C
C#•2y ago
akaJB

✅ 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
ero•2y 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
canton7•2y 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
akaJB
akaJBOP•2y 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
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canton7
canton7•2y ago
Ah yeah, IReadOnlyDictionary isn't covariant (Which is confusing, as IReadOnlyCollection and IReadOnlyList are)

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