typed as an IEnumerable<T>. null is after all a perfectly valid IEnumerable<T> and IQueryable<T>.

Another way of looking at this is, what's to be gained from throwing an exception if the source is null? The method can work perfectly fine it it's passed null, and that might be useful to someone.