Question

Follow up question to a previous question, this has been identified as a co-variance issue. Taking this one step further, if I modify IFactory as follows:

class Program
{
    static void Main(string[] args)
    {
        IFactory<IProduct> factory = new Factory();
    }
}

class Factory : IFactory<Product>
{
}

class Product : IProduct
{
}

interface IFactory<out T> where T : IProduct
{
    List<T> MakeStuff();
}

interface IProduct
{
}

I get:

Invalid variance: The type parameter T must be invariantly valid on Sandbox.IFactory.MakeStuff(). T is covariant.

Why is this not invariantly valid? How can/should this be resolved?

Answer 1

@Craig’s answer is correct. To resolve, change it to:

IEnumerable<T> MakeStuff()

EDIT: As to the why, look at the definition of IEnumerable<T> Interface:

public interface IEnumerable<out T> : IEnumerable

Note that the IList<T> Interface does not have the out keyword. Variance is supported for generic type parameters in interfaces and delegates, not classes, so it doesn’t apply to List<T>.