#1,139 – The Problem with Comparisons of Objects in Generic Types

If you don’t constrain a type parameter in a generic class, the compiler will not let you compare two instances of objects of that type using the == or != operators.

In the example below, we store a collection of objects whose type is the type parameter T.  

    public class Pile<T>
    {
        List<T> pile = new List<T>();

        public void Add(T item)
        {
            if (!pile.Contains(item))
                pile.Add(item);
        }

        public bool IsFirst(T item)
        {
            // Compare to null allowed
            bool isnull = (item == null);

            return (pile[0] == item) ;
        }

        public void Dump()
        {
            foreach (T item in pile)
                Console.WriteLine(item);
        }
    }

If we try compiling this code, we’ll get a compile-time error in the IsFirst method, indicating that we can’t apply the == operator.  The compiler doesn’t have enough information about the type T to know that we can use the == operator.