Summary - Task and ContinueWith
Combine WhenAll, WhenAny and CancellationToken to write a parallel downloader.
Summary - Task and ContinueWith
In this group you learned the basics of Task operations: Task.Run sends work to the pool; ContinueWith registers a continuation; Task.WhenAll waits for all Tasks; Task.WhenAny waits for the first; CancellationToken cancels work from outside.
The key tip: in new code, await is usually cleaner than ContinueWith. For cancellation use ThrowIfCancellationRequested. Now we will combine all of this in a small app.
Exercise Combine two methods with WhenAll
Define static Task<int> GetA() (return Task.FromResult(10);). Define static Task<int> GetB() (return Task.FromResult(20);). Make Main a static async Task Main. Await with int[] arr = await Task.WhenAll(GetA(), GetB()); and print the sum with Console.WriteLine(arr[0] + arr[1]);.
Show example solution
return Task.FromResult(10); and return Task.FromResult(20);. Make Main static async Task Main() and write int[] arr = await Task.WhenAll(GetA(), GetB()); Console.WriteLine(arr[0] + arr[1]);
Example solution:
using System;
using System.Threading.Tasks;
class Program
{
static Task<int> GetA()
{
return Task.FromResult(10);
}
static Task<int> GetB()
{
return Task.FromResult(20);
}
static async Task Main()
{
int[] arr = await Task.WhenAll(GetA(), GetB());
Console.WriteLine(arr[0] + arr[1]);
}
}Tips
A few tips: Task.Run only for CPU work; for I/O call an async method directly. Prefer await over ContinueWith. Use WhenAll to wait for many Tasks, WhenAny for races.
When writing cancellable methods, always take a CancellationToken parameter and call ThrowIfCancellationRequested inside loops. Cancellation is not an error, it is an expected flow.
Exercise Mini app: Parallel squares
Define static Task<int> Compute(int x) (return Task.FromResult(x * x);). Make Main a static async Task Main. Compute two squares in parallel with int[] arr = await Task.WhenAll(Compute(3), Compute(4)); and print the sum with Console.WriteLine(arr[0] + arr[1]); (3² + 4² = 9 + 16 = 25).
Show example solution
In Compute body write return Task.FromResult(x * x);. Make Main static async Task Main() and write int[] arr = await Task.WhenAll(Compute(3), Compute(4)); Console.WriteLine(arr[0] + arr[1]);
Example solution:
using System;
using System.Threading.Tasks;
class Program
{
static Task<int> Compute(int x)
{
return Task.FromResult(x * x);
}
static async Task Main()
{
int[] arr = await Task.WhenAll(Compute(3), Compute(4));
Console.WriteLine(arr[0] + arr[1]);
}
}Test yourself
-
What does Task.WhenAll do?
- Waits for all Tasks
- Waits for the first Task
- Cancels the Tasks
- Runs the Tasks one by one
Show answer
Waits for all Tasks — Correct. WhenAll waits until all Tasks finish.