Generic method returns nullReferenceException

Question

I have this Generic Method that always returns a nullReferenceException. I've taken all the unnecessary code out, and it still give me the exception.

    public T RetrieveInformationFromApi<T>(int Id) where T : IMovie
    {
        T result = default(T);

        result.Title = "test";
     }

The method can be called with three different classes that all have the attribute Title and the all implement the interface IMovie but the method still give me an exception. What am i doing wrong?

Scratching my head over this one. What did you expect this to do? — Jim Mischel, Nov 29 '13 at 22:27
Almost all cases of `NullReferenceException` are the same. Please see "[What is a NullReferenceException in .NET?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-in-net)" for some hints. — John Saunders, Nov 29 '13 at 22:37
It won't compile until you `return` something (method return type is not `void`). Looks like you meant to `return result;`. — Jeppe Stig Nielsen, Nov 30 '13 at 08:11

score 5 · Accepted Answer · answered Nov 29 '13 at 22:28

5

You should add a constructor constraint on you T parameter so you can create a new instance.

public T RetrieveInformationFromApi<T>(int Id) where T : IMovie, new()
{
    T result = new T();

    result.Title = "test";
}

answered Nov 29 '13 at 22:28

Johnbot

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Szymon · Answer 2 · 2013-11-29T22:57:20.093

4

You're using default which returns null for reference types (and I'm guessing you're using a reference type.

From MSDN:

Given a variable t of a parameterized type T, the statement t = null is only valid if T is a reference type and t = 0 will only work for numeric value types but not for structs. The solution is to use the default keyword, which will return null for reference types and zero for numeric value types.

I'm not sure what you're trying to achieve but this way you will always get that exception for reference types.

edited Nov 29 '13 at 22:57

answered Nov 29 '13 at 22:25

Szymon

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He will not "always" get the exception. When he calls that method with `T` being a `struct` (value type) implementing `IMovie`, then the setter of `result.Title` *will* be called. – Jeppe Stig Nielsen Nov 29 '13 at 22:52
@JeppeStigNielsen Very true, I edited the answer to make it correct. – Szymon Nov 29 '13 at 22:57

score 3 · Answer 3 · answered Nov 29 '13 at 22:25

3

When the T is a reference type (class), the default (T) returns null, as it is the default value of all reference types.

Then obviously trying to access its property results in NullReferenceException

answered Nov 29 '13 at 22:25

Honza Brestan

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Generic method returns nullReferenceException

3 Answers3