C#•17mo ago

✅ How do I deal with generic covariance and collections types?

Let's say I have osmething like the following:

public interface IBoxCollection<out T> : IBox
    where T : IBox
{
    public Dictionary<string, T> Collection { get; }
}

public interface IBoxCollection<out T> : IBox
    where T : IBox
{
    public Dictionary<string, T> Collection { get; }
}

I get the following error: Invalid variance: The type parameter 'T' must be invariantly valid on 'IBoxCollection<T>.Collection'. 'T' is covariant I've looked around on how to resolve this issue but haven't found anything. Is there an easy solution to this problem, or is there a pattern which will accomplish what I'm looking for?

4 Replies

ero•17mo ago

IReadOnlyDictionary would probably work? A normal dictionary lets you both insert (in) and retrieve (out) values A readonly dictionary would only allow you to retrieve them

canton7•17mo ago

The problem is that someone can do:

class BoxCollection<T> : IBoxCollection<T> { ... }

var x = new BoxCollection<string>();
IBoxCollection<object> y = x;
y.Collection["test"] = 3;

class BoxCollection<T> : IBoxCollection<T> { ... }

var x = new BoxCollection<string>();
IBoxCollection<object> y = x;
y.Collection["test"] = 3;

Now your BoxCollection<string>, whose dictionary can only contain strings, contains an int

akaJBOP•17mo ago

Can you verify if I'm I misunderstanding something? It seems to not compile:

using System.Collections.Generic;

namespace Test
{
    public interface IBox
    {
    }

    public interface IBoxCollection<out T>
        where T : IBox
    {
        IReadOnlyDictionary<string, T> Collection { get; }
    }
}

using System.Collections.Generic;

namespace Test
{
    public interface IBox
    {
    }

    public interface IBoxCollection<out T>
        where T : IBox
    {
        IReadOnlyDictionary<string, T> Collection { get; }
    }
}

https://dotnetfiddle.net/v9UvPa

C# Online Compiler | .NET Fiddle

Test your C# code online with .NET Fiddle code editor.

canton7•17mo ago

Ah yeah, IReadOnlyDictionary isn't covariant (Which is confusing, as IReadOnlyCollection and IReadOnlyList are)

Gaming

Programming

✅ How do I deal with generic covariance and collections types?