Asynchronous Programming with C#

What is Asynchronous Programming?

In synchronous programming, operations execute sequentially one after another. Each line of code waits for the previous one to finish before running.

This can cause problems when dealing with long-running operations like network calls, disk I/O, and calculation-heavy workloads. Synchronous code has to halt everything while it waits on the blocking task.

Asynchronous programming solves this by allowing long-running tasks to execute concurrently without blocking the rest of the program. This enables highly responsive and performant applications.

Instead of waiting for a task to finish, asynchronous code will initiate the operation and continue executing other logic. The async task runs in the background until it completes.

C#’s async/await model provides an elegant syntax for dealing with asynchronous operations and managing non-blocking concurrency.

Why Asynchronous Programming Matters

There are two major reasons why asynchronous programming is crucial:

1. Improved Responsiveness

Synchronous code has to stop everything and wait until a long operation is happening. This leads to an unresponsive app.

With async code, the UI thread is freed up to remain active while background I/O proceeds. Users see a more responsive experience.

2. Better Scalability

Synchronous code is very inefficient in high-load scenarios. Each operation hogs a thread while waiting, limiting overall throughput.

By shifting work to async background tasks, your C# app can scale to handle many more concurrent operations with a limited thread pool.

Other Benefits

Here are some other advantages of asynchronous programming:

-Simplified code flow compared to traditional multithreading
-Easy to add concurrency to existing synchronous methods
-Integrates well with GUI event loops
-Utilities like Task.WhenAll reduce callback complexity

The async/await model makes asynchronous logic easy to implement in C#. Let’s look at how it works under the hood.

Understanding Async/Await in C#

The async and await keywords enable asynchronous programming in C#. Here is a high-level overview:

• async marks a method as asynchronous. This enables the await keyword within the method body.
• await pauses async method execution until a non-blocking Task completes.
• The async method returns a Task or Task instance that runs concurrently.
• Control returns to the caller without blocking.

This allows long-running logic to be partitioned into discrete asynchronous operations. Externally it appears synchronous since you can await each async method sequentially.

Internally, await handles all the complexity of managing non-blocking concurrency behind the scenes.

Let’s walk through a basic example:

async Task DownloadDataAsync() 
{
  // Initiate download
  var data = await DownloadFromWebAsync();

  // Process download
  ProcessData(data);
}

void CallerMethod()
{
  DownloadDataAsync();

  // More logic executes here without waiting
} 

• DownloadDataAsync runs asynchronously when called.
• Execution pauses at await but control returns immediately to caller.
• Once download completes, execution resumes and ProcessData runs.

So externally it operates synchronously while handling concurrency under the hood. This blended approach makes asynchronous logic easier to work with.

Next let’s go over the common patterns used in async methods.

Async Method Patterns

There are a few common patterns seen in asynchronous C# methods:

Async Void Methods

async void methods are used for event handlers and other scenarios where a result is not needed:

private async void ButtonClick(object sender, EventArgs e)
{
  await HandleClickAsync(); // No return value needed
}

Avoid async void for functions that do return values. Use async Task instead.

Async Task Methods

async Task methods are the most common form. They perform async work and return a Task instance:

async Task<Data> FetchDataAsync()
{
  var data = await HttpClient.GetStringAsync(url);
  return data; 
}

The Task object represents the pending operation. You resume execution by awaiting it.

Async Task Methods

For async methods that return values, use async Task<T>:

async Task<User> GetUserAsync(int id)
{
  var user = await _dbContext.FindAsync(id);
  return user;
}

This allows you to await the Task and access the return value once complete.

Now let’s explore how to properly handle Task objects.

Working with Task Objects

Async methods return an instance of the Task or Task class. This object represents the pending asynchronous operation.

Task provides methods like .Wait() and .Result to synchronously block until completion, but should generally be avoided in asynchronous code.

Here are some best practices for working with Task objects:

Await the Task

Prefer awaiting Tasks instead of blocking methods like.Wait() or . Result. This keeps the code asynchronous:

// Blocking
var user = GetUserAsync(1).Result; 

// Async
var user = await GetUserAsync(1);

Handle Exceptions with try/catch

Wrap awaited code in try/catch blocks to handle exceptions:

try 
{
  var user = await GetUserAsync(1);
}
catch (Exception ex)
{
  // Handle error
}

Exceptions will bubble up once you await the task.

Return Task Directly If Possible

Having async methods return Task objects allows greater reusability:

async Task<User> GetUserAsync()
{
  //...
}

// Elsewhere in code

Task task = GetUserAsync(); // No need to await

This allows the caller to await or otherwise process the task as needed.

Use WhenAll for Multiple Tasks

Task.WhenAll can combine multiple Tasks into one:

await Task.WhenAll(
  GetUserAsync(1), 
  GetPostsForUser(1)
);

This awaits both completing before continuing, without nesting many awaits.

Properly handling Task objects is key to asynchronous logic in C#. Next we’ll cover some common pitfalls.

Common Async/Await Pitfalls

Here are some common mistakes and issues that can occur with async code:

Blocking Instead of Awaiting

Don’t block on async code with Task.Wait() or Task.Result. This eliminates the benefits of async:

// Blocks
var user = GetUserAsync(1).Result; 

// Proper way
var user = await GetUserAsync(1);

Always await async methods to keep code non-blocking.

Forgetting Async Keyword

All methods that contain await must be marked async:

// Compiler error
Task DownloadAsync() // Missing async keyword
{
  await DownloadFile();
}

This applies to both void and Task-returning async methods.

Fire and Forget Tasks

Don’t start Tasks without awaiting them somewhere later. This can lead to unobserved exceptions and orphaned threads:

// Bad practice
SomeTask().Start();

// Proper handling
await SomeTask();

Always retain a reference to track started Tasks.

Assuming Order is Preserved

Async methods may actually complete in a different order than called:

var task1 = SomeLongTaskAsync(); 
var task2 = AnotherLongTaskAsync();

await task2; // May actually complete after task1!

If order matters, await tasks sequentially or use Task.WhenAll.

By being aware of these common pitfalls, you can avoid shooting yourself in the foot with asynchronous code!

Now let’s cover some key best practices.

Async Programming Best Practices

Here are some best practices to keep in mind when writing asynchronous C# code:

Clearly Name Async Methods

Follow naming conventions like appending “Async” to async method names:

async Task<User> GetUserAsync(int id) { }

This indicates at the call site that the method is asynchronous.

Document Async Methods

In XML doc comments, note that a method is async and consider potential pitfalls like order of operations.

Handle Exceptions Properly

Wrap awaited calls in try/catch blocks and handle any exceptions raised. Catch specific exceptions when possible.

Avoid Async Void When Possible

Prefer async Task methods over async void. Only use async void for event handlers where no result is needed.

Use Async All the Way

Don’t mix blocking calls like .Result with async code. Keep async flows async throughout.

Limit Concurrency for I/O Bound Work

Limit concurrency to around 256 max tasks for I/O bound async code to avoid thread exhaustion.

Test Async Code Thoroughly

Rigorously test async code flows with unit tests. Test failures and cancellation cases.

Following these best practices will result in robust and idiomatic asynchronous C# code.

Let’s now look at some real-world examples.

Async Programming Examples

Here are some examples of asynchronous programming with C# in action:

Async Web Requests

async Task<int> GetWebPageSizeAsync(string url)
{
  HttpClient client = new HttpClient();

  var response = await client.GetAsync(url);

  return response.ContentLength;
}

This allows initiating web requests without blocking the calling thread.

Async Database Queries

async Task<User> GetUserAsync(int userId)  
{
  using (var db = new AppContext())
  {
    return await db.Users.FindAsync(userId); 
  }
}

Database operations like queries and inserts can be easily made asynchronous.

Parallel Async Work

await Task.WhenAll(
  ProcessDataAsync(data1),
  ProcessDataAsync(data2),
  ProcessDataAsync(data3)
);

// All complete before continuing

Task.WhenAll allows running async methods in parallel very easily.

Fire and Forget Background Tasks

_ = LongTaskAsync(); // Discard returned Task

// Don't await, allow to run in background

Sometimes fire-and-forget background jobs are useful in async flows.

Proper usage of async/await unlocks powerful asynchronous patterns like these.

Conclusion

Asynchronous programming is essential for building high-performance applications with C#. The async/await model makes it easy to write asynchronous code while handling all the complexity behind the scenes.

Here are some key points:

• Use async methods to perform long-running work without blocking
• await Tasks to pause execution until completion
• Return Task or Task objects from async methods
• Handle exceptions and adhere to async best practices
• Avoid common pitfalls like blocking Tasks

Learning to properly leverage async/await will take your C# skills to the next level. Your apps will gain huge benefits in responsiveness and scalability.

Hopefully, this comprehensive guide has provided a deep understanding of asynchronous programming in modern C#. Happy coding!