Writing a class that represents a sorted list of filenames

Question

I have a question:

Write a class that represents a sorted list of filenames. The filenames are sorted by most recently added first. Duplicates are not allowed.

And I wrote this solution:

public class RecentlyUsedList

{

    private readonly List<string> items;

    public RecentlyUsedList()

    {

        items = new List<string>();

    }

    public void Add(string newItem)

    {

        if (items.Contains(newItem))

        {

            int position = items.IndexOf(newItem);



            string existingItem = items[position];

            items.RemoveAt(position);

            items.Insert(0, existingItem);

        }

        else

        {

            items.Insert(0, newItem);

        }

    }

    public int Count

    {

        get

        {

            int size = items.Count;

            return size;

        }

    }

    public string this[int index]

    {

        get

        {

            int position = 0;

            foreach (string item in items)

            {

                if (position == index)

                    return item;

                ++position;

            }

            throw new ArgumentOutOfRangeException();

        }

    }

}

Now, other person asked me, do the code review of your own code and tell the answers of following questions:

Three operations are required:

• How long is the list?

• Access filename at a given position

• Add a filename to the list; if it already exists in the list it gets moved to the top otherwise the new filename is simply prepended to the top.

I got confused and tried to look on Google but I didn't get much from there. What would be the best answers to these questions?

Answer 1

Empty lines as separation around code lines that are related are always a good idea. Your code has a little too much empty lines where it is not common in C# (or C/C++, Java, JavaScript):

public class RecentlyUsedList

{

This in fact decreases readability. Instead just write:

public class RecentlyUsedList
{

---

      private readonly List<string> items;

      public RecentlyUsedList()

      {

        items = new List<string>();

      }

Here the constructor is not necessary. Just do:

private readonly List<string> items = new List<string>();

---

Your Add(...) method can be simplified to this:

  public void Add(string newItem)
  {
    items.Remove(newItem);
    items.Insert(0, newItem);
  }

items.Remove(newItem) just returns false, if the item is not present, so it's safe to use in any case. There is no need to be concerned about the existing string because it is the same as the newItem.

---

public int Count... can be simplified to:

public int Count => items.Count;

---

List<string> items

has an indexer it self, so you can use that when implementing the indexer:

  public string this[int index]
  {
    get
    {
      if (index < 0 || index >= items.Count)
      {
        throw new ArgumentOutOfRangeException();
      }
      return items[index];
    }
  }

Here I throw an exception if the index argument is out of range. You could let items handle that as well...

In fact your foreach-loop is potentially much slower than the List<T>[index] because you make a kind of search where List<T>[index] just performs a look up. So you hide an efficient behavior with a not so efficient one.

Answer 2

In addition to Henrik Hansen's excellent answer, I'll add a note about encapsulation.

Your class is currently acting as a wrapper around a List, hiding that list from public view. The only way for other actors to access that list is through your Add, Count, and indexer methods. This is a good thing, because it allows you to enforce the guarantee that the order of the elements in the list will be meaningful.

The danger that I see is this: Perhaps in the future someone will want to iterate through this List and, having no way to do that, will simply pop in and make your private List into a protected, internal, or even public List. This will break the encapsulation, and you will no longer be able to guarantee that the order of elements is meaningful: an external class with a reference to the list itself will be able to add and remove elements at will.

For that reason, I would consider implementing IEnumerable<string> on your class. It can be as simple as adding these two lines:

public IEnumerator<string> GetEnumerator() => items.GetEnumerator();
public IEnumerator GetEnumerator() => this.GetEnumerator();

Now, adding this behavior before it's actually required is toying with YAGNI, so you should consider carefully before you do so. But, you may find that it's a good idea and a good fit for your class.