Count the items from a IEnumerable without iterating?


Question

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.

1
298
2/22/2018 5:23:10 PM

Accepted Answer

IEnumerable doesn't support this. This is by design. IEnumerable uses lazy evaluation to get the elements you ask for just before you need them.

If you want to know the number of items without iterating over them you can use ICollection<T>, it has a Count property.

314
3/3/2017 3:02:20 AM

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.


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