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

51

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

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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
12

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
@JimBalter: PJTraill's comments still is the right answer in certain contexts, when only the "incidental thing" is wrong. At least is the first thing to check even now in 2023. if you do a lot of drafts in LinqPad and DotFiddleNet because they do not automatically import the correct Namespace, and the error msg is far from helpful. – Marcelo Scofano Diniz May 30 '23 at 19:45
My comment was factually correct. – Jim Balter May 31 '23 at 20:36

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 5 · 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

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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

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