Question

I have a custom class derived from List with an Add method that adds only if a certain condition is satisfied.

Do I also need to override* AddRange, or does AddRange simply call Add on each element of the given range?

*: Yes, new is hiding and not overriding in the context of C#.

Answer 1

If you want to create custom collection. Don’t derive it from List<T> but from Collection<T> or directly implement IList<T> or ICollection<T>. Indeed, the Add method in the List<T> class is not virtual.

Note: List<T>.AddRange uses Array.Copy.

UPDATE

When inheriting Collection you just have to override 2 methods!

public class MyCollection : Collection<string>
{
    private bool IsValidItem(string item)
    {
        return; // Your condition : true if valid; false, otherwise.
    }

    // This method will be called when you call MyCollection.Add or MyCollection.Insert
    protected override void InsertItem(int index, string item)
    {
        if(IsValidItem(item))
            base.InsertItem(index, item);
    }

    // This method will be called when you call MyCollection[index] = newItem
    protected override void SetItem(int index, string item)
    {
        if(IsValidItem(item))
            base.SetItem(index, item);
    }
}

If your items to validate are not string replace string in the code above by the correct type.