Extending List<T> and Violating The Open/Closed Principle

Question

I just created the following method in one of my classes

public static bool Assimilate(this List<Card> first, List<Card> second)
{
    // Trivial
    if (first.Count == 0 || second.Count == 0)
    {
        return false;
    }

    // Sort the lists, so I can do a binarySearch
    first.Sort();
    second.Sort();

    // Copia only the new elements
    int index;
    for (int i = 0; i < second.Count; i++)
    {
        index = first.BinarySearch(second[i]);
        if (index < 0)
        {
            first.Insert(~index, second[i]);
        }
    }

    // Edit
    second = null;

    return true;
}

And a friend of mine, reviewing my code, said that I shouldn't create methods that 'extend the List class', since that violates the open/closed principle. If I want to extend the class List I should create a new class that inherits from List and implement my "merge" method in that new class. Is he right? Extending the List class violates the Open/Closed principle?

Answer 1

I don't think this violates open/close principle. I think about it in terms of if I have to 'change' existing code to add functionality to an object then I'm violating open/close, but extending an object is exactly what you should do to add functionality.

You can extend the object in different ways in different languages, inheritance is just one way; c# provides you the ability to add extension methods to an existing class.

Remember 'open for extension - close for modification'