Fixing "CS8603: Possible null reference return" in method override

Question

With the following code

abstract class Base
{
    protected abstract T? GetValue<T>([CallerMemberName] string propertyName = "");
}

class Derived : Base
{
    protected override T GetValue<T>([CallerMemberName] string propertyName = "")
    {
        return default;
    }
}

the compiler tells me at return default; that I have a CS8603 "Possible null reference return" which is true. However if I append a ? to the return type of that method so that reads (like the abstract method) protected override T? GetValue<T>([CallerMemberName] string propertyName = "") the compiler tells me

• CS0508 'Derived.GetValue(string)': return type must be 'T' to match overridden member 'Base.GetValue(string)'.
• CS0453 The type 'T' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'Nullable'.

How do I tell the compiler my intent that GetValue<T> might return a null reference and not that the return type of that method should be Nullable<T>?

Answer 1

The solution was to place the [return: MaybeNull] attribute on the method override as follows:

[return: MaybeNull]
protected override T GetValue<T>([CallerMemberName] string propertyName = "")
{
  return default;
}

The compiler now no more lists warning CS8603.

Answer 2

You can use default generic constraint to make it compile:

abstract class Base
{
    protected abstract T? GetValue<T>([CallerMemberName] string propertyName = "");
}

class Derived : Base
{
    protected override T? GetValue<T>([CallerMemberName] string propertyName = "") where T : default
    {
        return default;
    }
}

But for value types default(T) will be returned not default(Nullable<T>) so it needs to be invoked with nullable value type for type parameter to achieve desired behaviour.

You can try to workaround by having 2 methods with different type constraints and dummy T? == null parameter:

abstract class Base
{
    public abstract T? GetValue<T>([CallerMemberName] string propertyName = "", T? _ = null) where T : struct;
    public abstract T? GetValue<T>([CallerMemberName] string propertyName = "", T? _ = null) where T : class;
}

class Derived : Base
{
    public override T? GetValue<T>([CallerMemberName] string propertyName = "", T? _ = null) where T : struct
    {
        return default;
    }
    public override T? GetValue<T>([CallerMemberName] string propertyName = "", T? _ = null) where T : class
    {
        return default;
    }
}

But it's up to you to decide how usable/ugly this is)