In C#, what happens when you call an extension method on a null object?

Question

Does the method get called with a null value or does it give a null reference exception?

MyObject myObject = null; myObject.MyExtensionMethod(); // <-- is this a null reference exception? 

If this is the case I will never need to check my 'this' parameter for null?

Answer 1

That will work fine (no exception). Extension methods don't use virtual calls (i.e. it uses the "call" il instruction, not "callvirt") so there is no null check unless you write it yourself in the extension method. This is actually useful in a few cases:

public static bool IsNullOrEmpty(this string value) {     return string.IsNullOrEmpty(value); } public static void ThrowIfNull<T>(this T obj, string parameterName)         where T : class {     if(obj == null) throw new ArgumentNullException(parameterName); } 

etc

Fundamentally, calls to static calls are very literal - i.e.

string s = ... if(s.IsNullOrEmpty()) {...} 

becomes:

string s = ... if(YourExtensionClass.IsNullOrEmpty(s)) {...} 

where there is obviously no null check.

Answer 2

Addition to the correct answer from Marc Gravell.

You could get a warning from the compiler if it is obvious that the this argument is null:

default(string).MyExtension(); 

Works well at runtime, but produces the warning "Expression will always cause a System.NullReferenceException, because the default value of string is null".