Constraint.LinkedList<K,T>' does not implement interface member 'System.Collections.IEnumerable.GetEnumerator()'. 'Constraint.LinkedList<K,T>.GetEnumerator()' cannot implement 'System.Collections.IEnumerable.GetEnumerator()' because it does not have the m

Question

How to correct it? The code is from a sample on MSDN

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections;

namespace Constraint
{
    class Node<K, T>
    {
        public K Key;
        public T Item;
        public Node<K, T> NextNode;
        public Node()
        {
            Key = default(K);
            Item = default(T);
            NextNode = null;
        }
        public Node(K key, T item, Node<K, T> nextNode)
        {
            Key = key;
            Item = item;
            NextNode = nextNode;
        }
    }

 

    public class LinkedList<K, T> : IEnumerable<T> where K : IComparable<K>
    {
        Node<K, T> m_Head;

        public LinkedList()
        {
            m_Head = new Node<K, T>();
        }

        public void AddHead(K key, T item)
        {
            Node<K, T> newNode = new Node<K, T>(key, item, m_Head.NextNode);
            m_Head.NextNode = newNode;
        }

        public T this[K key]
        {
            get
            {
                return Find(key);
            }
        }

        T Find(K key)
        {
            Node<K, T> current = m_Head;
            while (current.NextNode != null)
            {
                if (current.Key.CompareTo(key) == 0)
                {
                    break;
                }
                else
                {
                    current = current.NextNode;
                }
            }
            return current.Item;
        }

        public IEnumerator<T> GetEnumerator()
        {
            Node<K, T> current = m_Head;
            while (current != null)
            {
                yield return current.Item;
                current = current.NextNode;
            }
        }

        public static LinkedList<K, T> operator +(LinkedList<K, T> lhs, LinkedList<K, T> rhs)
        {
            return concatenate(lhs, rhs);
        }

        static LinkedList<K, T> concatenate(LinkedList<K, T> list1, LinkedList<K, T> list2)
        {
            LinkedList<K, T> newList = new LinkedList<K, T>();
            Node<K, T> current;

            current = list1.m_Head;
            while (current != null)
            {
                newList.AddHead(current.Key, current.Item);
                current = current.NextNode;
            }
            current = list2.m_Head;
            while (current != null)
            {
                newList.AddHead(current.Key, current.Item);
                current = current.NextNode;
            }
            return newList;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            LinkedList<int,string>list = new LinkedList<int,string>();
            list.AddHead(123, "AAA");
            list.AddHead(244, "BBB");
            string item = list[123];           
        }
    }
}
      

 

 

Answer 1

How to correct it? The code is from a sample on MSDN

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections;

namespace Constraint
{
    class Node<K, T>
    {
        public K Key;
        public T Item;
        public Node<K, T> NextNode;
        public Node()
        {
            Key = default(K);
            Item = default(T);
            NextNode = null;
        }
        public Node(K key, T item, Node<K, T> nextNode)
        {
            Key = key;
            Item = item;
            NextNode = nextNode;
        }
    }

 

    public class LinkedList<K, T> : IEnumerable<T> where K : IComparable<K>
    {
        Node<K, T> m_Head;

        public LinkedList()
        {
            m_Head = new Node<K, T>();
        }

        public void AddHead(K key, T item)
        {
            Node<K, T> newNode = new Node<K, T>(key, item, m_Head.NextNode);
            m_Head.NextNode = newNode;
        }

        public T this[K key]
        {
            get
            {
                return Find(key);
            }
        }

        T Find(K key)
        {
            Node<K, T> current = m_Head;
            while (current.NextNode != null)
            {
                if (current.Key.CompareTo(key) == 0)
                {
                    break;
                }
                else
                {
                    current = current.NextNode;
                }
            }
            return current.Item;
        }

        public IEnumerator<T> GetEnumerator()
        {
            Node<K, T> current = m_Head;
            while (current != null)
            {
                yield return current.Item;
                current = current.NextNode;
            }
        }

        public static LinkedList<K, T> operator +(LinkedList<K, T> lhs, LinkedList<K, T> rhs)
        {
            return concatenate(lhs, rhs);
        }

        static LinkedList<K, T> concatenate(LinkedList<K, T> list1, LinkedList<K, T> list2)
        {
            LinkedList<K, T> newList = new LinkedList<K, T>();
            Node<K, T> current;

            current = list1.m_Head;
            while (current != null)
            {
                newList.AddHead(current.Key, current.Item);
                current = current.NextNode;
            }
            current = list2.m_Head;
            while (current != null)
            {
                newList.AddHead(current.Key, current.Item);
                current = current.NextNode;
            }
            return newList;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            LinkedList<int,string>list = new LinkedList<int,string>();
            list.AddHead(123, "AAA");
            list.AddHead(244, "BBB");
            string item = list[123];           
        }
    }
}
      

 

 

You forgot to implement the non-generic version of GetEnumerator.

Here is the corrected code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections;

namespace Constraint
{
    class Node<K, T>
    {
        public K Key;
        public T Item;
        public Node<K, T> NextNode;
        public Node()
        {
            Key = default(K);
            Item = default(T);
            NextNode = null;
        }
        public Node(K key, T item, Node<K, T> nextNode)
        {
            Key = key;
            Item = item;
            NextNode = nextNode;
        }
    }



    public class LinkedList<K, T> : IEnumerable<T> where K : IComparable<K>
    {
        Node<K, T> m_Head;

        public LinkedList()
        {
            m_Head = new Node<K, T>();
        }

        public void AddHead(K key, T item)
        {
            Node<K, T> newNode = new Node<K, T>(key, item, m_Head.NextNode);
            m_Head.NextNode = newNode;
        }

        public T this[K key]
        {
            get
            {
                return Find(key);
            }
        }

        T Find(K key)
        {
            Node<K, T> current = m_Head;
            while (current.NextNode != null)
            {
                if (current.Key.CompareTo(key) == 0)
                {
                    break;
                }
                else
                {
                    current = current.NextNode;
                }
            }
            return current.Item;
        }

        public IEnumerator<T> GetEnumerator()
        {
            Node<K, T> current = m_Head;
            while (current != null)
            {
                yield return current.Item;
                current = current.NextNode;
            }
        }

        public static LinkedList<K, T> operator +(LinkedList<K, T> lhs, LinkedList<K, T> rhs)
        {
            return concatenate(lhs, rhs);
        }

        static LinkedList<K, T> concatenate(LinkedList<K, T> list1, LinkedList<K, T> list2)
        {
            LinkedList<K, T> newList = new LinkedList<K, T>();
            Node<K, T> current;

            current = list1.m_Head;
            while (current != null)
            {
                newList.AddHead(current.Key, current.Item);
                current = current.NextNode;
            }
            current = list2.m_Head;
            while (current != null)
            {
                newList.AddHead(current.Key, current.Item);
                current = current.NextNode;
            }
            return newList;
        }

        #region IEnumerable Members

        IEnumerator IEnumerable.GetEnumerator()
        {
            throw new NotImplementedException();
        }

        #endregion
    }

    class Program
    {
        static void Main(string[] args)
        {
            LinkedList<int, string> list = new LinkedList<int, string>();
            list.AddHead(123, "AAA");
            list.AddHead(244, "BBB");
            string item = list[123];
        }
    }
}

If you open your project, right-click the class name "LinkedList" then select "Implement Interface" the IDE will add this empty interface member for you.

Hugh

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Hugh Moran - http://www.morantex.com

Answer 2

Can you esplain the code "#region IEnumerable Members

        IEnumerator IEnumerable.GetEnumerator()
        {
            throw new NotImplementedException();
        }

        #endregion"

I don't know why it is necessary.

 . Thank you

Answer 3

Hi,

The nongeneric IEnumerator is required when implementing generic IEnumerable.

The following is experpted from: http://msdn.microsoft.com/en-us/library/78dfe2yb.aspx

Notes to Implementers:

Implementing this interface requires implementing the nongeneric IEnumerator interface. The MoveNext and Reset methods do not depend on T, and appear only on the nongeneric interface. The Current property appears on both interfaces, and has different return types. Implement the nongeneric IEnumerator..::.Current property as an explicit interface implementation. This allows any consumer of the nongeneric interface to consume the generic interface.

Harry

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