Cannot implicitly convert List to Collection

Question

This is a compiler error (slightly changed for readability).

This one always puzzled me. FxCop tells that this is a bad thing to return List and classes that are\\de

Answer 1

Here is a generic extension method written in C# 3.0 used to convert List to Collection

using System.Collections.Generic;
using System.Collections.ObjectModel;

public static class ExtensionMethods
{
    public static Collection ToCollection(this List items)
    {
        Collection collection = new Collection();

        for (int i = 0; i < items.Count; i++)
        {
            collection.Add(items[i]);
        }

        return collection;
    }
}

and it is used like this…

List entities = new List();
entities.Add("Value 1");
entities.Add("Value 2");
entities.Add("Value 3");
entities.Add("Value 4");

Collection convertedEntities = entities.ToCollection();