Is it necessary to throw a NullReferenceException from an extension method? [duplicate]

Question

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• ArgumentNullException or NullReferenceException from extension method? 5 answers

If I define an extension method such as this:

static public String ToTitleCase(this string instance, CultureInfo culture)
{
    if (instance == null)
        throw new NullReferenceException();

    if (culture == null)
        throw new ArgumentNullException("culture");

    return culture.TextInfo.ToTitleCase(instance);
}

Is it necessary for me to check the string instance for null and throw an null reference exception myself? Or does the CLR treat extension methods like instance methods in this case and handle the checking/throwing for me?

I know extension methods are just syntactic sugar for static methods, perhaps the C# compiler adds in the check at compile time? Please clarify :)

Answer 1

No. You should never throw a NullReferenceException manually. It should only ever be thrown by the framework itself.

In this context, you should be throwing ArgumentNullException for both instance and culture:

static public String ToTitleCase(this string instance, CultureInfo culture)
{
    if (instance == null)
        throw new ArgumentNullException("instance");

    if (culture == null)
        throw new ArgumentNullException("culture");

   return culture.TextInfo.ToTitleCase(instance);
}

From the NullReferenceException documentation:

Note that applications throw the ArgumentNullException exception rather than the NullReferenceException exception discussed here.

Answer 2

It most certainly is not. However, "fail fast" and, what some people forget ... "fail helpfully". However, I believe the reasons for not throwing an ArgumentNullException (debate vs. NullReferenceException left to other posts) are limited and usually related to over-cleverness :-) One hypothetical use-case may be IsNullOrEmpty. As long as it actually serves a purpose and makes code cleaner: go for it.

There is no check from the CLR. As far as the runtime is concerned it just passed a (possibly null) argument to a static method. The rest is sugar -- and none of that sugar involves adding extra null checks :-)

Happy coding.