MaybeNull attribute warns on null reference return

5
votes

As part of moving towards C# 8 nullable reference types, I encountered the following code (simplified):

public string GetIfExists(string key)
{
    dict.TryGetValue(key, out var value);

    return value;
}

The return line warns on a possible null reference return, and it makes sense. So I tried to annotate the method with a [return: MaybeNull] attribute, but the warning remained to my surprise. From the documentation, I understand that this attribute marks the return type as optional null, even when the actual type doesn't allow it.

It seems that my only option to avoid warnings is to mark the return type string?. So what is the use of [return: MaybeNull]?

2
c#-8.0nullable-reference-types
docs.microsoft.com/en-us/dotnet/csharp/nullable-attributes: "you can apply a number of attributes: ... to completely describe the null states of argument and return values. ... The more information you provide to the compiler about when a null value is allowed or prohibited, the better warnings users of your API will get." - GSerg
I tried using [return: MaybeNull] on your method, and the warning went away. Can you show the exact code you tried, and then also show a screenshot of the warning still being there after a rebuild? - Lasse V. Karlsen
@LasseV.Karlsen I think the issue was resolved in the compiler since I asked this question. I can no longer repro it. - Tomer
OK, good that you managed to resolve it :) - Lasse V. Karlsen

2 Answers

6
votes

From the documentation:

[return: MaybeNull] informs callers that the contract implies a non-nullable type, but the return value may actually be null. Use the MaybeNull attribute when your API should be a non-nullable type, typically a generic type parameter, but there may be instances where null would be returned.

This means that you want to use the attribute when you want the warning to be shown without changing the contract.

It seems that the attribute has been created to help enforce non-nullability (propagating a warning that forces the user to check for null) in two scenarios:

  • in situations where it is not possible to change the signature of a function.
  • in situations in which generics are involved. Generics are, due to their nature, a bit more difficult to handle and you might find yourself in a situation in which you are not allowed to have a T?, but you would still like the users of the method to be aware that the result needs to be checked.

The only way you have to get rid of the warning is to change the return type to string? (or return value!;, but it would be a lie :p).

0
votes

Your issue is irreproducible.

public string GetIfExists ( string key )
{
  new Dictionary<string, string>().TryGetValue (key, out string value); // Warning  CS8600  Converting null literal or possible null value to non-nullable type.
  return value; // Warning  CS8603  Possible null reference return.
}

[return: MaybeNull]
public string GetIfExists ( string key )
{
  new Dictionary<string, string>().TryGetValue (key, out string? value);
  return value;
}

(No issue‼)

From usage perspective for generics there is no difference between T? return type and [return: MaybeNull]. (Applies also to reference type.)

[return: MaybeNull]
public T Find<T> ( ) => default;
public T? Find2<T> () => default;    
public T Find3<T> ( ) => default;  // Warning   CS8603  Possible null reference return.

void Test ()
{
  object x = Find<object> (); // Warning    CS8600  Converting null literal or possible null value to non-nullable type.
  object x2 = Find2<object> (); // Warning  CS8600  Converting null literal or possible null value to non-nullable type.
  object x3 = Find3<object> ();
}