Async in C# 5.0

Chapter 4. Writing Async Methods

Now we know how great asynchronous code is, but how hard it is to write? It’s time to look at the C# 5.0 async feature. As we saw previously in What Async Does, a method marked async is allowed to contain the await keyword.

private async void DumpWebPageAsync(string uri)
{
    WebClient webClient = new WebClient();
    string page = await webClient.DownloadStringTaskAsync(uri);
    Console.WriteLine(page);
}

The await expression in this example transforms the method, so it pauses during the download, then resumes when the download is done. This transformation makes the method asynchronous. In this chapter, we’ll explore writing async methods like this one.

Converting the Favicon Example to Async

We’ll now modify the favicon browser example from earlier to make use of async. If you can, open the original version of the example (the default branch) and try to convert it by adding async and await keywords before reading any further.

The important method is AddAFavicon, which downloads the icon, then adds it to the UI. We want to make this method asynchronous, so that the UI thread is free to respond to user actions during the download. The first step is to add the async keyword to the method. It appears in the method signature in the same way that the static keyword does.

Then, we need to wait for the download using the await keyword. In terms of C# syntax, await acts as a unary operator, like the ! not operator, or the (type) cast operator. It is placed to the left ...