I'm using an API client that is completely asynchrounous, that is, each operation either returns Task
or Task<T>
, e.g:
static async Task DoSomething(int siteId, int postId, IBlogClient client)
{
await client.DeletePost(siteId, postId); // call API client
Console.WriteLine("Deleted post {0}.", siteId);
}
Using the C# 5 async/await operators, what is the correct/most efficient way to start multiple tasks and wait for them all to complete:
int[] ids = new[] { 1, 2, 3, 4, 5 };
Parallel.ForEach(ids, i => DoSomething(1, i, blogClient).Wait());
or:
int[] ids = new[] { 1, 2, 3, 4, 5 };
Task.WaitAll(ids.Select(i => DoSomething(1, i, blogClient)).ToArray());
Since the API client is using HttpClient internally, I would expect this to issue 5 HTTP requests immediately, writing to the console as each one completes.
int[] ids = new[] { 1, 2, 3, 4, 5 };
Parallel.ForEach(ids, i => DoSomething(1, i, blogClient).Wait());
Although you run the operations in parallel with the above code, this code blocks each thread that each operation runs on. For example, if the network call takes 2 seconds, each thread hangs for 2 seconds w/o doing anything but waiting.
int[] ids = new[] { 1, 2, 3, 4, 5 };
Task.WaitAll(ids.Select(i => DoSomething(1, i, blogClient)).ToArray());
On the other hand, the above code with WaitAll
also blocks the threads and your threads won't be free to process any other work till the operation ends.
I would prefer WhenAll
which will perform your operations asynchronously in Parallel.
public async Task DoWork() {
int[] ids = new[] { 1, 2, 3, 4, 5 };
await Task.WhenAll(ids.Select(i => DoSomething(1, i, blogClient)));
}
In fact, in the above case, you don't even need to
await
, you can just directly return from the method as you don't have any continuations:public Task DoWork() { int[] ids = new[] { 1, 2, 3, 4, 5 }; return Task.WhenAll(ids.Select(i => DoSomething(1, i, blogClient))); }
To back this up, here is a detailed blog post going through all the alternatives and their advantages/disadvantages: How and Where Concurrent Asynchronous I/O with ASP.NET Web API