How to await a list of tasks asynchronously using LINQ?

Question

I have a list of tasks that I created like this:

public async Task<IList<Foo>> GetFoosAndDoSomethingAsync() {     var foos = await GetFoosAsync();      var tasks = foos.Select(async foo => await DoSomethingAsync(foo)).ToList();      ... } 

By using .ToList(), the tasks should all start. Now I want to await their completion and return the results.

This works in the above ... block:

var list = new List<Foo>(); foreach (var task in tasks)     list.Add(await task); return list; 

It does what I want, but this seems rather clumsy. I'd much rather write something simpler like this:

return tasks.Select(async task => await task).ToList(); 

... but this doesn't compile. What am I missing? Or is it just not possible to express things this way?

Answer 1

LINQ doesn't work perfectly with async code, but you can do this:

var tasks = foos.Select(DoSomethingAsync).ToList(); await Task.WhenAll(tasks); 

If your tasks all return the same type of value, then you can even do this:

var results = await Task.WhenAll(tasks); 

which is quite nice. WhenAll returns an array, so I believe your method can return the results directly:

return await Task.WhenAll(tasks); 

Answer 2

To expand on Stephen's answer, I've created the following extension method to keep the fluent style of LINQ. You can then do

await someTasks.WhenAll()  namespace System.Linq {     public static class IEnumerableExtensions     {         public static Task<T[]> WhenAll<T>(this IEnumerable<Task<T>> source)         {             return Task.WhenAll(source);         }     } }