Using linked lists in .NET with C#

Linked lists are very efficient if you want to insert and remove items in a collection frequently. Linked lists do not store their elements in contiguous regions in memory, like e.g. the List of T object. Instead elements in linked lists can be stored anywhere.

The elements are linked by their “next” and “previous” references. The first element will have a “next” pointer to the element that comes after that. The second element in turn will have a reference to the previous and next elements. The first element will of course only have a reference to the “next” element and “previous” will be null, whereas the last element will point to the previous element and “next” will be null. All elements in between will have two references. Such a linked list is also called a doubly linked list.

One big difference between linked lists and “normal” lists is that linked lists have no index accessors. You just cannot extract the element #3 for example without looping through the linked list and stopping at the third element.

Linked lists are represented by the generic LinkedList of T class in .NET. The elements are stored within objects of type LinkedListNode of T. Each node will keep track of its “next” and “previous” references. LinkedList provides 4 Add methods: AddFirst, AddLast, AddAfter and AddBefore.

AddFirst and AddLast will, as expected, add the element to the first and last position of the linked list. They also return a LinkedListNode object which come in handy when using the AddAfter and AddBefore methods. Those two require a LinkedListNode parameter that indicates the reference point of “before” and “after”.

Here’s an example of AddFirst and AddLast:

LinkedList<Guest> guestList = new LinkedList<Guest>();
LinkedListNode<Guest> john25 = guestList.AddFirst(new Guest() { Age = 25, Name = "John" });
LinkedListNode<Guest> barbara24 = guestList.AddFirst(new Guest() { Age = 24, Name = "Barbara" });
LinkedListNode<Guest> phil24 = guestList.AddFirst(new Guest() { Age = 24, Name = "Phil" });
LinkedListNode<Guest> fred23 = guestList.AddLast(new Guest() { Age = 23, Name = "Fred" });
LinkedListNode<Guest> hannah26 = guestList.AddFirst(new Guest() { Age = 26, Name = "Hannah" });
LinkedListNode<Guest> cindy27 = guestList.AddLast(new Guest() { Age = 27, Name = "Cindy" });

foreach (Guest guest in guestList)
{
	Debug.WriteLine("{0}, {1}", guest.Name, guest.Age);
}

…where Guest look as follows:

public class Guest
{
	public string Name { get; set; }
	public int Age { get; set; }
}

The code prints…

Hannah, 26
Phil, 24
Barbara, 24
John, 25
Fred, 23
Cindy, 27

Let’s see how we can insert other elements before and after:

LinkedList<Guest> guestList = new LinkedList<Guest>();
LinkedListNode<Guest> john25 = guestList.AddFirst(new Guest() { Age = 25, Name = "John" });
LinkedListNode<Guest> barbara24 = guestList.AddFirst(new Guest() { Age = 24, Name = "Barbara" });
LinkedListNode<Guest> phil24 = guestList.AddFirst(new Guest() { Age = 24, Name = "Phil" });
LinkedListNode<Guest> fred23 = guestList.AddLast(new Guest() { Age = 23, Name = "Fred" });
LinkedListNode<Guest> hannah26 = guestList.AddFirst(new Guest() { Age = 26, Name = "Hannah" });
LinkedListNode<Guest> cindy27 = guestList.AddLast(new Guest() { Age = 27, Name = "Cindy" });

LinkedListNode<Guest> ron21 = guestList.AddAfter(hannah26, new Guest() { Age = 21, Name = "Ron" });
LinkedListNode<Guest> mary26 = guestList.AddBefore(barbara24, new Guest() { Age = 26, Name = "Mary" });

foreach (Guest guest in guestList)
{
	Debug.WriteLine("{0}, {1}", guest.Name, guest.Age);
}

Here’s the printout:

Hannah, 26
Ron, 21
Phil, 24
Mary, 26
Barbara, 24
John, 25
Fred, 23
Cindy, 27

If you don’t have these references then you can use the Find method. Keep in mind that the linked list will need to search through the elements until it fins the first matching one. If you have several identical items in the list then Find will return the first one only.

By the way the result for Find will be based on equality. As Guest is a reference type it will need to implement the IEquatable interface so that the Find will know how to equate two Guest objects. Here’s an example:

public class Guest : IEquatable<Guest>
{
	public string Name { get; set; }
	public int Age { get; set; }

	public bool Equals(Guest other)
	{
		return Name.Equals(other.Name);
	}
}

The Find method will then correctly locate john25 by simply providing a Guest object with name = John:

LinkedListNode<Guest> find = guestList.Find(new Guest() { Name = "John" });

View all various C# language feature related posts here.