Why doesn't allow covariation here？

[LeaFrock]

I find an interesting thing and I'm confused whether it's due to limit of compiler or is just by design. Codes see below,

    public class SingleHashFunctionList<T> : IHashFunctionList  
        where T : IHashFunction //Notice this 'where'
    {
        private readonly IReadOnlyList<T> _hashFunctions;

        public SingleHashFunctionList()
        {
            // Skip init work of '_hashFunctions'
            HashFunctions = _hashFunctions; //Compiler error: ...cannot implictly convert...
        }

        public IReadOnlyList<IHashFunction> HashFunctions { get; }
    }

Since the interface IReadOnlyList<out T> supports covariation and the compiler should know T must inherit the interface IHashFunction, why is IReadOnlyList<T> disallowed to be implictly converted to IReadOnlyList<IHashFunction> ?

[YairHalberstadt]

You need to add the class constraint, since covariant type parameters don't work with structs:

    public class SingleHashFunctionList<T> : IHashFunctionList  
        where T : class, IHashFunction
    {
        private readonly IReadOnlyList<T> _hashFunctions;

        public SingleHashFunctionList()
        {
            // Skip init work of '_hashFunctions'
            HashFunctions = _hashFunctions;
        }

        public IReadOnlyList<IHashFunction> HashFunctions { get; }
    }

[LeaFrock]

Wow thanks! Maybe I need to create an issue for dotnet/roslyn that the analyzer should be smarter and the error message can be more friendly here...

[yaakov-h]

I, for one, would welcome an improved diagnostic.