Async and Await

In simple words#

Learning objectives#

• Write async methods that return Task or Task<T>
• Await results instead of blocking
• Run independent calls in parallel with Task.WhenAll
• Catch exceptions from async methods

Explanation#

An async method returns Task (no result) or Task<T> (a result of type T). By convention its name ends in Async: GetUserAsync.

await pauses the method until the task finishes, then continues with the result. While it waits, the thread is free to do other work — that is the whole point.

Awaiting calls one after another runs them in sequence. If calls do not depend on each other, start them all first and await Task.WhenAll to run them at the same time.

Exceptions thrown inside an async method are stored in the Task and re-thrown when you await it, so a normal try/catch around await works.

Never block on async code with .Result or .Wait() in ASP.NET — it wastes threads and can deadlock. Make the caller async too (“async all the way”).

Syntax#

1async Task<User> GetUserAsync(int id)
2{
3    var user = await db.Users.FindAsync(id);
4    return user;
5}
6

7var user = await GetUserAsync(1);
8await Task.WhenAll(taskA, taskB);

Code example: Sequential, parallel and failing calls#

1using System.Diagnostics;
2

3string user = await GetUserAsync(1);
4Console.WriteLine(user);
5

6var timer = Stopwatch.StartNew();
7Task<int> ordersTask = GetOrderCountAsync(1);   // both start now
8Task<decimal> balanceTask = GetBalanceAsync(1);
9await Task.WhenAll(ordersTask, balanceTask);
10Console.WriteLine(
quot;Orders: {await ordersTask}, balance: {await balanceTask}");
11Console.WriteLine(
quot;Ran in parallel: {timer.ElapsedMilliseconds < 900}");
12

13try
14{
15    await GetUserAsync(-1);
16}
17catch (ArgumentException ex)
18{
19    Console.WriteLine(
quot;Caught: {ex.Message}");
20}
21

22static async Task<string> GetUserAsync(int id)
23{
24    await Task.Delay(300);   // pretend this is a database call
25    if (id < 0) throw new ArgumentException("Invalid id");
26    return 
quot;User {id}: Asha";
27}
28

29static async Task<int> GetOrderCountAsync(int userId)
30{
31    await Task.Delay(500);
32    return 7;
33}
34

35static async Task<decimal> GetBalanceAsync(int userId)
36{
37    await Task.Delay(500);
38    return 2500m;
39}

User 1: Asha
Orders: 7, balance: 2500
Ran in parallel: True
Caught: Invalid id

This example is illustrative. Copy or download it and run it in your own environment; the expected output is shown above.

Step-by-step explanation#

1. await GetUserAsync(1) waits about 300 ms without blocking a thread, then gives back the string.
2. GetOrderCountAsync and GetBalanceAsync are started before either is awaited, so their 500 ms delays overlap.
3. Task.WhenAll finishes when both are done — about 500 ms in total instead of 1000 ms.
4. Awaiting an already-finished task just returns its result immediately.
5. The exception from GetUserAsync(-1) is re-thrown at the await and caught normally.

Real-world use case#

Common mistakes#

Best practices#

• Go async all the way from controller to database
• Name async methods with the Async suffix
• Pass CancellationToken through long operations
• Use Task.WhenAll for independent work

Remember this#

Practice exercise#

1string[] results = await Task.WhenAll(
2    DownloadAsync("a.pdf", 300),
3    DownloadAsync("b.pdf", 200),
4    DownloadAsync("c.pdf", 100));
5Console.WriteLine(string.Join(", ", results));   // a.pdf done, b.pdf done, c.pdf done
6

7static async Task<string> DownloadAsync(string name, int ms)
8{
9    await Task.Delay(ms);
10    return 
quot;{name} done";
11}

Mini quiz#

Want more? Practise all C# MCQs · C# interview questions

Related topics#

My notes#

Saved in this browser.