namespace MyNamespace
{
    interface ISomeInterface<in T> {
        Task M(T arg);
    }

    interface IBase : ISomeInterface<object>;

    interface IDerived : IBase, ISomeInterface<string> {
        Task ISomeInterface<string>.M(string obj) => M(obj as object);
    }

    interface ISomeGrain : IDerived, IGrainWithIntegerKey;

    class SomeGrain : ISomeGrain
    {
        public Task M(object arg)
        {
            throw new NotImplementedException();
        }
    }
}

        Task Task global::MyNamespace.IDerived.MyNamespace.ISomeInterface<System.String>.M(string arg0)