LINQ, Where() vs FindAll()

Can someone explain how the开发者_Python百科 LINQ functions Where(..) and FindAll(..) differ? They both seem to do the same thing...

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FindAll() is a function on the List<T> type, it's not a LINQ extension method like Where. The LINQ extension methods work on any type that implements IEnumerable, whereas FindAll can only be used on List<T> instances (or instances of classes that inherit from it, of course).

Additionally, they differ in actual purpose. Where returns an instance of IEnumerable that is executed on-demand when the object is enumerated. FindAll returns a new List<T> that contains the requested elements. FindAll is more like calling Where(...).ToList() on an instance of IEnumerable.

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The biggest difference to me is that .FindAll is also available in .Net 2.0. I don't always have the luxury to program in .Net 3.5, so I try to remember the 'native' methods of the .Net generic collections.

It happened several times that I implemented an already available List method myself because I couldn't LINQ it.

What I find handy in this case is that, using VS2008, I can use type inference and the lambda syntax. These are compiler features, not framework features. This means I can write this and still remain within .Net 2.0:

var myOddNums = myNums.FindAll(n => n%2==1);

But if you do have LINQ available, keeping the difference between deferred execution and immediate execution is important.

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If I recall correctly, the main difference (besides what they're implemented on: IEnumerable<T> vs. List<T>) is that Where implements deferred execution, where it doesn't actually do the lookup until you need it -- using it in a foreach loop for example. FindAll is an immediate execution method.

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I did some tests on a list of 80K objects and found that Find() can be up to 1000% faster than using a Where with FirstOrDefault(). I didn't know that until testing a timer before and after each call. Sometimes it was the same time, other times it was faster.

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Performance wise FindAll() is better, Here is an example below. The FindAll takes 3millisecs while the Where took 11millisecs.

    public class SortedSearch
    {
        public static int[] CountNumbersUsingFindAll(int[] sortedArray, int lessThan)
        {
            var smaller = Array.FindAll(sortedArray,x => x < lessThan);
            return smaller;
        }
        public static IEnumerable<int> CountNumbersUsingWhere(int[] sortedArray,int lessThan)
        {
            var smaller = sortedArray.Where(x => x < lessThan);
            return smaller;
        }
   }

class Program
{
        static void Main(string[] args)
        {
            Stopwatch s = Stopwatch.StartNew();
            Console.WriteLine(SortedSearch.CountNumbersUsingFindAll(new int[]{1,3,5,7},4));
            Console.WriteLine(s.ElapsedMilliseconds);
            s.Stop();

            s.Restart();
            Console.WriteLine(SortedSearch.CountNumbersUsingWhere(new int[] { 1, 3, 5, 7 }, 4));
            Console.WriteLine(s.ElapsedMilliseconds);
            s.Stop();
            Console.ReadKey();
        }
}