C#•3w ago

Purpose and use of nested classes in C#

Hello guys, sorry to disturb you all; can someone explain the idea of using nested classes in C# please. For example, consider the following code:

C#
// Type parameter T in angle brackets.
public class GenericList<T>
{
    // The nested class is also generic, and
    // holds a data item of type T.
    private class Node(T t)
    {
        // T as property type.
        public T Data { get; set; } = t;

        public Node? Next { get; set; }
    }

    // First item in the linked list
    private Node? head;

    // T as parameter type.
    public void AddHead(T t)
    {
        Node n = new(t);
        n.Next = head;
        head = n;
    }

    // T in method return type.
    public IEnumerator<T> GetEnumerator()
    {
        Node? current = head;

        while (current is not null)
        {
            yield return current.Data;
            current = current.Next;
        }
    }
}

C#
// Type parameter T in angle brackets.
public class GenericList<T>
{
    // The nested class is also generic, and
    // holds a data item of type T.
    private class Node(T t)
    {
        // T as property type.
        public T Data { get; set; } = t;

        public Node? Next { get; set; }
    }

    // First item in the linked list
    private Node? head;

    // T as parameter type.
    public void AddHead(T t)
    {
        Node n = new(t);
        n.Next = head;
        head = n;
    }

    // T in method return type.
    public IEnumerator<T> GetEnumerator()
    {
        Node? current = head;

        while (current is not null)
        {
            yield return current.Data;
            current = current.Next;
        }
    }
}

I want to implement a linkedList by scratch in C#. Why not just use a class of "Node" or "LinkedList" ? why we should have both, what if we don't have them?

35 Replies

FakerOP•3w ago

wrong code sorry

Pobiega•3w ago

There are no nested classes in your example.

FakerOP•3w ago

2sec yeah sorry 2sec here

Pobiega•3w ago

Do you know what a linked list is and how it works?

FakerOP•3w ago

yep it doesn't need "contiguous" memory like an array

Pobiega•3w ago

Then you know the answer :p

FakerOP•3w ago

hmm

Pobiega•3w ago

as to specifically nested classes, we nest Node here because it doesnt make sense outside the context of the list

FakerOP•3w ago

from my understanding, a linked list is a connection of nodes, that is we are modelling the collection as an object (kind of an "Array" of nodes), then the nodes is what contains the data/pointers

Pobiega•3w ago

notice how the public API of the list doesnt expose Node sort of

FakerOP•3w ago

yeah, why is it important to make it private here

Pobiega•3w ago

because it can be Node serves no purpose on the outside so we nest it and hide it with private

FakerOP•3w ago

yeah I see but the thing is hmm, couldn't we omit the GenericList class? like only have the node class ?

Pobiega•3w ago

FakerOP•3w ago

hmm why it would break things?

Pobiega•3w ago

... are you trolling?

FakerOP•3w ago

😭 no :c

Pobiega•3w ago

Where did you get the above code from?

FakerOP•3w ago

it was from microsoft doc but I don't remember exactly on which article

Pobiega•3w ago

Thats essentially the "standard" linked list implementation

FakerOP•3w ago

it's not written by me

Pobiega•3w ago

Ok so lets pretend we only keep Node

    public class Node(T t)
    {
        // T as property type.
        public T Data { get; set; } = t;

        public Node? Next { get; set; }
    }

    public class Node(T t)
    {
        // T as property type.
        public T Data { get; set; } = t;

        public Node? Next { get; set; }
    }

this is clearly not a list

FakerOP•3w ago

yep it's just an object yeah like person

Pobiega•3w ago

not like person but yeah, its just an object it doesnt have an Add method. It cant be iterated over. its just a class. what makes a list a list?

FakerOP•3w ago

ahh didn't see that yeah, it must have several nodes that can be traversed

Pobiega•3w ago

it must be able to have multiple nodes, yes a list can be empty, or have a single element too

FakerOP•3w ago

yep will just refresh a bit just give me 2min, try to make sense of all that

Thinker•3w ago

You know how namespaces work, right?

FakerOP•3w ago

Hmm I know it organises our projects I think or I'm interchanging it with solution 2sec yeah I think both are use to organise things

Thinker•3w ago

Yeah Again you can think of namespaces are 'folders' you put classes into, purely for organizational purposes Nested classes are pretty much the same. When we nest a class inside another class, it's usually because (as Pobiega said previously) we want to group that nested class together with the other class. So for GenericList here (which again is a linked list), we put Node inside of it because (again, as previously said) it doesn't make sense to use outside of GenericList.

Pobiega•3w ago

yeah, Node existing is just an implementation detail so we hide it, like we would any other "helper"

FakerOP•3w ago

yeah I see, the thing is we could have done something like this:

C#
public class Node<T>
{
    public T Data { get; set; }
    public Node<T>? Next { get; set; }

    public Node(T data)
    {
        Data = data;
    }

    // Adding a node after the current node
    public void AddAfter(T newData)
    {
        Node<T> newNode = new Node<T>(newData);
        newNode.Next = this.Next;
        this.Next = newNode;
    }
    
    // Display the list starting from this node
    public void Display()
    {
        Node<T>? current = this;
        while (current != null)
        {
            Console.WriteLine(current.Data);
            current = current.Next;
        }
    }
}

C#
public class Node<T>
{
    public T Data { get; set; }
    public Node<T>? Next { get; set; }

    public Node(T data)
    {
        Data = data;
    }

    // Adding a node after the current node
    public void AddAfter(T newData)
    {
        Node<T> newNode = new Node<T>(newData);
        newNode.Next = this.Next;
        this.Next = newNode;
    }
    
    // Display the list starting from this node
    public void Display()
    {
        Node<T>? current = this;
        while (current != null)
        {
            Console.WriteLine(current.Data);
            current = current.Next;
        }
    }
}

we just omit the outer class, create a new class Node and implements everything here it would be bad practice ?

Pobiega•3w ago

sure you could do that, but its... weird like, if you keep a reference to the first node, you are continously adding to the second place of the link and the "list" itself has data, since its all a node like, sure you CAN do this, but I see no reason why you would

FakerOP•3w ago

Yeah I see like we divide the idea into 2 entities; node implies holding data and reference, while GenericList to handle the linkedlist, like adding new node etc... just read about Single Responsibility Princile (SRP)... the thing is we should always try to implement a class that adheres to a specific function ? Like in this case, GenericList would have the task of managing the list, while the class of Node would have the task of defining the data contained into the node ?

glhays•3w ago

And then came partial classes, where we separate nested classes elegantly🤔 . Oh sorry different topic.

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Purpose and use of nested classes in C#

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