Async in C# 5.0

Chapter 1. Introduction

Let’s start with a high-level introduction to the async feature in C# 5.0, and what it means for you.

Asynchronous Programming

Code is asynchronous if it starts some long-running operation, but then doesn’t wait while it’s happening. In this way, it is the opposite of blocking code, which sits there, doing nothing, during an operation.

These long-running operations include:

• Network requests

• Disk accesses

• Delays for a length of time

The distinction is all about the thread that’s running the code. In all widely used programming languages, your code runs inside an operating system thread. If that thread continues to do other things while the long-running operation is happening, your code is asynchronous. If the thread is still in your code, but isn’t doing any work, it is blocked, and you’ve written blocking code.

Note

Of course, there is a third strategy for waiting for long-running operations, called polling, where you repeatedly ask whether the job is complete. While it has its place for very short operations, it’s usually a bad idea.

You’ve probably used asynchronous code before in your work. If you’ve ever started a new thread, or used the ThreadPool, that was asynchronous programming, because the thread you did it on is free to continue with other things. If you’ve ever made a web page that a user can access another web page from, that was asynchronous, because there’s no thread on the web server waiting for the user’s input. That may seem completely obvious, but ...