C#•3d ago

NRT: How to correctly annotate my generic-class when inheriting from another generic class?

Hello. I'm trying to correctly annotate nullable reference types (NRT) to a generic class derived from another generic class. Here is some simplified code:

// Some dependency
interface IOther {
    K? Get<K>();
}

// The derived class which I want to annotate correctly
public class MyDictionary<T,K> : Dictionary<T, K> where T: notnull {
    public void Foo(T key) {
        IOther x = null!;
        Add(key, x.Get<K>()); // This triggers a CS8604
    }
}

// Usage of the class
var tmp = new MyDictionary<Guid, string>();
tmp.Add(Guid.NewGuid(), null);
tmp.Add(Guid.NewGuid(), "null");

// Some dependency
interface IOther {
    K? Get<K>();
}

// The derived class which I want to annotate correctly
public class MyDictionary<T,K> : Dictionary<T, K> where T: notnull {
    public void Foo(T key) {
        IOther x = null!;
        Add(key, x.Get<K>()); // This triggers a CS8604
    }
}

// Usage of the class
var tmp = new MyDictionary<Guid, string>();
tmp.Add(Guid.NewGuid(), null);
tmp.Add(Guid.NewGuid(), "null");

The shown implementation triggers the CS8604 as commented which I understand. But how do I tell the compiler: I don't know if the user will use a nullable type or not for K? When I change Dictionary<T, K> to Dictionary<T, K?> intellisense says that even my instance tmp can have nulls added but my type parameter clearly states that I don't want that to be allowed. So how do I correctly annotate this kind of scenario (I have a couple more like that)? Thanks in advance

31 Replies

ero•3d ago

what do you mean by

my type parameter clearly states that I don't want that to be allowed

Aaron•3d ago

the problem is that IOther.Get always returns a nullable value

PhereneticOP•3d ago

MyDictionary<Guid, string> <- No ? on string.

Aaron•3d ago

if I do MyDictionary<string, object>, then IOther.Get returns object? then you try to call Add, which takes K, which is object (not nullable) aka you're trying to pass K? as K

PhereneticOP•3d ago

Exactly

Aaron•3d ago

what exactly do you want the compiler to do about that you're doing something you explicitly said you wouldn't do like, it doesn't matter what the actual user of your class does, you are allowing them to pass in object as K your type parameter doesn't say at all that you don't want nulls you have to pick one, either make Get not return nulls or the dictionary has to be allowed to contain nulls you can't have both

PhereneticOP•3d ago

I can't decide whether K will be nullable or not

ero•3d ago

it depdends on the impl of the dependency really. if it can return a null value regardless of whether K is nullable or not, then there's nothing you can do about it. you have to assume that Get<K> will return null in some cases. that's what the dependency says.

Aaron•3d ago

. you are the one picking this it has nothing to do with the users of the class

ero•3d ago

it's a dependency, they're not the one picking it

Aaron•3d ago

they can always throw on nulls or make a new interface

PhereneticOP•3d ago

My problem/question is: How do I implement MyDictionary<T, K> : Dictionary<T, K> where I can leverage NRT and it's up to an actual consumer of that type if he wants to allow null or not.

ero•3d ago

it's not up to you. the dependency says it can return null

Aaron•3d ago

what do you do when IOther returns null when the user said it can't contain nulls

PhereneticOP•3d ago

That's my problem xD I can't just check against null...

Aaron•3d ago

I mean, that's not something the compiler can help you with at all it's a design choice yes you can? x.Get<K>() is null works just fine

PhereneticOP•3d ago

That's true. But I can't decide whether the used K from the consumer allows null or not.

ero•3d ago

that doesn't matter here at all even when K in MyDictionary<T, K> is not nullable, the return value of Get<K> is they're unrelated

Aaron•3d ago

yes, it has to purely be a design consideration of your implementation I believe they essentially want

var k = x.Get<K>();
if (K is not nullable && k is null)
    throw new Whatever();

var k = x.Get<K>();
if (K is not nullable && k is null)
    throw new Whatever();

which yeah, you can't do that information isn't available to the implementation, NRTs are purely compile time

PhereneticOP•3d ago

I just skimmed through the List<T> implementation. Maybe I have to use <T, K> and use something like this https://github.com/dotnet/runtime/blob/5535e31a712343a63f5d7d796cd874e563e5ac14/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/List.cs#L225

GitHub

runtime/src/libraries/System.Private.CoreLib/src/System/Collections...

.NET is a cross-platform runtime for cloud, mobile, desktop, and IoT apps. - dotnet/runtime

Aaron•3d ago

that throw helper is for value types it only works when assigning object to T where T is a non-nullable value type aka it doesn't work for you I meant it when I said this information is not available at runtime

PhereneticOP•3d ago

Yeah I know it's only compile time 🤔 So I have to either try change my dependency or suppress the warning with ! and hope for the best 🥴

ero•3d ago

if you can control the dependency, then you should change the implementation, yes Get<K> should only return K, and a TryGet method would be nice to have as well i assume K? Get<K> means a potential failure in getting that info should be conveyed through other means

PhereneticOP•3d ago

No it really means K could be null here.

ero•3d ago

i'm not really sure what you mean. Get<K> can return null even when K may not be nullable? that's what it currently means

PhereneticOP•3d ago

💡 let me check And that's exactly what it does. Returning null even if K would be string 🤔

ero•3d ago

in what circumstance? and is there no TryGet method?

PhereneticOP•3d ago

There is no TryGet and if it fails to get & convert that value.

ero•3d ago

that's bad design by that api then a failure like that should throw, not return null

PhereneticOP•3d ago

I agree Thank you for your answers. That was very enlightening.

ero•3d ago

i can't really think of a good way to handle this in that case. you would need to provide your own implementation of IOther

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NRT: How to correctly annotate my generic-class when inheriting from another generic class?