WebService --> XML Cache Repository
I tend to prefer composition over inheritance, but I see why you inherited from List&lt;T&gt;, and I feel like it's okay, but you'll run into some trouble if you decide that some other type of collection would be better suited to your task. I would spend a little time thinking about what you'd need to do in your client code if you changed the implementation. It may be worth implementing IList by delegating to a private instance of List&lt;IVideo&gt;. Then again, it may not be. 

XmlSerializer and StreamWriter both implement IDisposable, so it'd be better to put your usages of these into a using block. For example:

public void SaveChanges()
{
    using (var xmlSerializer = new XmlSerializer(Collection.GetType()))
    using (StreamWriter writer = new StreamWriter(RepoPath))
    {
        xmlSerializer.Serialize(writer, Collection);
    }
}