Lesson Weekend

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# and .NET handle 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. For instance, take at a sample Edit() action that one may use in their to do list:

public ActionResult Edit(int id)
    Item thisItem = db.Items.FirstOrDefault(item => item.ItemId == id);
    return View(thisItem);

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

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

  2. After locating the item, it returns the Edit() view.

  3. While .NET 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 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> task = ExampleAsyncMethodThatTakesAWhile();

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

  int x = await task;

  Console.WriteLine("Return value of ExampleAsyncMethodThatTakesAWhile(): " + x);
  1. First, the async keyword is included in the method's signature. 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 task. As its name suggests, this keyword makes the application wait until the specified async action task is completed. Once task 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. Finally, our hypothetical async method ExampleAsyncMethodThatTakesAWhile() has a return type of Task<int>. This line stores the return in the variable task. 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, there are a few different options available, including the following:

    • Task<int> returns an int;
    • Task<string> returns a string;
    • Task<IActionResult> returns an ActionResult.

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");
  int giftNumber = await determineHowManyGiftsIShouldBuy;
  for (int index = 0; index < giftNumber.Length; 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.

  • However, there are different types of Tasks to choose from, including Task<int>, Task<string> returns a string, and Task<IActionResult>.

  • 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.

Additional Resources

Lesson 2 of 22
Last updated March 29, 2022