Linq, Where() VS Findall()

LINQ, Where() vs FindAll()

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.

FindAll Vs Where

You can find your answer here: LINQ, Where() vs FindAll() . Basically if you call .ToList() on your "Where" they would be the same.

You can find more about the differences between deferred and immediate execution: https://code.msdn.microsoft.com/LINQ-Query-Execution-ce0d3b95

FindAll vs Where extension-method

Well, FindAll copies the matching elements to a new list, whereas Where just returns a lazily evaluated sequence - no copying is required.

I'd therefore expect Where to be slightly faster than FindAll even when the resulting sequence is fully evaluated - and of course the lazy evaluation strategy of Where means that if you only look at (say) the first match, it won't need to check the remainder of the list. (As Matthew points out, there's work in maintaining the state machine for Where. However, this will only have a fixed memory cost - whereas constructing a new list may require multiple array allocations etc.)

Basically, FindAll(predicate) is closer to Where(predicate).ToList() than to just Where(predicate).

Just to react a bit more to Matthew's answer, I don't think he's tested it quite thoroughly enough. His predicate happens to pick half the items. Here's a short but complete program which tests the same list but with three different predicates - one picks no items, one picks all the items, and one picks half of them. In each case I run the test fifty times to get longer timing.

I'm using Count() to make sure that the Where result is fully evaluated. The results show that collecting around half the results, the two are neck and neck. Collecting no results, FindAll wins. Collecting all the results, Where wins. I find this intriguing: all of the solutions become slower as more and more matches are found: FindAll has more copying to do, and Where has to return the matched values instead of just looping within the MoveNext() implementation. However, FindAll gets slower faster than Where does, so loses its early lead. Very interesting.

Results:

FindAll: All: 11994
Where: All: 8176
FindAll: Half: 6887
Where: Half: 6844
FindAll: None: 3253
Where: None: 4891

(Compiled with /o+ /debug- and run from the command line, .NET 3.5.)

Code:

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;

class Test
{
    static List<int> ints = Enumerable.Range(0, 10000000).ToList();

    static void Main(string[] args)
    {
        Benchmark("All", i => i >= 0); // Match all
        Benchmark("Half", i => i % 2 == 0); // Match half
        Benchmark("None", i => i < 0); // Match none
    }

    static void Benchmark(string name, Predicate<int> predicate)
    {
        // We could just use new Func<int, bool>(predicate) but that
        // would create one delegate wrapping another.
        Func<int, bool> func = (Func<int, bool>) 
            Delegate.CreateDelegate(typeof(Func<int, bool>), predicate.Target,
                                    predicate.Method);
        Benchmark("FindAll: " + name, () => ints.FindAll(predicate));
        Benchmark("Where: " + name, () => ints.Where(func).Count());
    }

    static void Benchmark(string name, Action action)
    {
        GC.Collect();
        Stopwatch sw = Stopwatch.StartNew();
        for (int i = 0; i < 50; i++)
        {
            action();
        }
        sw.Stop();
        Console.WriteLine("{0}: {1}", name, sw.ElapsedMilliseconds);
    }
}

C# FindAll VS Where Speed

The FindAll method of the List<T> class actually constructs a new list object, and adds results to it. The Where extension method for IEnumerable<T> will simply iterate over an existing list and yield an enumeration of the matching results without creating or adding anything (other than the enumerator itself.)

Given a small set, the two would likely perform comparably. However, given a larger set, Where should outperform FindAll, as the new List created to contain the results will have to dynamically grow to contain additional results. Memory usage of FindAll will also start to grow exponentially as the number of matching results increases, where as Where should have constant minimal memory usage (in and of itself...excluding whatever you do with the results.)

Linq FindAll and Where returns different results

Where and FindAll are equivalent, except that in terms of execution, Where is deferred, but FindAll is immediate.

Source: This SO thread.

Find() vs. Where().FirstOrDefault()

Where is the Find method on IEnumerable<T>? (Rhetorical question.)

The Where and FirstOrDefault methods are applicable against multiple kinds of sequences, including List<T>, T[], Collection<T>, etc. Any sequence that implements IEnumerable<T> can use these methods. Find is available only for the List<T>. Methods that are generally more applicable, are then more reusable and have a greater impact.

I guess my next question would be why did they add the find at all. That is a good tip. The only thing I can think of is that the FirstOrDefault could return a different default value other than null. Otherwise it just seems like a pointless addition

Find on List<T> predates the other methods. List<T> was added with generics in .NET 2.0, and Find was part of the API for that class. Where and FirstOrDefault were added as extension methods for IEnumerable<T> with Linq, which is a later .NET version. I cannot say with certainty that if Linq existed with the 2.0 release that Find would never have been added, but that is arguably the case for many other features that came in earlier .NET versions that were made obsolete or redundant by later versions.

Using List in FindAll function -C# LinQ

You can use LINQ Contains() extension method like this.

The Linq Contains Method in C# is used to check whether a sequence or collection (i.e. data source) contains a specified element or not. If the data source contains the specified element, then it returns true else return false

List<int> locations= // code to get location id's here

items = ctx.listName.Where(x=>locations.Contains(x.locationId)).ToList();

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