Parallel.ForEach with adding to list

Question

I'm trying to run multiple functions that connect to a remote site (by network) and return a generic list. But I want to run them simultaneously.

For example:

public static List<SearchResult> Search(string title)
{
    //Initialize a new temp list to hold all search results
    List<SearchResult> results = new List<SearchResult>();

    //Loop all providers simultaneously
    Parallel.ForEach(Providers, currentProvider =>
    {
        List<SearchResult> tmpResults = currentProvider.SearchTitle((title));

        //Add results from current provider
        results.AddRange(tmpResults);
    });

    //Return all combined results
    return results;
}

As I see it, multiple insertions to 'results' may happend at the same time... Which may crash my application.

How can I avoid this?

It would have to be at least .Net 4; Parallel was introduced there. — Matt Mills, Nov 03 '11 at 20:58

Mark Byers · Answer 1 · 2011-11-03T21:16:36.280

170

You can use a concurrent collection.

The System.Collections.Concurrent namespace provides several thread-safe collection classes that should be used in place of the corresponding types in the System.Collections and System.Collections.Generic namespaces whenever multiple threads are accessing the collection concurrently.

You could for example use ConcurrentBag since you have no guarantee which order the items will be added.

Represents a thread-safe, unordered collection of objects.

edited Nov 03 '11 at 21:16

answered Nov 03 '11 at 20:58

Mark Byers

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Yes, this is the actual answer. You'll get better performance (generally) with concurrent collections. – lkg May 24 '17 at 16:54
1

A [`ConcurrentQueue`](https://learn.microsoft.com/en-us/dotnet/api/system.collections.concurrent.concurrentqueue-1) might be preferable to a `ConcurrentBag`, because the former preserves the insertion order. The `ConcurrentBag` is an [extremely specialized](https://stackoverflow.com/questions/15400133/when-to-use-blockingcollection-and-when-concurrentbag-instead-of-listt/64823123#64823123) collection intended for mixed producer consumer scenarios, and adding items in parallel (without taking items from the collection) is not such a scenario. – Theodor Zoulias Mar 28 '22 at 20:40

Haedrian · Accepted Answer · 2013-12-03T21:13:51.507

70

//In the class scope:
Object lockMe = new Object();    

//In the function
lock (lockMe)
{    
     results.AddRange(tmpResults);
}

Basically a lock means that only one thread can have access to that critical section at the same time.

edited Dec 03 '13 at 21:13

answered Nov 03 '11 at 20:57

Haedrian

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1

But what will happend if WHILE those results are being added the results from another provider try to add to? will they FAIL or WAIT until possible? – shaharmor Nov 03 '11 at 20:58
4

When there's a lock, the thread will wait until it can get the lock. – Haedrian Nov 03 '11 at 20:59
So basically its like saying: Wait untill !results.isLocked, and when its free lock it and write? – shaharmor Nov 03 '11 at 21:01
Locks work on pieces of code not on the variables. You can put much more code there. Its important to realise that. The lock won't protect the variable outside of the lock. Basically the thread will wait at the lock, if the lock is locked, it will wait. If its not, it'll lock it for itself, and enter that section. Then when its done it releases the lock and other threads can enter. If you need more flexibility there are other kinds of locks but this will work for you. – Haedrian Nov 03 '11 at 21:05
8

A minor point: `this` is not the safest choice for the lock object. Better to use a special private object: `lock(resultsLock)` . – H H Nov 03 '11 at 21:28
2

`locks` can slow down the overall execution time though.. concurrent collections seem better to avoid that – Piotr Kula Mar 29 '17 at 14:17
1

Why do we need a lock at class level if the object modified is declared locally ? If we `lock(results)` and then add to it what is wrong ? Lock at class level means, if multiple requests are calling `Search` then you have degraded the performance – Frank Q. May 17 '18 at 20:44

score 43 · Answer 3 · answered Dec 01 '15 at 10:17

43

For those who prefer code:

public static ConcurrentBag<SearchResult> Search(string title)
{
    var results = new ConcurrentBag<SearchResult>();
    Parallel.ForEach(Providers, currentProvider =>
    {
        results.Add(currentProvider.SearchTitle((title)));
    });

    return results;
}

answered Dec 01 '15 at 10:17

Matas Vaitkevicius

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Have to use a loop: `foreach (var item in currentProvider.SearchTitle((title))) results.Add(item);` – Anthony McGrath Jul 10 '20 at 19:56

score 26 · Answer 4 · edited Jun 20 '20 at 09:12

The Concurrent Collections are new for .Net 4; they are designed to work with the new parallel functionality.

See Concurrent Collections in the .NET Framework 4:

Before .NET 4, you had to provide your own synchronization mechanisms if multiple threads might be accessing a single shared collection. You had to lock the collection ...

... the [new] classes and interfaces in System.Collections.Concurrent [added in .NET 4] provide a consistent implementation for [...] multi-threaded programming problems involving shared data across threads.

Douglas · Answer 5 · 2017-06-08T18:33:56.993

16

This could be expressed concisely using PLINQ's AsParallel and SelectMany:

public static List<SearchResult> Search(string title)
{
    return Providers.AsParallel()
                    .SelectMany(p => p.SearchTitle(title))
                    .ToList();
}

edited Jun 08 '17 at 18:33

answered Nov 16 '14 at 21:19

Douglas

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linq selectMany is great, sadly linq is slower than normal foreach. :( – Kugan Kumar Jun 07 '18 at 18:32
9

Don't micro-optimize. The OP implied that `SearchTitle` connects to a remote site. Its latency will be several orders of magnitude slower than the difference between LINQ and foreach. – Douglas Jun 08 '18 at 19:21

Parallel.ForEach with adding to list

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