Question

I have an interface:

interface IKey<TId, TName>
    where TId: IEquatable<TId>
    where TName: IEquatable<TName>
{
    TId Id { get; set; }
    TName Name { get; set; }
}

Then I implement IKey like this:

class Item : IKey<int, string>
{
    int Id { get; set; }
    string Name { get; set; }
    //...
}

And I have collection that should work with these items

class ItemCollection<T>
    where T : IKey<TId, TName> //Any type that implements IEquatable<...>
    where TId: IEquatable<TId>
    where TName: IEquatable<TName>
{
    //...
}

And the problem is that it doesn’t work. Is there a way I can do this correctly?

There is another implementation whithout IEquatable, using IKey<out TId, out TName> and IKey<object, object> but it doesn’t work with value-types and uses Object.Equals.

Answer 1

The problem is your attempting to use TId and TName in ItemCollection without ever defining them. Because they are part of the interface constraint on T they need to either be concrete types are specified as type parameters.

class ItemCollection<T, TId, TName>
    where T : IKey<TId, TName>
    where TId : IEquatable<TId>
    where TName : IEquatable<TName>
{
    //...
}

Example using hard coded types

class ItemCollection<T>
    where T : IKey<string, string>
{
    //...
}