Writing asynchronous code in .NET

Over the years there have been multiple ways to write asynchronous code in .NET with async/await being the latest attempt to make it less error-prone and more developer friendly.

We started the asynchronous journey in .NET 1.0 with the Asynchronous Programming Model where you had to write a Begin and End method following a specific pattern:

	class Handler
	{
	public int DoStuff(string arg);
	public IAsyncResult BeginDoStuff(string arg, AsyncCallback? callback, object? state);
	public int EndDoStuff(IAsyncResult asyncResult);
	}

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This was improved in .NET 2.0 with the introduction of the Event-Based Asynchronous pattern(focussing mainly on client applications) in combination with the SynchronizationContext simplifying the scheduling of work on the UI thread:

	class Handler
	{
	public int DoStuff(string arg);
	public void DoStuffAsync(string arg, object? userToken);
	public event DoStuffEventHandler? DoStuffCompleted;
	}
	public delegate void DoStuffEventHandler(object sender, DoStuffEventArgs e);

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In .NET 4.0 the System.Threading.Tasks.Task type was introduced, a data structure that represents the eventual completion of some asynchronous operation:

	Action<object> action = (object obj) =>
	{
	Console.WriteLine("Task={0}, obj={1}, Thread={2}",
	Task.CurrentId, obj,
	Thread.CurrentThread.ManagedThreadId);
	};
	// Create a task but do not start it.
	Task t1 = new Task(action, "alpha");
	// Launch t1
	t1.Start();

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And finally we arrived at async/await where we use the power of iterators to generate a state machine that handles all the continuations and callbacks for us. This allows us to write asynchronous code in a way that almost feel as synchronous simplifying the mental model and readability of our code:

	static async Task Main(string[] args)
	{
	Coffee cup = PourCoffee();
	Console.WriteLine("coffee is ready");
	var eggsTask = FryEggsAsync(2);
	var baconTask = FryBaconAsync(3);
	var toastTask = MakeToastWithButterAndJamAsync(2);
	var eggs = await eggsTask;
	Console.WriteLine("eggs are ready");
	var bacon = await baconTask;
	Console.WriteLine("bacon is ready");
	var toast = await toastTask;
	Console.WriteLine("toast is ready");
	Juice oj = PourOJ();
	Console.WriteLine("oj is ready");
	Console.WriteLine("Breakfast is ready!");
	}

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If you want to learn about all the details, check out the amazingly detailed post by Stephen Toub: How Async/Await Really Works in C# - .NET Blog (microsoft.com)

More information: Asynchronous programming - C# | Microsoft Learn