The best way to get a count of IEnumerable<T>

In any case, you have to loop through it. Linq offers the Count method:

var result = myenum.Count();

The solution depends on why you don't want to enumerate through the collection.

If it's because enumerating the collection might be slow, then there is no solution that will be faster. You might want to consider using an ICollection instead if possible. Unless the enumeration is remarkably slow (e.g. it reads items from disk) speed shouldn't be a problem though.

If it's because enumerating the collection will require more code then it's already been written for you in the form of the .Count() extension method. Just use MyEnumerable.Count().

If it's because you want to be able to enumerate the collection after you've counted then the .Count() extension method allows for this. You can even call .Count() on a collection you're in the middle of enumerating and it will carry on from where it was before the count. For example:

foreach (int item in Series.Generate(5))
{
    Console.WriteLine(item + "(" + myEnumerable.Count() + ")");
}

will give the results

0 (5)

1 (5)

2 (5)

3 (5)

4 (5)

If it's because the enumeration has side effects (e.g. writes to disk/console) or is dependant on variables that may change between counting and enumerating (e.g. reads from disk) [N.B. If possible, I would suggest rethinking the architecture as this can cause a lot of problems] then one possibility to consider is reading the enumeration into an intermittent storage. For example:

List<int> seriesAsList = Series.Generate(5).ToList();

All of the above assume you can't change the type (i.e. it is returned from a library that you do not own). If possible you might want to consider changing to use an ICollection or IList (ICollection being more widely scoped than IList) which has a Count property on it.

You will have to enumerate to get a count. Other constructs like the List keep a running count.