Compiler transformation into state machine

Context: The C# compiler transforms async methods into a state machine. This allows the method to suspend and resume at await points without blocking.

using System;
using System.Threading.Tasks;

public class Example
{
    public async Task ExampleAsync()
    {
        Console.WriteLine("Start");
        await Task.Delay(1000);
        Console.WriteLine("Resumed");
    }
}

The compiler generates a struct with states (0 = before first await, 1 = after, etc.), preserving local variables.

Why this matters

• No threading magic: the method runs on the original synchronization context after the await.
• Efficient: no extra thread is created for waiting.

Real-world usage example

UI applications: In WPF or WinForms, await returns to the UI thread automatically (if not using ConfigureAwait(false)).

Example: Async in depth explains the state machine.

Related sections

• 9.2 async and await
• 9.6 ConfigureAwait false