Asynchronous Programming in .NET Core

Date: 12 February 2018

Asynchronous programming is a cornerstone of modern .NET applications, allowing you to write non-blocking, responsive, and scalable code. In .NET Core, asynchronous programming is incredibly powerful thanks to the async and await keywords. These tools help you run potentially blocking I/O operations (such as database calls, web requests, file reads, etc.) in the background, keeping your application responsive.

In today’s post, we’ll dive into the basics of asynchronous programming in .NET Core and explore best practices for building efficient asynchronous controllers and services.

What is Asynchronous Programming?

Asynchronous programming allows a program to perform multiple operations simultaneously without blocking the main execution thread. This is particularly useful when dealing with I/O-bound tasks, such as making HTTP requests, reading from a file, or interacting with a database.

In a traditional synchronous approach, if you are performing an I/O-bound operation, the program has to wait for the operation to complete before moving on to the next one. This can lead to poor performance and unresponsive applications, especially in web applications with multiple concurrent users.

Asynchronous programming solves this by allowing the application to “continue” executing other operations while waiting for I/O-bound tasks to complete. This results in better resource utilization, improved scalability, and a more responsive user experience.

Async and Await in .NET Core

The async and await keywords are the core building blocks for asynchronous programming in .NET Core.

The async Keyword:

When you mark a method with the async keyword, it tells the compiler that the method will contain one or more await expressions and should be executed asynchronously.
The return type of an async method is usually Task or Task<T> (for methods returning a value). If the method doesn’t return any result, the return type will be Task; if it returns a value, the return type will be Task<T>, where T is the type of the result. Example:

   public async Task<string> GetDataAsync()
   {
       // Simulate an I/O-bound operation (e.g., HTTP request)
       await Task.Delay(2000);
       return "Data loaded";
   }

The await Keyword:

The await keyword is used to “pause” the method’s execution until the awaited task completes. While the method is awaiting the task, other tasks or operations can continue executing.
You cannot use await outside of an async method. Example:

   public async Task MainAsync()
   {
       string data = await GetDataAsync();  // Asynchronous call
       Console.WriteLine(data);
   }

Why Use Asynchronous Programming?

Asynchronous programming is especially beneficial in web applications for the following reasons:

Non-blocking I/O operations: In traditional synchronous applications, requests that involve I/O operations (such as database queries, API calls, or file access) can block the thread until the task completes. In an asynchronous application, the thread is freed up to process other requests while waiting for the I/O operation to complete.
Better Scalability: Since asynchronous methods allow your application to handle more requests concurrently without blocking threads, it results in better scalability and performance, especially in high-load applications like APIs or web services.
Improved User Experience: For UI applications, asynchronous programming ensures that the interface remains responsive while performing long-running tasks in the background.

Best Practices for Async Controllers and Services

Use Async for I/O-bound Operations:

Async programming is most beneficial for I/O-bound operations. For CPU-bound operations, consider using parallel programming approaches. For database calls, HTTP requests, or file operations, always prefer asynchronous methods.

Avoid Async Void Methods:

In general, avoid using async void methods except for event handlers. When using async void, the caller cannot await the method, and any exceptions thrown inside the method will be unhandled. Always use Task or Task<T> as the return type. Example of bad practice:

   public async void BadMethod()
   {
       await Task.Delay(1000);  // Unhandled exception if something goes wrong
   }

Example of good practice:

   public async Task GoodMethod()
   {
       await Task.Delay(1000);
   }

Return Task in Controller Actions:

In Web API controllers, it’s best practice to return a Task or Task<T> from action methods that are asynchronous. This ensures that the framework can properly manage the asynchronous execution pipeline. Example:

   [HttpGet]
   public async Task<IActionResult> GetDataAsync()
   {
       var data = await _dataService.GetDataFromDatabaseAsync();
       return Ok(data);
   }

Use ConfigureAwait(false) for Library Code:

When you’re working in a library or background task (e.g., not in an ASP.NET Core context), use ConfigureAwait(false) to avoid deadlocks. It tells the compiler not to attempt to marshal the continuation back to the original context (like the UI thread or HTTP request context). Example:

   public async Task ProcessDataAsync()
   {
       // Avoid deadlocks by not marshalling the continuation to the original context
       await Task.Delay(1000).ConfigureAwait(false);
       // Process data here
   }

Be Aware of Thread Contexts:

In a web application, ASP.NET Core manages the request thread context, which can impact how asynchronous code is executed. Be mindful of the thread context, especially when using await inside critical sections of your code.

Exception Handling in Async Methods:

Just like synchronous methods, async methods can throw exceptions. However, exceptions thrown in async methods won’t be caught unless you await them. Use try-catch blocks to handle exceptions in async code. Example:

   try
   {
       await SomeAsyncMethod();
   }
   catch (Exception ex)
   {
       // Handle exception here
   }

Conclusion

Asynchronous programming is a powerful tool for improving performance and scalability in .NET Core applications, especially in I/O-bound scenarios like database interactions or HTTP requests. By using the async and await keywords effectively, and following best practices such as avoiding async void and ensuring proper exception handling, you can build more responsive and scalable applications.

Next time, we’ll take a closer look at how to apply asynchronous programming patterns to complex applications. Stay tuned!