c# - Finalizer for Parallel.ForEach

Question

How do I add a finalizer that runs once all parallels have completed?

Parallel.ForEach(entries, new ParallelOptions { MaxDegreeOfParallelism = 15 }, async (entry) =>
    // Do something with the entry.
});

I have tried like this but it doesn't compile:

Parallel.ForEach(entries, new ParallelOptions { MaxDegreeOfParallelism = 15 }, async (entry) =>
    // Do something with the entry.
}, () => { // Was hoping this would work. });

Answer 1

1. You should not declare the action for the Parallel.ForEach as async. If you use an await inside that action, the control flow is returned to Parallel.ForEach and its implementation "thinks" that the action is finished. This will lead to a very different behaviour than you expect.

2. The call to Parallel.ForEach returns when the loop is completed. It returns when all actions have been done for all the elements in the enumeration. So whatever you want to do "when all parallels have completed" can be done right after that call:

   Parallel.ForEach(entries, new ParallelOptions { MaxDegreeOfParallelism = 15 }, 
            (entry) =>
            // Do something with the entry.
   );
   DoSomethingWhenAllParallelsHaveCompleted();