Capturing exceptions on multiple threads

Running multiple threads at once can be a good idea. However, if exceptions are encountered, they should be handled properly.
In this example, create a set of tasks that are running at the same time.

// create a list to hold all the tasks
var taskList = new List<Task>();

// start our tasks
var tsk1 = this.DoTask1();
var tsk2 = this.DoTask2();
var tsk3 = this.DoTask3();

// add the tasks to our list
taskList.Add(tsk1);
taskList.Add(tsk2);
taskList.Add(tsk3);

At this point, we have all the tasks running and we have a list of all the tasks in a list. We need to wait until they have all completed. The following illustrates what may be the most obvious way to do this. BUT, this is bad. Don't do this.

// wait for all the tasks to complete.
await Task.WhenAll(taskList);

The problem with this is that if an exception occurs, we don't get back good information on the exception. If we get exceptions in multiple tasks, we will only get back one of the exceptions. The remaining ones will be forever lost.

The proper way to wait for all the tasks to complete:

// Call the WhenAll() method, but don't await it here. Store it into a variable.
var allTasks = Task.WhenAll(taskList);

try
{
   // await the allTasks task here
   await allTasks;
}
catch (Exception ex)
{
   Exception ex2;
   // catch the exception here
   if (allTasks?.Exception == null)
   {
      ex2 = new Exception("Non aggregate exception caught waiting for WhenAll() to complete in Foo()", ex);
   }
   else
   {
      // handle the Aggregate Exception
      // get a reference to the Aggregate Exception
      AggregateException aggrEx = allTasks.Exception;

      // get collection of exceptions live in the InnerExceptions property
      ReadOnlyCollection exceptions = aggrEx.InnerExceptions;
      
      // build a new exception object with the aggregate exception as the inner exception
      ex2 = new Exception("Aggregate exception caught in function Foo()", aggrEx);
   }
   
   // throw our newly created exception
   throw ex2;
}

The above example will give details on each of the exceptions in each of the threads / tasks.

Related

Running X Tasks at a time using Semaphore -

Last modified by Mohit @ 4/16/2025 4:41:29 AM