Custom Collection Implementing IEnumerable

Question

I know that technically, an Interface is used for reading and not writting or editing however, I want to add an add and addrange function to the following class, here is what I currently have which is not working

public class HrefCollection : IEnumerable<Href> 
{
    private IEnumerable<Href> hrefs;

    public IEnumerable<Href> Add( Href href )
    {
        yield return href;
    }

    public IEnumerable<Href> AddRange( List<Href> hrefs )
    {
        foreach( Href href in hrefs )
        {
            yield return href;
        }
    }

    public IEnumerator<Href> GetEnumerator()
    {
        return hrefs.GetEnumerator();
    }

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

I'm not quite sure how to associate the yield return with the private list.

Thanks for your help!

You may be a little confused here. Is there a reason for you not to simply use a List? — Eric Mickelsen, May 21 '10 at 14:41
"an Interface is used for reading and not writting or editing" ?? I don't understand that statement. You mean IEnumerable? — brickner, May 21 '10 at 14:43
I kinda misexplained, I mean that The Ienumerable interface does not contain an "add" method because it's used for returning objects from the current collection not for adding or removing something from it. — Pierluc SS, May 21 '10 at 14:50
I dont want to use List (even though I could implement IList), I want to use yield return and kinda understand how it works and how i can use it. — Pierluc SS, May 21 '10 at 14:52

score 7 · Accepted Answer · answered May 21 '10 at 15:04

The IEnumerable<T> and IEnumerable interfaces are used to generate a read-only sequence or provide a read-only view of the items in a collection.

If you want to be able to add items to your collection then, internally, you'll need to use a data structure that allows items to be added -- for example List<T>. You simply can't add items using the IEnumerable<T> or IEnumerable interfaces.

public class HrefCollection : IEnumerable<Href>
{
    private readonly List<Href> _hrefs = new List<Href>();

    public void Add(Href href)
    {
        _hrefs.Add(href);
    }

    public void AddRange(IEnumerable<Href> hrefs)
    {
        _hrefs.AddRange(hrefs);
    }

    public IEnumerator<Href> GetEnumerator()
    {
        return _hrefs.GetEnumerator();
    }

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

This works, but it's still pretty silly and it doesn't serve the purpose of learning how yield works. — Eric Mickelsen, May 21 '10 at 15:14
@tehMick: True, but the question says "I want to add an add and addrange function to the following class". I know that the question also mentions using `yield return`, but the answer is that you just can't use `yield return` to add items to a collection. — LukeH, May 21 '10 at 15:55

score 1 · Answer 2 · edited May 21 '10 at 15:06

1

foreach( Href href in hrefs )
{
    yield return href;
}

should be

foreach( Href href in this.hrefs )
{
    yield return href;
}
foreach( Href href in hrefs )
{
    yield return href;
}

edited May 21 '10 at 15:06

abatishchev

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answered May 21 '10 at 14:42

Eric Mickelsen

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@Burnzy: could you be more specific? – Eric Mickelsen May 21 '10 at 14:54
well if I call addrange, the Collection remains null. – Pierluc SS May 21 '10 at 14:56
Actually, you should get rid of the private IEnumerable hrefs; . You're declaring one IEnumerable inside another. Instead, you should be referencing this. – Eric Mickelsen May 21 '10 at 14:56
1

@Burzy, but using yield will never add to the collection you have, only output a new collection. – Eric Mickelsen May 21 '10 at 14:58

score 0 · Answer 3 · answered Mar 08 '12 at 09:58

0

public class HrefCollection<T> : ObservableCollection<T>, IEnumerable<T>  {


// access via this[index]
}

answered Mar 08 '12 at 09:58

Pascal

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Custom Collection Implementing IEnumerable

3 Answers3