Troubles implementing IEnumerable

Question

I'm trying to write my own (simple) implementation of List. This is what I did so far:

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

namespace provaIEnum
{
    class MyList<T> : IEnumerable<T>
    {
        private T[] _array;
        public int Count { get; private set; }

        public MyList() { /* ... */ }
        public void Add(T element) { /* ... */ }

        // ...

        public IEnumerator<T> GetEnumerator()
        {
            for (int i = 0; i < Count; i++)
                yield return _array[i];
        }
    }

I'm getting an error about GetEnumerator though:

'provaIEnum.Lista' does not implement interface member 'System.Collections.IEnumerable.GetEnumerator()'. 'provaIEnum.Lista.GetEnumerator()' cannot implement 'System.Collections.IEnumerable.GetEnumerator()' because it does not have the matching return type of 'System.Collections.IEnumerator'.

I'm not sure if I understand what VS's trying to tell me and I have no idea how to fix it.

Thanks for your time

Note that you shouldn't write your enumerator like that since currently you can modify your internal array in a `foreach` (calling `Add` in foreach wont cause an exception). [See](http://stackoverflow.com/questions/24142614/when-should-i-separately-implement-ienumeratort) — nawfal, Jul 08 '14 at 16:59

score 49 · Accepted Answer · answered Jan 06 '12 at 15:40

49

Since IEnumerable<T> implements IEnumerable you need to implement this interface as well in your class which has the non-generic version of the GetEnumerator method. To avoid conflicts you could implement it explicitly:

IEnumerator IEnumerable.GetEnumerator()
{
    // call the generic version of the method
    return this.GetEnumerator();
}

public IEnumerator<T> GetEnumerator()
{
    for (int i = 0; i < Count; i++)
        yield return _array[i];
}

answered Jan 06 '12 at 15:40

Darin Dimitrov

994,864
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1

Thank you, I didn't know IEnumerable implements IEnumerable (though it sounds obvious :P) – BlackBear Jan 06 '12 at 15:54
11

Since you have not explicitly qualified `IEnumerator` and `IEnumerable.GetEnumerator` with `System.Collections.`, this will fail if you do not have `using System.Collections`, saying (pretty unhelpfully) that `IEnumerator` needs a generic argument! – PJTraill Apr 04 '16 at 16:42
1

@PJTraill This comment is the right answer (`IEnumerator` needs to be fully qualified to resolve conflicting with its generic variant). – yacc Aug 19 '18 at 19:46
1

^ No, PJTraill's comment is not "the right answer", Darin gave the right answer. That you need the right using statements or else need to fully qualify the interface names is a good point, but is incidental. – Jim Balter Nov 13 '18 at 04:35

score 40 · Answer 2 · answered Jan 06 '12 at 15:40

Read the error message more carefully; it is telling you exactly what you must do. You did not implement System.Collections.IEnumerable.GetEnumerator.

When you implement the generic IEnumerable<T> you have to also implement System.Collections.IEnumerable's GetEnumerator.

The standard way to do so is:

public IEnumerator<T> GetEnumerator () { whatever }
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
    return this.GetEnumerator();
}

Rich O'Kelly · Answer 3 · 2014-07-09T20:33:42.957

11

You'll need to implement the non-generic GetEnumerator method as well:

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

Since the IEnumerable<T> interface extends IEnumerable, you have to implement the methods declared on both.

edited Jul 09 '14 at 20:33

answered Jan 06 '12 at 15:40

Rich O'Kelly

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Explicit interface implementations must not have an access modifier – Nuffin Jan 06 '12 at 15:44

score 4 · Answer 4 · answered Nov 09 '15 at 12:22

I got similar problem,though I implemented also IEnumerable and IEnumerable. I was using custom type (Person class) than generic type T. For me the reason was, namespace was not included in the top of file.

using System.Collections;

After adding the namespace it was fine for me.

score 4 · Answer 5 · answered Jan 06 '12 at 15:41

4

Add the following method:

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

answered Jan 06 '12 at 15:41

Chris Shain

50,081
5
91
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score 2 · Answer 6 · answered Jan 06 '12 at 15:42

There are two overloads of GetEnumerator() that you must implement. One comes from IEnumerable<T> and returns IEnumerator<T>. The other comes from IEnumerable and returns IEnumerator. The override should look like this:

IEnumerator IEnumerable.GetEnumerator()

This is because IEnumerable<T> implements IEnumerable.

score 1 · Answer 7 · answered Jan 06 '12 at 15:50

Since your class, MyList<T> inherits IEnumerable<out T> which in turn inherits the non-generic IEnumerable, you have two methods you need to implement:

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

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

An easy way to do this is when you declare your class:

class MyList<T> : IEnumerable<T>

Right click the IEnumerable<T> text and select Implement Interface > Implement Interface Explictly from the context menu.

Troubles implementing IEnumerable

7 Answers7

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