Question

Possible Duplicate:
Implementing C# IEnumerable<T> for a LinkedList class

After searching the web for some hours now I still can’t understand how IEnumerable/IEnumerator works and how to implement it.

I’ve constructed a simple LinkedList from scratch but now I want to implement IEnumerable for it so I can foreach it. How do I do that?

class Program
{
    LL myList = new LL();

    static void Main()
    {
        var gogo = new Program();
    }
    public Program()
    {

        myList.Add("test");
        myList.Add("test1");

        foreach (var item in myList) //This doesn't work because I havn't implemented Ienumerable
            Console.WriteLine(item);

        Console.Read();
    }
}


class LL
{

    private LLNode first;

    public void Add(string s)
    {
        if (this.first == null)
            this.first = new LLNode() { Value = s };
        else
        {
            var node = this.first;
            while (node.Next != null)
                node = node.Next;

            node.Next = new LLNode() { Value = s };
        }
    }


class LLNode
{
    public string Value { get; set; }
    public LLNode Next { get; set; }
}

Answer 1

What you need to do is:

(1) Make your class implement IEnumerable<T> where T is the type of the enumerated items. (In your case, it looks like it would be LLNode).

(2) Write a public IEnumerator<T> GetEnumerator. Implement it using the “yield” keyword.

(3) Add a IEnumerator IEnumerable.GetEnumerator() method and just return GetEnumerator().

The following code should make this clear. Where I have <int>, you should put <LLNode>, assuming that is the correct type.

using System;
using System.Collections;
using System.Collections.Generic;

namespace Demo
{
    internal class Program
    {
        private static void Main()
        {
            var test = new MyDemo();

            foreach (int item in test)
            {
                Console.WriteLine(item);
            }
        }
    }

    public class MyDemo: IEnumerable<int>
    {
        public IEnumerator<int> GetEnumerator()
        {
            // Your implementation of this method will iterate over your nodes
            // and use "yield return" to return each one in turn.

            for (int i = 10; i <= 20; ++i)
            {
                yield return i;
            }
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
    }
}

I would have modified your code to do it properly, but the code you posted won’t compile.

[EDIT]

Now you’ve updated your code, I can see that you want to enumerate the values. Here’s the completed code:

using System;
using System.Collections;
using System.Collections.Generic;

namespace Demo
{
    internal class Program
    {
        private LL myList = new LL();

        private static void Main()
        {
            var gogo = new Program();
        }

        public Program()
        {
            myList.Add("test");
            myList.Add("test1");

            foreach (var item in myList) // This now works.
                Console.WriteLine(item);

            Console.Read();
        }
    }


    internal class LL: IEnumerable<string>
    {
        private LLNode first;

        public void Add(string s)
        {
            if (this.first == null)
                this.first = new LLNode
                {
                    Value = s
                };
            else
            {
                var node = this.first;
                while (node.Next != null)
                    node = node.Next;

                node.Next = new LLNode
                {
                    Value = s
                };
            }
        }

        public IEnumerator<string> GetEnumerator()
        {
            for (var node = first; node != null; node = node.Next)
            {
                yield return node.Value;
            }
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }

        private class LLNode
        {
            public string Value { get; set; }
            public LLNode Next { get; set; }
        }
    }
}