C# get results from Task.WhenAll

The C# method Task.WhenAll can run a bunch of async methods in parallel and returns when every one finished.

But how do you collect the return values?

Imagine that you have this pseudo-async-method:

private async Task<string> GetAsync(int number)
{
  return DoMagic();
}

And you wish to call that method 20 times, and then collect all the results in a list?

That is a 3 step rocket:

1. Create a list of tasks to run
2. Run the tasks in parallel using Task.WhenAll.
3. Collect the results in a list

// Create a list of tasks to run
List<Task> tasks = new List<Task>();
foreach (int i=0;i<20;i++)
{
  tasks.Add(GetAsync(i));
}

// Run the tasks in parallel, and
// wait until all have been run
await Task.WhenAll(tasks);

// Get the values from the tasks
// and put them in a list
List<string> results = new List<string>();
foreach (var task in tasks)
{
  var result = ((Task<string>)task).Result;
  results.Add(result);
}

MORE TO READ:

• Task.WhenAll from microsoft
• Run tasks in parallel using .NET Core, C# and async coding by briancaos
• Using C# HttpClient from Sync and Async code by briancaos
• Awaiting multiple Tasks with different results from stackoverflow
• Get the result of multiple tasks in a ValueTuple and WhenAll by Gérald Barré
• Using Task.WhenAny And Task.WhenAll by Hamid Mosalla