How to add an item to the list if it doesn't exist in C#?

A thumbnail showing C# code.

To add an item to the list, if it doesn't exist:

  1. Use the Contains method to check if the item exists in the list. It returns true if the item exists in the list.
  2. If Contains is true, use the Add method to add the item to the list.
  3. Create a generic extension method to make it easy to add an item to the list if it doesn't exist.
csharp
List<string> list = new List<string>() { "exists" };

list.AddIfNotExists("exists");
list.AddIfNotExists("doesn't exist");

Console.WriteLine(string.Join(", ", list)); // exists, doesn't exist

public static bool AddIfNotExists<T>(this List<T> list, T value)
{

  if (!list.Contains(value))
  {

    list.Add(value);
    return true;
  }
  return false;
}

Performance

The Contains method does a linear, O(n) search. If you have a lot of items in the list, this can be slow.

To get a faster collection search, consider to:

In most cases, you won't run into performance problems. If you have a list with over 10,000 items, do some performance testing to find which method performs the best.

IEnumerable

To implement the "add if not exists" method using IEnumerable:

  1. Use Contains on the IEnumerable to check if the item exists in the list.
  2. If Contains is false, use Append on the IEnumerable to add the item.
  3. Use a new variable to store the returned value because Append creates a new collection.
csharp
public static IEnumerable<T> AddIfNotExists<T>(this IEnumerable<T> list, T value)
{
  if (!list.Contains(value))
  {
    return list.Append(value);
  }
  return list;
}

We need to create a new variable because in C#, IEnumerable is an immutable collection. It means that we cannot add, modify, or remove items.

The Append method does not change the elements in the collection, it just makes a copy of the collection with the extra element added.

The benefit of using the IEnumerable interface is that it's more general. The code will work with any type that implements IEnumerable, not just List.

For example, we could use the same code to add an item to a string list:

csharp
string[] words = { "apple", "banana", "carrot" };

IEnumerable<string> newWords = words.AddIfNotExists("celery")
                                    .AddIfNotExists("apple")
                                    .AddIfNotExists("banana");

Console.WriteLine(string.Join(", ", newWords)); // apple, banana, carrot, celery

HashSet

Consider using a HashSet to prevent duplicate values in a collection.

In C#, HashSet cannot have duplicate elements. The implementation takes care of it for you.

So, if you keep calling the Add method with an existing item, nothing will happen:

csharp
HashSet<string> items = new (); 
items.Add("1");
items.Add("1");
items.Add("1");
items.Add("2");
Console.WriteLine(string.Join(", ", items)); // 1, 2

However, unlike List, HashSet doesn't guarantee order of items.

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