Generic Interfaces and Covariance

pairofstrings · Dec 23, 2023

Hi. I have the following code:

C#:

public interface ICovariance<out T>
{
    void Add(T item);
    T Get();
}

How to remove the compile-time error without changing the code of method declaration and by only changing the interface signature?

The error is in:

C#:

void Add(T item);

Thanks.

Filip Larsen · Dec 23, 2023

It sounds a bit like an exercise. Perhaps the compile error will give you a hint or perhaps you can simply search for how a generic covariant interface usually are defined (I haven't used C# in years, but I assume the compile error relates to you mixing covariant and contravariant signatures)?

pairofstrings · Dec 23, 2023

Filip Larsen said:

It sounds a bit like an exercise. Perhaps the compile error will give you a hint or perhaps you can simply search for how a generic interface usually are defined?

Yes, I did but the search says that it is not possible to have covariance and contravariance in the single interface like this:

Code:

public interface IContainer<out T>
{
    void Add(T item);
    T Get();
}

If I remove the following statement then it works fine:

C#:

void Add(T item);

But I need both statements like this:

Code:

public interface IContainer<out T>
{
    void Add(T item);
    T Get();
}

Is it really undoable or Am I missing something?

Filip Larsen · Dec 23, 2023

pairofstrings said:

But I need both statements

Then T can only be a specific type, i.e. remove out (which signals covariance).

pairofstrings · Dec 23, 2023

Filip Larsen said:

Then T can only be a specific type, i.e. remove out (which signals covariance).

I need both covariance and contravariance..

pairofstrings · Dec 23, 2023

I found the code on ChatGPT:

Code:

public interface IContainer<out T>
{
    void Add(T item);
    T Get();
}

Filip Larsen · Dec 23, 2023

pairofstrings said:

I need both covariance and contravariance.

Do you know what these two concept mean in C#? (hint: if you did, you would know why you can't have both for the interface you gave).

pairofstrings said:

I found the code on ChatGPT

Not sure what would be the most constructive comment to people getting surprised that code conjured up by ChatGPT doesn't work, but in this case perhaps you can ask it to explain why it is wrong and you can't have both co- and contra-variance for the same type generic type? If that fails there are plenty of regular search hits on both wikipedia and microsoft that explains the two variance concept.

pairofstrings · Dec 24, 2023

Filip Larsen said:

hint: if you did, you would know why you can't have both for the interface you gave

Thanks. I used multiple interfaces to achieve the desired result.

Generic Interfaces and Covariance

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