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?

Answer 1

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.

Answer 2

//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.