Is the Linq Count() faster or slower than List.Cou

Question

Is the LINQ Count() method any faster or slower than List<>.Count or Array.Length?

Answer 1

Some additional info - LINQ Count - the difference between using it and not can be huge - and this doesn't have to be over 'large' collections either. I have a collection formed from linq to objects with about 6500 items (big.. but not huge by any means) . Count() in my case takes several seconds. Converting to a list (or array, whatver) the count is then virtually immediate. Having this count in an inner loop means the impact could be huge. Count enumerates through everything. An array and a list are both 'self aware' of their lengths and do not need to enumerate them. Any debug statements (log4net for ex) that reference this count() will also then slow everything down considerably more. Do yourself a favor and if you need to reference this often save the count size and only call it once on a LINQ collection unless you convert it to a list and then can reference away without a performance hit.

Here is a quick test of what I was talking about above. Note every time we call Count() our collection size changes.. hence evaluation happens, which is more than an expected 'count' operation. Just something to be aware of : )

    
    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;

    namespace LinqTest
    {
        class TestClass
        {
            public TestClass()
            {
                CreateDate = DateTime.Now;
            }
            public DateTime CreateDate;
        }

        class Program
        {

            static void Main(string[] args)
            {
                //Populate the test class
                List list = new List(1000);
                for (int i=0; i<1000; i++)
                {
                    System.Threading.Thread.Sleep(20);
                    list.Add(new TestClass());
                    if(i%100==0)
                    { 
                        Console.WriteLine(i.ToString() +  " items added");
                    }
                }

                //now query for items 
                var newList = list.Where(o=> o.CreateDate.AddSeconds(5)> DateTime.Now);
                while (newList.Count() > 0)
                {
                    //Note - are actual count keeps decreasing.. showing our 'execute' is running every time we call count.
                    Console.WriteLine(newList.Count());
                    System.Threading.Thread.Sleep(500);
                }
            }
        }
    }