How would you implement the IEnumerator interface?

Question

I have a class that map objects to objects, but unlike dictionary it maps them both ways. I am now trying to implement a custom IEnumerator interface that iterates through the values.

public class Mapper<K,T> : IEnumerable<T>, IEnumerator<T>

{
    C5.TreeDictionary<K,T> KToTMap = new TreeDictionary<K,T>();
    C5.HashDictionary<T,K> TToKMap = new HashDictionary<T,K>();

    public void Add(K key, T value)
    {
        KToTMap.Add(key, value);
        TToKMap.Add(value, key);

    }

    public int Count
    {
        get { return KToTMap.Count; }
    }
    

    public K this[T obj]
    {
        get
        {
            return TToKMap[obj];
        }
    }

    public T this[K obj]
    {
        get
        {
            return KToTMap[obj];
        }
    }

    public IEnumerator<T> GetEnumerator()
    {
        return KToTMap.Values.GetEnumerator();
    }

    public T Current
    {
        get { throw new NotImplementedException(); }
    }

    public void Dispose()
    {
        throw new NotImplementedException();
    }

    object System.Collections.IEnumerator.Current
    {
        get { throw new NotImplementedException(); }
    }

    public bool MoveNext()
    {
        ;
    }

    public void Reset()
    {
        throw new NotImplementedException();
    }
}

Answer 1

Just implement the IEnumerable<T> interface. No need to implement the IEnumerator<T> unless you want to do some special things in the enumerator, which for your case doesn't seem to be needed.

public class Mapper<K,T> : IEnumerable<T> {
    public IEnumerator<T> GetEnumerator()
    {
        return KToTMap.Values.GetEnumerator();
    }
}

and that's it.

Answer 2

First, don't make your collection object implement IEnumerator<>. This leads to bugs. (Consider the situation where two threads are iterating over the same collection).

Implementing an enumerator correctly turns out to be non-trivial, so C# 2.0 added special language support for doing it, based on the 'yield return' statement.

Raymond Chen's recent series of blog posts ("The implementation of iterators in C# and its consequences") is a good place to get up to speed.

• Part 1: https://web.archive.org/web/20081216071723/http://blogs.msdn.com/oldnewthing/archive/2008/08/12/8849519.aspx
• Part 2: https://web.archive.org/web/20080907004812/http://blogs.msdn.com/oldnewthing/archive/2008/08/13/8854601.aspx
• Part 3: https://web.archive.org/web/20080824210655/http://blogs.msdn.com/oldnewthing/archive/2008/08/14/8862242.aspx
• Part 4: https://web.archive.org/web/20090207130506/http://blogs.msdn.com/oldnewthing/archive/2008/08/15/8868267.aspx

Answer 3

Here's an example from the book "Algorithms (4th Edition) by Robert Sedgewick".

It was written in java and i basically rewrote it in C#.

public class Stack<T> : IEnumerable<T>
{
    private T[] array;

    public Stack(int n)
    {
        array = new T[n];
    }

    public Stack()
    {
        array = new T[16];
    }

    public void Push(T item)
    {
        if (Count == array.Length)
        {
            Grow(array.Length * 2);
        }

        array[Count++] = item;
    }

    public T Pop()
    {
        if (Count == array.Length/4)
        {
            Shrink(array.Length/2);
        }

        return array[--Count];
    }

    private void Grow(int size)
    {
        var temp = array;
        array = new T[size];
        Array.Copy(temp, array, temp.Length);
    }

    private void Shrink(int size)
    {
        Array temp = array;
        array = new T[size];
        Array.Copy(temp,0,array,0,size);
    }

    public int Count { get; private set; }
    public IEnumerator<T> GetEnumerator()
    {
        return new ReverseArrayIterator(Count,array);
    }

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


    // IEnumerator implementation
    private class ReverseArrayIterator : IEnumerator<T>
    {
        private int i;

        private readonly T[] array;

        public ReverseArrayIterator(int count,T[] array)
        {
            i = count;
            this.array = array;
        }

        public void Dispose()
        {

        }

        public bool MoveNext()
        {
            return i > 0;
        }

        public void Reset()
        {

        }

        public T Current { get { return array[--i]; } }

        object IEnumerator.Current
        {
            get { return Current; }
        }
    }
}

Answer 4

CreateEnumerable() returns an IEnumerable which implements GetEnumerator()

public class EasyEnumerable : IEnumerable<int> {

    IEnumerable<int> CreateEnumerable() {
        yield return 123;
        yield return 456;
        for (int i = 0; i < 6; i++) {
            yield return i;
        }//for
    }//method

    public IEnumerator<int> GetEnumerator() {
        return CreateEnumerable().GetEnumerator();
    }//method

    IEnumerator IEnumerable.GetEnumerator() {
        return CreateEnumerable().GetEnumerator();
    }//method

}//class

Answer 5

Use yield return.

What is the yield keyword used for in C#?