Understanding async and await

C# 5 introduced two C# keywords when working with the Task type that enable easy multithreading. The pair of keywords is especially useful for the following:

• Implementing multitasking for a graphical user interface (GUI).
• Improving the scalability of web applications and web services.
• Preventing blocking calls when interacting with the filesystem, databases, and remote services, all of which tend to take a long time to complete their work.

In an online section, Building Websites Using the Model-View-Controller Pattern, we will see how the async and await keywords can improve scalability for websites. But for now, let’s see an example of how they can be used in a console app, and then later you will see them used in a more practical example within web projects.

Improving responsiveness for console apps

One of the limitations with console apps is that you can only use the await keyword inside methods that are marked as async, but C# 7 and earlier do not allow the Main method to be marked as async! Luckily, a new feature introduced in C# 7.1 was support for async in Main.

Let’s see it in action:

1. Use your preferred code editor to add a new Console App / console project named AsyncConsole to the Chapter02 solution.
2. Open AsyncConsole.csproj, and after the <PropertyGroup> section, add a new <ItemGroup> section to statically import System.Console for all C# files using the implicit usings .NET SDK feature, as shown in the following markup:

   <ItemGroup>
     <Using Include="System.Console" Static="true" />
   </ItemGroup>
   

3. In Program.cs, delete the existing statements, and then add statements to create an HttpClient instance, make a request for Apple’s home page, and output how many bytes it has, as shown in the following code:

   HttpClient client = new();
   HttpResponseMessage response =
     await client.GetAsync("http://www.apple.com/");
   WriteLine("Apple's home page has {0:N0} bytes.",
     response.Content.Headers.ContentLength);
   

4. Navigate to Build | Build AsyncConsole and note that the project builds successfully.

   In .NET 5 and earlier, you would have seen an error message, as shown in the following output:

   Program.cs(14,9): error CS4033: The 'await' operator can only be used within an async method. Consider marking this method with the 'async' modifier and changing its return type to 'Task'. [/Users/markjprice/Code/ Chapter02/AsyncConsole/AsyncConsole.csproj]

   You would have had to add the async keyword for your Main method and change its return type from void to Task. With .NET 6 and later, the console app project template uses the top-level program feature to automatically define the Program class with an asynchronous <Main>$ method for you.

5. Run the code and view the result, which is likely to have a different number of bytes since Apple changes its home page frequently, as shown in the following output:

   Apple's home page has 170,688 bytes.