Disposing an IDisposable in Linq

Question

(This is a follow on from a comment on an answer to this question)

18 months after I posted it, someone spotted a bug in one of my Linq examples, where I use an IDisposable half way through a method chain, which never gets disposed.

I attempted to write an extension method to handle this:

public static IEnumerable<R> Using<T, R>(
     this IEnumerable<T> list, Func<T, R> selector) where R : IDisposable
{
    foreach(var item in list)
        using(var disposable = selector(item))
            yield return disposable;
}

var q = Enumerable.Range(0, 10)
        .Using(i => new Disposable(i))
        .Select(d => d.Id);

After seeing Marc's answer to the question I referenced above, I wondered if I could overload the Select extension method directly, but if I do (by renaming Using to Select), the compiler squeals about Select(d => d.Id), because There is no implicit conversion from 'string' to 'System.IDisposable'.

Here's a test class...

public class Disposable : IDisposable
{
    private string _id;
    private bool _disposed = false;
    public Disposable(int id)
    {
        Id = id.ToString();
        Console.WriteLine("Creating " + Id);
    }
    public void Dispose()
    {
        Console.WriteLine("Disposing " + Id);
        _disposed = true;
    }
    public string Id 
    { 
        get 
        {
            if(_disposed) throw new Exception("Call to disposed object!");
            return _id; 
        }
        set { _id = value; }
    }
}

Answer 1

Like so? I haven't really changed much here, except the constructor usage...

    static void Main()
    {
        var q = from d in System.Linq.Enumerable.Range(0, 10)
                select new Disposable(d);
    //  alternatively:
    //  var q = System.Linq.Enumerable.Range(0, 10)
    //          .Select(d => new Disposable(d));

        foreach(var item in q)
        {
            Console.WriteLine("Hi");
        }
    }
    public static IEnumerable<R> Select<T, R>(
        this IEnumerable<T> list, Func<T, R> selector) where R : IDisposable
    {
        foreach (var item in list)
            using (var disposable = selector(item))
                yield return disposable;
    }

However! The thing to watch here is that we don't introduce any conflicting LINQ operations.