Question

I’d like to be able to create a static generic type with a base type constraint like

public static class Manager<T> where T : HasId
{
    public static T GetSingleById(ref List<T> items, Guid id)
    {
        // the Id is a property provided by HasId
        return (from i in items where i.Id == id select i).SingleOrDefault();
    }
}

Then add another method

...
    public static IEnumerable<T> GetManyByParentId(ref List<T> items, Guid parentId) where T : HasIdAndParentId
    {
        // the parentId is a property of HasIdAndParentId which subclasses HasId
        return from i in items where i.ParentId == parentId select i;
    }
...

Since HasIdAndParentId subclasses HasId the constraint T : HasId is met but the compiler won’t accept the where base type constraint on the method.

Any ideas?

Answer 1

In this case, you’re not redefining the type parameter on the method, so you can’t apply any new constraints. You should be able to do it this way:

public static IEnumerable<T2> GetManyByParentId<T2>(
    ref List<T2> items, Guid parentId) 
    where T2 : T, HasIdAndParentId { .. }