imnull

// Please write an sequence list implements the interface with the required
// time complexity described in the comments. The users can add the same
// element as many times as they want, but it doesn't support the null item.
// You can use any types in .NET BCL but cannot use any 3rd party libraries.
// PS: You don't need to consider the multi-threaded environment.
interface IMyList<T> : IEnumerable<T>
{
    // O(1)
    // Add an item at the beginning of the list.
    void AddFirst(T item);