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c#nullable-reference-types
David
David

Reputation: 2556

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

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

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>?

Upvotes: 4

Views: 9260

Answers (3)

Jay
Jay

Reputation: 2254

Returning T? instead of T worked for me.

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

Or, remove <Nullable>enable</Nullable> from your .csproj file

Upvotes: 0

David
David

Reputation: 2556

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.

Upvotes: 6

Guru Stron
Guru Stron

Reputation: 142288

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)

Upvotes: 1

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