Can extensions methods be called on no-object?

Question

Why does the following piece of code work?

call:

SomeObject sO = null;
bool test = sO.TestNull();

code:

public static bool TestNull(this SomeObject sO)
{
    return sO == null;
}

Is this allowed to work or just a bug?

Answer 1

Is this allowed to work or just a bug?

The code after you've edited the question (to call s0.TestNull() instead of null.TestNull() is meant to work, yes.

Extension methods are just syntactic sugar for calling static methods as if they're instance methods. So a call of:

s0.TestNull()

is converted into

ClassContainingExtensionMethod.TestNull(s0)

... and that's all. No nullity checks are performed automatically.

This can actually be really useful - imagine if string.IsNullOrEmpty had been an extension method - then instead of writing:

if (string.IsNullOrEmpty(foo))

you could write the more readable:

if (foo.IsNullOrEmpty())

However, this power should not be taken lightly - most extension methods should throw ArgumentNullException if the first parameter has a null value, and those which don't should be very clear about it. (It should be relatively rare for such a method not to include Null somewhere in the name.)

Answer 2

Assuming you meant

bool test = sO.TestNull();

Then the answer is just that static functions do not need an instance of the object. Also, calling an extension function is just syntactic sugar for calling the function with parameters.

edit:

I recommend also reading Jon Skeet's answer.

Answer 3

The code should be bool test = sO.TestNull();. And yes, that is supposed to work.

You are extending an object with a function, it doesn't matter if the object is null or contains a value. It's essentially the same as writing SomeStaticClass.TestNull(sO);

Answer 4

As Jon and others already pointed out, this is standard behaviour. It can be useful for

• Extension methods that raise events and check for nullity before doing so
• Extension methods that provide chains of Maybes where each part of the chain could be null.

Long time ago I wondered whether it is OK to send messages to null references but time has shown that this is a pretty pragmatic and useful, especially considering the fact that in other languages null references simply do not exist.