Question

I have a generic method with this (dummy) code (yes I’m aware IList has predicates, but my code is not using IList but some other collection, anyway this is irrelevant for the question…)

    static T FindThing<T>(IList collection, int id) where T : IThing, new()     {         foreach (T thing in collection)         {             if (thing.Id == id)                 return thing;         }         return null;  // ERROR: Cannot convert null to type parameter 'T' because it could be a value type. Consider using 'default(T)' instead.     } 

This gives me a build error

"Cannot convert null to type parameter ‘T’ because it could be a value type. Consider using ‘default(T)’ instead."

Can I avoid this error?

Answer 1

Three options:

• Return default (or default(T) for older versions of C#) which means you’ll return null if T is a reference type (or a nullable value type), 0 for int, '\0' for char, etc. (Default values table (C# Reference))
• If you’re happy to restrict T to be a reference type with the where T : class constraint and then return null as normal
• If you’re happy to restrict T to be a non-nullable value type with the where T : struct constraint, then again you can return null as normal from a method with a return value of T? – note that this is not returning a null reference, but the null value of the nullable value type.