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Count the items from a IEnumerable<T> without iterating?

Asked 2008-10-03T21:05:03.580
376
private IEnumerable<string> Tables
{
    get
    {
        yield return "Foo";
        yield return "Bar";
    }
}

Let's say I want iterate on those and write something like processing #n of #m.

Is there a way I can find out the value of m without iterating before my main iteration?

I hope I made myself clear.

c# .net ienumerable
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2 Answers

264

The System.Linq.Enumerable.Count extension method on IEnumerable<T> has the following implementation:

ICollection<T> c = source as ICollection<TSource>;
if (c != null)
    return c.Count;

int result = 0;
using (IEnumerator<T> enumerator = source.GetEnumerator())
{
    while (enumerator.MoveNext())
        result++;
}
return result;

So it tries to cast to ICollection<T>, which has a Count property, and uses that if possible. Otherwise it iterates.

So your best bet is to use the Count() extension method on your IEnumerable<T> object, as you will get the best performance possible that way.

answered 2009-05-12T15:48:20.857
5

Going beyond your immediate question (which has been thoroughly answered in the negative), if you're looking to report progress whilst processing an enumerable, you might want to look at my blog post Reporting Progress During Linq Queries.

It lets you do this:

BackgroundWorker worker = new BackgroundWorker();
worker.WorkerReportsProgress = true;
worker.DoWork += (sender, e) =>
      {
          // pretend we have a collection of 
          // items to process
          var items = 1.To(1000);
          items
              .WithProgressReporting(progress => worker.ReportProgress(progress))
              .ForEach(item => Thread.Sleep(10)); // simulate some real work
      };
answered 2009-05-12T15:56:18.550

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