How can I get a NullReferenceException in this code sample?

Question

??? o = new ???();

Console.WriteLine("ToString() -> " + o.ToString() ); //<--- Prints 'ToString() -> '
Console.WriteLine("GetType() -> " + o.GetType()); //<--- NullReferenceException

Output:

ToString() -> 

Unhandled Exception: System.NullReferenceException: Object reference not set 
to an instance of an object.
at System.Object.GetType()
at Program.Main(String[] args)

Question

What is the type ??? and why does o.ToString() return string.Empty and o.GetType() throws a NullReferenceException?

Note: GetType() is not redefined in the ??? type.

If the type is Nullable<T>, this will happen since GetType is not virtual and the object will have to be boxed in order to call Object.GetType in the base class, and a nullable type will box to null, and a method call on null will cause a NullReferenceException. This is not a real question--voted to close. — Mehrdad Afshari
– Mehrdad Afshari, Commented Feb 12, 2010 at 16:04
Completely disagree with the close. The question is clear - what type would give a NullReferenceException. Just because the OP implies they know the answer doesn't make this "not a real question". It's some C# trivia that others can learn from. HOwever, it is now answered so reopening seems futile. — Jeff Yates
– Jeff Yates, Commented Feb 12, 2010 at 16:21
There are two different examples on stackoverflow.com/questions/194484/… — Marc Gravell
– Marc Gravell, Commented Feb 12, 2010 at 16:35

Community · Accepted Answer · 2017-05-23 12:11:49Z

7

Any Nullable<T>.

Check Gravell's example to strange corner cases in C#

edited May 23, 2017 at 12:11

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answered Feb 12, 2010 at 16:03

Dynami Le Savard

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3 Comments

Marc Gravell Over a year ago

An even more interesting example is the MyFunnyType on the same answer.

Russell Giddings Over a year ago

A Nullable<T> doesn't produce the stack trace above. The stack trace line 'at System.Object.GetType()' implies that the error is thrown from within GetType(), the Nullable<T> situation will just throw the NullReferenceException in Program.Main.

Noël Widmer Over a year ago

@RussellGiddings This is not correct. The stack trace behaves as described above. Calling GetType() on a null reference type would produce a different trace but calling it on a null value type results in what is described above. See: dotnetfiddle.net/3zDtxc

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