Async / Task

• Call async method will return a Task / Task<T>, <T> is return value type
  • Task == JavaScript Promise
• C# async method must include await, otherwise will run sync
• Use Task.Delay(<ms>) to create a delay task
• Call await <Task> to wait the Task return result
• Task.CurrentId == Current task ID
• Task.Run() will create new thread
• Call async method does not create new thread (like yield)

async static Task<int> AddAsync(int x, int y)
{
    Console.WriteLine("Task Start");
    await Task.Delay(3000);
    Console.WriteLine("Task End");
    return x + y;
}

var task = AddAsync(1, 2);
Thread.Sleep(1000);
Console.WriteLine("Main");
Console.WriteLine("Result = " + await task);

Task Start
# ~ 1000ms
Main
# ~ 2000ms
Task End
Result = 3

Run sync method in async

int x = 1;
// Task.Factory.StartNew(...);
// same as
var task = Task.Run(() =>
{
    Console.WriteLine("Start");
    Thread.Sleep(2000);
    Console.WriteLine("End");
    return x * 2;
});

Thread.Sleep(1000);
Console.WriteLine("Waiting Task");
Console.WriteLine(await task);

Start
# ~ 1000ms
Waiting Task
# ~ 1000ms
End
Result

• Task.Run() can have return value
• Task.Run() Func can not have parameter

Task

// Create a task but do not start it
var task = new Task(() => "Task");

// Start task
task.Start();

// Wait the task finish
task.Wait();

// Wait the task 1000ms, return true if finish
var finished = task.Wait(1000);

Cancellation Token

• CancellationToken is like a bool, let asyn method to know if cancellation requested
• Cancellation request need to be set using CancellationTokenSource.Cancel(), CancellationToken cannot request cancellation
• Most of C# async method support
• You need handle CancellationToken by yourself in your method

async static Task LoopAsync(int seconds, CancellationToken ct)
{
    for (int i = 0; i < seconds; i += 1)
    {
        if (ct.IsCancellationRequested)
            break;
        Console.WriteLine($"Loop {i}");
        await Task.Delay(1000, ct);
    }
}

var cts = new CancellationTokenSource();

// Loop 100 seconds
var task = LoopAsync(100, cts.Token);

Thread.Sleep(10 * 1000);
// Cancel after 10 seconds
cts.Cancel();

Loop 0
Loop 1
Loop 2
Loop 3
Loop 4
Loop 5
Loop 6
Loop 7
Loop 8
Loop 9

Cancel after a time

var cts = new CancellationTokenSource();
cts.