Generic Type parameters

Question

Please explain why this doesn't work? What is it about the constraint IPoolable<T> that the compiler doesn't like? Is it because the generic IPoolable<T> is really just a "template" and there is no inheritance between PoolItem and IPollable<T>?

In the code

public class GenericPool<TPoolable> where TPoolable : IPoolable<T>

while can't T be interpreted as long?

Full code listing:

    public interface IPoolable<T> where T : new()
    {
        T PooledObject { get; }
    }

    public class PoolItem : IPoolable<long>
    {
        private readonly long _id;

        public long PooledObject
        {
            get { return _id; }
        }
    }

    public class GenericPool<TPoolable> where TPoolable : IPoolable<T>
    {
        public T Borrow()
        {
            var item = default(TPoolable);
            return item.PooledObject;
        }

        public void Return(T item)
        {

        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var pool = new GenericPool<PoolItem>();
        }
    }

Just to be clear, I know that I can change GenericPool to

public class GenericPool<TPoolable, T> where TPoolable : IPoolable<T> where T : new()

I am not asking how to fix this code. Instead I am asking the question; why do I have to add T as a type parameter to GenericPool in the first place?

dlev · Accepted Answer · 2011-08-06 06:25:14Z

3

You can't use IPoolable<T> as a constraint because the generic type parameter T isn't known in that context. As you surmise, IPoolable<T> is essentially a blueprint for an infinite number of types, so without binding T to something (for example, a type parameter of the GenericPool class) it doesn't make sense as a constraint.

The actual error message you should be getting is The type or namespace name 'T' could not be found, which makes sense: T isn't defined anywhere.

edited Aug 6, 2011 at 6:25

answered Aug 6, 2011 at 6:17

dlev

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3 Comments

Ryan Pedersen Over a year ago

I understand that T "isn't defined" in the GenericPool class but IPoolable<T> is defined. Why do I have to define T twice?

dlev Over a year ago

The T in IPoolable<T> is only relevant in the declaration of the interface itself. Outside of that, IPoolable<> has to be declared with a combination of actual types and valid type parameters between the < and the > (with the notable exception of typeof(IPoolable<>) ). Since T is neither a valid type, nor a currently useable type parameter, you don't have valid code.

Grozz Over a year ago

no, IPoolable<T> isn't defined either, because compiler doesn't know what T is. The only thing that is defined is generic IPoolable interface, which should be parametrized with a type before instantiation.

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