Error message regarding IEnumerable.GetEnumerator().

Question

I get this error message and I can't figure out why!

Error   1   'Exo5Chap12.ShortCollection<T>' does not implement interface member 
'System.Collections.IEnumerable.GetEnumerator()'. 
'Exo5Chap12.ShortCollection<T>.GetEnumerator()' cannot implement 
'System.Collections.IEnumerable.GetEnumerator()' because it does not have the matching 
return type of 'System.Collections.IEnumerator'.    
E:\MyFolders\Dev\c#\Chapter12\Exo5Chap12\Exo5Chap12\exo5.cs 9   18  Exo5Chap12

Here is the code with an implementation of GetEnumerator(). What is wrong?

 public class ShortCollection<T> : IList<T>
{
    protected Collection<T> innerCollection;
    protected int maxSize = 10;
    public IEnumerator<T> GetEnumerator()
    {
        return (innerCollection as IEnumerator<T>).GetEnumerator();
    }
}

Answer 1

IEnumerable and IEnumerable<T> are different interfaces and your code has to implement both. Your code only implements the GetEnumerator() of IEnumerable<T>, but not GetEnumerator() of IEnumerable. You should consider installing ReSharper which makes it easy to fix errors like this one.

Answer 2

You need to implement a few more things:

public class ShortCollection<T> : IList<T>
{
    protected Collection<T> innerCollection;
    protected int maxSize = 10;

    #region IList<T> Members

    public int IndexOf(T item)
    {
        return innerCollection.IndexOf(item);
    }

    public void Insert(int index, T item)
    {
        innerCollection.Insert(index, item);
    }

    public void RemoveAt(int index)
    {
        innerCollection.RemoveAt(index);
    }

    public T this[int index]
    {
        get
        {
            return innerCollection[index];
        }
        set
        {
            innerCollection[index] = value;
        }
    }

    #endregion

    #region ICollection<T> Members

    public void Add(T item)
    {
        innerCollection.Add(item);
    }

    public void Clear()
    {
        innerCollection.Clear();
    }

    public bool Contains(T item)
    {
        return innerCollection.Contains(item);
    }

    public void CopyTo(T[] array, int arrayIndex)
    {
        innerCollection.CopyTo(array, arrayIndex);
    }

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

    public bool IsReadOnly
    {
        get { return false; }
    }

    public bool Remove(T item)
    {
        return innerCollection.Remove(item);
    }

    #endregion

    #region IEnumerable<T> Members

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

    #endregion

    #region IEnumerable Members

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        return innerCollection.GetEnumerator();
    }

    #endregion
}

This code will compile... :)

Answer 3

As others have said, you need to implement IEnumerable as well as IEnumerable<T>. However, since IEnumberable<T> itself implemets IEnumerable this is trivial, just call your generic GetEnumerator():

public class ShortCollection<T> : IList<T>
{
    protected Collection<T> innerCollection;
    protected int maxSize = 10;
    public IEnumerator<T> GetEnumerator()
    {
        return innerCollection.GetEnumerator();
    }

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

I'm assuming that you have methods for actually adding and removing from the innerCollection and just left them out for brevity since they didn't relate to the question at hand.