C# – How to return NULL from a generic method in C#

cgenerics

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 collecion
    {
        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?

Best Answer

Two options:

  • Return default(T) 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))
  • Restrict T to be a reference type with the where T : class constraint and then return null as normal
Related Topic