Question

I have a collection class with an Equals method that I want to pass in a method to do equality checking between each item. Furthermore, I want to allow the delegate type to operate on superclasses of T as well as T itself:

public delegate bool EqualityComparer<T>(T x, T y);

public class Collection<T>
{
    //...

    public bool Equals<U>(Collection<T> other, EqualityComparer<U> eq) where T : U
    {
        // code using eq delegate to test equality between
        // members of this and other collection
    }
}

Unfortunately, the compiler borks over this (‘Collection.Equals()’ does not define type parameter ‘T’). Is there any way of specifying this type of constraint/operation?

Answer 1

No, I’m afraid you can’t specify a constraint like that. (I’ve wanted it too on occasion.)

You could write a static generic method with two type parameters in a non-generic class though:

public delegate bool EqualityComparer<T>(T x, T y);

public class Collection
{
    public static Equals<T, U>(Collection<T> first,
                               Collection<T> second,
                               EqualityComparer<U> comparer) where T : U
    {

    }
}

and you could even make that call an instance method on the generic class if you like:

// Implementing the static method:
return first.Equals(second, new EqualityComparer<T>(comparer));

where the collection’s instance method would just be:

public bool Equals(Collection<T> other, EqualityComparer<T> eq)
{
    // ...
}

This uses the contravariance available for creating delegates from C# 2 onwards.