Async/Await

Async/Await

The language keywords async and await are at the center of asynchronous programming in C#. Let's delve into how they work:

Async

The async keyword can be added to the signature of a method. It is syntactic sugar that hides away a lot of complexity that gets added for an async marked method. Under the hood, the compiler creates a state machine for an async method. We'll shortly explain the working of the state machine.

A method marked async has restrictions on what it can return. We can return the following from an async method:

• void

• Task

• Task<T>

• Starting in C# 7.0, we can also return generic types that satisfy the requirements for a type to be awaitable (more on that later).

Asynchronous methods either return tasks or nothing. Tasks are awaitable. Note that when a type is said to be awaitable, it implies that it can be legally used as an argument to the await expression.

Synchronous Sleep to Asynchronous Sleeep

To understand async/await better, we'll take a trivial program that executes synchronously and convert it into asynchronous execution. Along the way, we'll see how the various concepts tie-up together. In the code widget below, we have a method sleep() that performs a Thread.Sleep() for three seconds synchronously.