LINQ performance Count vs Where and Count

Question

public class Group
{
   public string Name { get; set; }
}  

Test:

List _groups = new List();

for (i         


        

Answer 1

It looks to me that the difference is in how the Linq extensions are coded. I suspect Where is using optimizations in the List<> class to speed up the operations, but Count just iterates through an IEnumerable<>.

If you do the same process, but with an IEnumerable, both methods are close, with Where being slightly slower.

List _groups = new List();

for (int i = 0; i < 10000; i++)
{
    var group = new Group();

    group.Name = i + "asdasdasd";
    _groups.Add(group);
}

IEnumerable _groupsEnumerable = from g in _groups select g;

Stopwatch _stopwatch2 = new Stopwatch();

_stopwatch2.Start();
foreach (var group in _groups)
{
    var count = _groupsEnumerable.Count(x => x.Name == group.Name);
}
_stopwatch2.Stop();

Console.WriteLine(_stopwatch2.ElapsedMilliseconds);
Stopwatch _stopwatch = new Stopwatch();

_stopwatch.Start();
foreach (var group in _groups)
{
    var count = _groupsEnumerable.Where(x => x.Name == group.Name).Count();
}
_stopwatch.Stop();

Console.WriteLine(_stopwatch.ElapsedMilliseconds);

Where extension method. Notice the if (source is List) case:

public static IEnumerable Where(this IEnumerable source, Func predicate)
{
    if (source == null)
    {
        throw Error.ArgumentNull("source");
    }
    if (predicate == null)
    {
        throw Error.ArgumentNull("predicate");
    }
    if (source is Enumerable.Iterator)
    {
        return ((Enumerable.Iterator)source).Where(predicate);
    }
    if (source is TSource[])
    {
        return new Enumerable.WhereArrayIterator((TSource[])source, predicate);
    }
    if (source is List)
    {
        return new Enumerable.WhereListIterator((List)source, predicate);
    }
    return new Enumerable.WhereEnumerableIterator(source, predicate);
}

Count method. Just iterates through the IEnumerable:

public static int Count(this IEnumerable source, Func predicate)
{
    if (source == null)
    {
        throw Error.ArgumentNull("source");
    }
    if (predicate == null)
    {
        throw Error.ArgumentNull("predicate");
    }
    int num = 0;
    checked
    {
        foreach (TSource current in source)
        {
            if (predicate(current))
            {
                num++;
            }
        }
        return num;
    }
}