Overview

In C#, Thread and Task are both types used for asynchronous operations. Task is a higher level of abstraction than Thread.

System.Threading.Tasks contains types for writing concurrent and asynchronous code.

• TaskFactory — static methods for creating and starting tasks.
• TaskScheduler — thread scheduling infrastructure.
• Task — a wrapper around a thread that enables easier management.

System.Threading.Tasks.Extensions.ValueTask is a lightweight implementation of a generalized task-returning value.

Task and Task<T> [ Documentation]

Task represents a single asynchronous operation that does not return a value. It is a reference type (so it allocates an object on the heap).

• Task.CompletedTask holds a task that is already completed. If you implement a class that must return a Task (equivalent to void in a synchronous method), return Task.CompletedTask.
• Task<TResult>’s Result property is of type TResult and holds the value the task returns.
  • It is a blocking property: accessing it before the task is finished results in blocking the currently active thread until it finishes.
  • Access the value by using await instead.

Task Lifecycle

A task’s lifecycle is represented by the [TaskStatus] enumeration and contains these states:

• (0) Created — initialized, but not yet scheduled.
• (1) WaitingForActivation — waiting to be activated and scheduled internally.
• (2) WaitingToRun — scheduled, but not yet started.
• (3) Running — started, but not yet completed.
• (4) WaitingForChildrenToComplete — completed, but waiting for attached child tasks to complete.
• (5) RanToCompletion — completed.
• (6) Canceled — either:
  • The task has acknowledged cancellation by throwing an OperationCanceledException with its own CancellationToken, or;
  • The task’s CancellationToken was signaled before the task started.
• (7) Faulted — the task completed due to an unhandled exception.

Creating and Starting

• Tasks that are created with public Task constructors are cold tasks — they are in the Created state. They must be Start()ed.
• All tasks that are returned from TAP methods must be started.
  • If, internally, a TAP method uses a task’s constructor to instantiate the task to be returned, the TAP method must still call Start on the Task object returned.

public static async Task Main()
{
    await Task.Run( () =>
    {
        …
    }
}

or, equivalently using a TaskFactory.StartNew, but this method is no longer recommended.

• Task t1 = new(MethodA); — instantiated, but not started.
  • t1.Start() — start the task then continue executing.
    • Caution: calling Start on an active task throws an InvalidOperationException. Check Task.TaskStatus to determine its state.
• Task t2 = Task.Factory.StartNew(MethodB); — instantiated and started.
• Task t3 = Task.Run(MethodC); — instantiated and started.

Waiting

Waiting Decision Table

Task.Wait (Blocking)

Wait to block the thread that started the Task. Otherwise, if the calling thread is the Main app thread, the app may terminate before the Task completes:

t1.Wait();
// or:
t1.Wait(n); // Wait for n milliseconds.
// or:
t1.Wait(TimeSpan)
// or:
t1.Wait(CancellationToken) // If a CancellationToken is fulfilled while waiting, OperationCanceledException is thrown.
// or:
t1.Wait(n, CancellationToken)

t4.RunSynchronously(); // Start the task on the main thread.
t4.Wait(); // Should still wait in case an error is thrown.

Task.WaitAny (Blocking)

To wait on the first of a series of Tasks to finish:

Task.WaitAny(t1, t2, …) // WaitAny() returns the index (an Int32) of which Task in the list finished.

To wait on all the Tasks in a series to finish:

Task.WaitAll(t1, t2, …)

await Task.WhenAll (Non-blocking)

public async Task<User> GetUserAsync(int userId)
{
    // Given a user Id {userId}, retrieves a User object corresponding
    // to the entry in the database with {userId} as its Id.
}

public static async Task<IEnumerable<User>> GetUsersAsync(IEnumerable<int> userIds)
{
    var getUserTasks = new List<Task<User>>();
 
    foreach (int userId in userIds)
        getUserTasks.Add(GetUserAsync(userId));

    return await Task.WhenAll(getUserTasks);
}

await Task.WhenAny (Non-blocking)

This method accepts Task.Delay as a parameter to implement a timeout:

Task<Bitmap> download = GetBitmapAsync(url);

if (download == await Task.WhenAny(download, Task.Delay(3000)))
{
    // Downloaded the bitmap successfully
}
else
{
    // Timed out
}

Exceptions While Waiting

While waiting on one or more Tasks to complete, exceptions thrown are propagated to the thread that called the Wait method:

try
{
    Task.WaitAll(t1, t2, …);
}
catch (AggregateException ae)
{
    // One or more exceptions were thrown:
    foreach (var ex in ae.InnerExceptions)
    {
        // ex.GetType().Name, ex.Message
    }
}