public class Foo<T>
{
    public T Member { get; set; }
}

[MessagePack.MessagePackObject]
public class Bar
{
    [MessagePack.Key(0)]
    public Foo<int> MemberUserGeneric { get; set; }

    [MessagePack.Key(1)]
    public System.Collections.Generic.List<int> MemberKnownGeneric { get; set; }
}

static void Test()
{
    MessagePackSerializer.Serialize(new Bar());
}