Collections in C# are a great example of polymorphism in action. Many collections utilize generics to allow the collection to hold an arbitrary type. For example, the List<T> can be used to hold strings, integers, or even specific objects:

List<string> strings = new List<string>();
List<int> ints = new List<int>();
List<Person> persons = new List<Person>();

We can also use an interface as the type, as we did with the IJumpable interface as we discussed in the generics section, i.e.:

List<IJumpable> jumpables = new List<IJumpable>();
jumpables.Add(new Kangaroo());
jumpables.Add(new Car());
jumpables.Add(new Kangaroo());

Collection Interfaces

The C# language and system libraries also define a number of interfaces that apply to custom collections. Implementing these interfaces allows different kinds of data structures to be utilized in a standardized way.

The IEnumerable<T> Interface

The first of these is the IEnumerable<T> interface, which requires the collection to implement one method:

  • public IEnumerator<T> GetEnumerator()

Implementing this interface allows the collection to be used in a foreach loop.

The ICollection<T> Interface

C# Collections also typically implement the ICollection<T> interface, which extends the IEnumerable<T> interface and adds additional methods:

  • public void Add<T>(T item) adds item to the collection
  • public void Clear() empties the collection
  • public bool Contains(T item) returns true if item is in the collection, false if not.
  • public void CopyTo(T[] array, int arrayIndex) copies the collection contents into array, starting at arrayIndex.
  • public bool Remove(T item) removes item from the collection, returning true if item was removed, false otherwise

Additionally, the collection must implement the following properties:

  • int Count the number of items in the collection
  • bool IsReadOnly the collection is read-only (can’t be added to or removed from)

The IList<T> Interface

Finally, collections that have an implied order and are intended to be accessed by a specific index should probably implement the IList<T> interface, which extends ICollection<T> and IEnumerable<T>. This interface adds these additional methods:

  • public int IndexOf(T item) returns the index of item in the list, or -1 if not found
  • public void Insert(int index, T item) Inserts item into the list at position index
  • public void RemoveAt(int index) Removes the item from the list at position index

The interface also adds the property:

  • Item[int index] which gets or sets the item at index.

Collection Implementation Strategies

When writing a C# collection, there are three general strategies you can follow to ensure you implement the corresponding interfaces:

  1. Write the entire class by scratch
  2. Implement the interface methods as a pass-through to a system library collection
  3. Inherit from a system library collection

Writing collections from scratch was the strategy you utilized in CIS 300 - Data Structures and Algorithms. While this strategy gives you the most control, it is also the most time-consuming.

The pass-through strategy involves creating a system library collection, such as a List<T>, as a private field in your collection class. Then, when you implement the necessary interface methods, you simply pass through the call to the private collection. I.e.:

public class PassThroughList<T> : IList<T>
  private List<T> _list = new List<T>;

  public IEnumerator<T> GetEnumerator() 
    return _list.GetEnumerator();

  // TODO: Implement remaining methods and properties...

Using this approach, you can add whatever additional logic your collection needs into your pass-through methods without needing to re-implement the basic collection functionality.

Using inheritance gives your derived class all of the methods of the base class, so if you extend a class that already implements the collection interfaces, you’ve already got all the methods!

public class InheritedList<T> : List<T>
  // All IList<T>, ICollection<T>, and IEnumerable<T> methods 
  // from List<T> are already defined on InheritedList<T>

However, most system collection class methods are not declared as virtual, so you cannot override them to add custom functionality.