Run async await tasks in parallel in Asp.net Core

Question

List<Employee> employeeList = new List<Employee>();
List<Event> eventList = new List<Event>();

//Wanted to execute each foreach call asynchronously, to improve performance
foreach (var account in accounts)
{
    var context = await GetContext(account); 
    
    //following two await is depends on above result
    var employees = await _employeeService.List(context);
    var events = await _eventService.List(context).ConfigureAwait(false);
    //Question is how to execute above two await calls parallely
    employeeList.AddRange(employees);
    eventList.AddRange(events);
}

I want to fire off two long-running tasks(employees and Events) at the same time. So maybe can we achieve parallel execution for each account above to solve this?

Answer 1

If you want only each call inside loop to be in parallel -

List<Employee> employeeList = new List<Employee>();
List<Event> eventList = new List<Event>();
foreach (var account in accounts)
{
    var context = await GetContext(account);
    var employeesTask = _employeeService.List(context);
    var eventsTask = _eventService.List(context);
    await Task.WhenAll(employeesTask , events).ConfigureAwait(false);
    var employees = await employeesTask.ConfigureAwait(false);
    var events= await eventsTask.ConfigureAwait(false);    
    employeeList.AddRange(employees);
    eventList.AddRange(events);
}

Answer 2

You could just schedule execution of each iteration as TAsk and put it array and then use Task.WhenAll , like below:

var tasks = new List<Task>();
//Wanted to execute each foreach call asynchronously, to improve performance
foreach (var account in new int[] { 1 })
{
    tasks.Add(
        Task.Run(async () =>
        {
            var context = await GetContext(account);

            //following two await is depends on above result
            var employees = await _employeeService.List(context);
            var events = await _eventService.List(context).ConfigureAwait(false);
            //Question is how to execute above two await calls parallely
            employeeList.AddRange(employees);
            eventList.AddRange(events);
        }));
}
await Task.WhenAll(tasks);

WARNING: you have to make sure that employeeList and eventList are thread safe.

You could even wrap loop iteration in a separate method (you can define itinside your method) to make code cleaner:

await Task.WhenAll(accounts.Select(a => loopIteration(a)));

async Task loopIteration(Account account)
{
    var context = await GetContext(account);

    //following two await is depends on above result
    var employees = await _employeeService.List(context);
    var events = await _eventService.List(context).ConfigureAwait(false);
    //Question is how to execute above two await calls parallely
    employeeList.AddRange(employees);
    eventList.AddRange(events);
}