Why can't I use LINQ methods on my class implementing IEnumerable?

Question

I have to complete a project but I have a simple problem

I have a class defined in this way

using System;
using System.Collections;
using System.Collections.Generic;

..

public class GroupId: XmlDataTransferObject, IEnumerable
{
    private IList _groupId;
    private string _nameId;

    public GroupId() : this("GroupId", "Id")
    {
    }

    public GroupId(string rootName, string nomeId) : base(rootName)
    {
        _groupId = new ArrayList();
        _nomeId = nomeId;
    }

    public override bool IsNull
    {
        get { return _groupId.Count == 0; }
    }

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

    public int Add(Intero i)
    {
        return _groupId.Add(i);
    }

    public Intero this[int index]
    {
        get { return (Intero)_groupId[index]; }
        set { _groupId[index] = value; }
    }
...

            public IEnumerator GetEnumerator()
            {
                    return _groupId.GetEnumerator();
            }
}

}

and I need to find the intersection between an instance of two GroupId objects.

Why can't I see in the available methods the Linq Intersect even if I have declared the statement:

Using System.Linq 

...

var x = _groupId1.Intersect(_groupId2);

...

Error 1 '....GroupId' does not contain a definition for 'Intersect' and no extension method 'Intersect' accepting a first argument of type '...GroupId' could be found (are you missing a using directive or an assembly reference?)

Answer 1

Your GroupId class only implements the non-generic IEnumerable class - you should implement IEnumerable<T> if you want to use the LINQ extension methods. (It'll generally be a better experience anyway.)

Note that that will also be easier if you use a generic IList<T> instead of the non-generic IList - basically try to avoid the non-generic collections entirely in new code, if at all possible.

You could use Cast to convert your IEnumerable to IEnumerable<T> like this:

var x = _groupId1.Cast<Intero>().Intersect(_groupId2.Cast<Intero>());

... but it would be much nicer to just make your class implement IEnumerable<Intero>.

Answer 2

Because your _groupId1 is declared as an IList, which is only an IEnumerable.

You need a generic enumerable (IEnumerable<T>) for nearly all the linq extension methods. Your choice is change the declaration, or use one of the Cast<T> or OfType<T> extensions.

try _groupId1.OfType<Intero>().Intersect(...)

or declaring it as IList<Intero>