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📓 Async and Await

During Intermediate JavaScript, we learned how to write asynchronous code and manage asynchronicity while performing complex actions like API calls. In this lesson, we'll discuss how C# handles asynchronous code.

Why is this relevant now? In the next few lessons, we'll learn how to authenticate users with Identity. This will require our applications to manage asynchronous actions, so we need to learn how to recognize and write asynchronous methods.

Synchronous Operations​

So far the C# code we've written is synchronous and our code executes a single line at a time. The Edit() action in the ItemsController.cs is an example of a synchronous method:

...
    public ActionResult Edit(int id)
    {
      Item thisItem = _db.Items.FirstOrDefault(item => item.ItemId == id);
      ViewBag.CategoryId = new SelectList(_db.Categories, "CategoryId", "Name");
      return View(thisItem);
    }
...

Let's walk through what occurs when this synchronous method is called:

  1. When Edit() is called, our program looks in the database to find a specific item.

  2. After locating the item, our program returns the Edit View().

  3. While our program is looking in the database, the browser waits. It cannot return the view until thisItem is found because the line of code attempting to locate thisItem appears before we return the view.

  4. When thisItem is located, the application can return the view to be displayed in the browser.

Because the lines of code are executed in order, this is a synchronous operation.

Asynchronous Operations​

On the other hand, an asynchronous (also, "async") operation allows other code to run while a method is waiting to return. This is very similar to what we learned in JavaScript. However, the code we write to manage this process looks much different.

There are three primary parts to an async C# method:

  1. The async keyword

  2. The await keyword

  3. A special Task class that represents an action or actions the program may not have completed yet because they're async.

Let's walk through an example async method:

static async void ProcessTextFileAsync()
{
  Task<int> asyncTask = ExampleAsyncMethodThatTakesAWhile();

  Console.WriteLine("Please wait patiently while I run the ExampleAsyncMethodThatTakesAWhile().");

  int x = await asyncTask;

  Console.WriteLine("Return value of ExampleAsyncMethodThatTakesAWhile(): " + x);
}

  1. First, the async keyword is included in the method's signature: static async void ProcessTextFileAsync(). Async methods should always have an async modifier in their declaration. It tells our application the method should run asynchronously.

  2. Next, notice the await keyword in the line int x = await asyncTask. As its name suggests, this keyword makes the application wait until the specified async action asyncTask is completed. Once asyncTask is completed, it will define x. This gives us a great deal of control over our code. We can allow multiple lines of code to run asynchronously and then add multiple "waiting points" with await.

  3. Our hypothetical async method ExampleAsyncMethodThatTakesAWhile() stores the return data in the variable asyncTask. Notice that asyncTask has the type of Task<int>. In C#, an asynchronous method can only return void or a Task object representing the asynchronous operation itself. A Task represents ongoing work. While a Task is void by default, we can specify what type the task will eventually return by using the Task<TResult> class. For example:

  • Task<int> returns an int;
  • Task<string> returns a string;
  • Task<ActionResult> returns an ActionResult.
  1. In the code above we want the result to be the integer x. In order to turn a Task<int> into an int, we use the await keyword. This forces the program to wait until task is appropriately defined as an int before moving on to subsequent lines of code.

Let's consider one more pseudocode example:

static async void DoMyHolidayErrandsForMeAsync()
{
  Task<int> determineHowManyGiftsIShouldBuy = ProcessSantasList("C:\\naughty_or_nice.txt");
  BakeSugarCookies();
  HangLights();
  int giftNumber = await determineHowManyGiftsIShouldBuy;
  for (int index = 0; index < giftNumber; index++)
  {
    BuyHolidayGifts();
    MarkOffList(index);
  }
}

Here, we have an async method called DoMyHolidayErrandsForMeAsync(). We could describe the method as doing the following: "I need you to do holiday errands. Start determining how many gifts we need to buy and feel free to multi-task and bake the cookies and hang lights while you're doing that, too. But wait to define giftNumber until after the ProcessSantasList() method fully finishes because we need its results before continuing. After we have that number, we can buy the necessary number of gifts and start marking them off our list."

Rules and Conventions​

Before we wrap up, let's review the rules and conventions for async methods in C#:

  • Async methods always have an async modifier in their declaration, as seen above.

  • It's also a best practice to include the term Async at the end of the method name as seen in the ProcessTextFileAsync() and DoMyHolidayErrandsForMeAsync() methods.

  • Again, an asynchronous method can only return void or a Task object representing the asynchronous operation itself.

  • We can specify what type the Task will eventually become. For example, the type Task<int> will resolve to an int, Task<string> returns a string, and Task<ActionResult> returns an ActionResult. When we do this, we're using the Task<TResult> class.

  • The await keyword can only be used in a method that includes the async modifier in its signature.

  • Because await waits for a Task to finish and return its specified data type, we can use it to "unwrap" a Task. For instance, if we await something declared as a Task<int>, we'll receive an int value, because that's what that Task returns when its async code is finished running. Similarly, if we use await on a Task<string>, we'd receive a string value.

Namespace​

The Task and Task<TResult> classes belong to the System.Threading.Tasks namespace. So anytime we use a Task in our code, we'll need to add the following using directive.

using System.Threading.Tasks;

The System.Threading.Tasks namespace contains a lot of classes to help us manage concurrent and asynchronous code. To learn more, visit the MS Docs for the System.Threading.Tasks namespace.

Additional Resources​