Nullable type GetType() throws exception

Question

I just got this quiz from a colleague that is driving me crazy. For this snippet of code:

var x = new Int32?();
string text = x.ToString(); // No exception
Console.WriteLine(text);
Type type = x.GetType(); // Bang!

Why does the first part .ToString() works without throwing an exception and then the call to GetType() throws a NullReferenceException ?

Answer 1

ToString is overridden in Nullable<T>, so no boxing is involved to make the call.

GetType() isn't a virtual method, so isn't (and can't be) overridden, so the value is boxed before the call is made... and boxing a null value of a nullable value type gives a null reference.

The reason for boxing is in section 7.5.5 of the C# 4 spec:

If M is an instance function member declared in a reference-type:

• ...
• If the type of E is a value-type, a boxing conversion (4.3.1) is performed to convert E to type object, and E is considered to be of type object in the following steps. In this case, M could only be a member of System.Object

Note that if you had:

var x = new Int32?(10);

you'd end up with the type being the same as typeof(int), again due to boxing. There is no way of creating a value foo such that foo.GetType() returns a nullable value type, using the normal GetType() method. (You could create a new GetType() method of course, but that's a side issue :)

(The use of "Bang!" suggests the author of said quiz may be me. Apologies for driving you crazy if that's the case.)