Usage of ConcurrentBag<T> vs list<T>-CodePudding

Currently I was working on parallel threads in C#. So, I have confusion of using ConcurrentBag<T> and List<T>.

Here is my code:

public async Task<ConcurrentDictionary<string, ConcurrentBag<T>>> MethodA(SearchResults<Result> response)
{
     var a = new ConcurrentDictionary<string, ConcurrentBag<DeviceAlertAlarm>>();
    
     var tasks = response.GetResults().Select(async result) =>
     {
            var b = new List <T>();
            // do something
            a["xxx"] = b;
            
     });
     await Task.WhenAll(tasks);
     return a;
}

For var b = new List ();

Is it mandatory of ConcurrentBag<T> in multi-threading or can I use List<T> which is best way of writing the code with respective of performance.

which one is better Concurrentbag<T> or List<T> in the above part of code?

Thanks in Advance!

CodePudding user response：

Because your inner list is never used concurrently you also do not need to use a ConcurrentBag<T> here.

Commented your example a bit. From what I expect your code is doing I would take ICollection<T> or IEnumerable<T> in the ConcurrentDictionary<string, IEnumerable<T>>.

// IEnumerable, List, Collection ... is enough here.
public async Task<ConcurrentDictionary<string, IEnumerable<T>>> MethodA(SearchResults<Result> response)
{
     var a = new ConcurrentDictionary<string, IEnumerable<DeviceAlertAlarm>>();
    
     var tasks = response.GetResults().Select(async (result) =>
     {    
            //
            // This list just exists and is accessed in this 'task/thread/delegate'
            var b = new List<T>();
            //
            // do something ...
            //
            // The surrounding IDictionary as you have it here 
            // is used *concurrently* so this is enough to be thread-safe
            a["xxx"] = b;
            
     });
     await Task.WhenAll(tasks);
     return a;
}