Asynchronous Code – Behind the Scenes – 002

In the earlier part of this series, we reviewed the generic structure of decompiled async code, especially the stub method. In this part, we would continue our explore of async code and look into the State Machine. We would not delve deep into the most important MoveNext() method yet, we will first familiar with the different parts of the State Machine first.

State Machine

Let us go back to the ILSpy and see how State Machine looks like.

[StructLayout(LayoutKind.Auto)]
[CompilerGenerated]
private struct <Foo>d__1 : IAsyncStateMachine
{
	public int <>1__state;

	public AsyncTaskMethodBuilder <>t__builder;

	public int delay;

	private TaskAwaiter <>u__1;

	private void MoveNext()
	{
		// To be discussed later
	}

	void IAsyncStateMachine.MoveNext()
	{
		//ILSpy generated this explicit interface implementation from .override directive in MoveNext
		this.MoveNext();
	}

	[DebuggerHidden]
	private void SetStateMachine(IAsyncStateMachine stateMachine)
	{
		<>t__builder.SetStateMachine(stateMachine);
	}

	void IAsyncStateMachine.SetStateMachine(IAsyncStateMachine stateMachine)
	{
		//ILSpy generated this explicit interface implementation from .override directive in SetStateMachine
		this.SetStateMachine(stateMachine);
	}
}

• IAsyncStateMachine Interface

One of the first things we would notice is the implementation of IAsyncStateMachine interface. The IAsyncStateMachine interface, which is defined under the System.Runtime.CompilerServices namespace, represents the state machine generated for the async method. The interface itself is a simple one, with just two methods in it.

public interface IAsyncStateMachine
{
    /// <summary>Moves the state machine to its next state.</summary>
    void MoveNext();
    /// <summary>Configures the state machine with a heap-allocated replica.</summary>
    /// <param name="stateMachine">The heap-allocated replica.</param>
    void SetStateMachine(IAsyncStateMachine stateMachine);
}

The MoveNext() as explained earlier, represents the heart of asynchronous code. We would, for time being, delay visiting the method for a bit longer. However, the key point to remember at this point of time is that each time the State Machine is starts or resumes (after a pause), the MoveNext() method would be called. The SetStateMachine() method associates the builder with the specific state machine.

The importance of the implementation of the interface and how it binds the state machine with the stub method could be understood by looking at the signature of the AsyncTaskMethodBuilder.Start(). The method accepts a single generic parameter, which has a constraint of having implemented the IAsyncStateMachine.

public void Start<TStateMachine>(ref TStateMachine stateMachine) where TStateMachine : IAsyncStateMachine
{
	if (stateMachine == null)
	{
		throw new ArgumentNullException("stateMachine");
	}
	ExecutionContextSwitcher ecsw = default(ExecutionContextSwitcher);
	RuntimeHelpers.PrepareConstrainedRegions();
	try
	{
		ExecutionContext.EstablishCopyOnWriteScope(ref ecsw);
		stateMachine.MoveNext();
	}
	finally
	{
		ecsw.Undo();
	}
}

We would not go too deep into AsyncTaskMethodBuilder.Start(), but key take away would be

• The constraint applied to parameter where TStateMachine : IAsyncStateMachine
• The method is responsible for calling IAsyncStateMachine.MoveNext()

There is another interesting fact to this look at this point. The generated State Machine has a small but significant difference depending on whether your are looking at debug/release mode code. When in release mode, the compiler optimizes the code and creates a stuct based State Machine, while in debug mode, it creates a class. This is supposed to an optimization done to so that the compiler would skip allocating memory when the awaitable has already been completed awaited. The following code displays the State Machine when decompiled in debug mode.

[CompilerGenerated]
private sealed class <Foo>d__1 : IAsyncStateMachine
{
	public int <>1__state;

	public AsyncTaskMethodBuilder <>t__builder;

	public int delay;

	private TaskAwaiter <>u__1;

	private void MoveNext()
	{
		// To be discussed later
	}

	void IAsyncStateMachine.MoveNext()
	{
		//ILSpy generated this explicit interface implementation from .override directive in MoveNext
		this.MoveNext();
	}

	[DebuggerHidden]
	private void SetStateMachine(IAsyncStateMachine stateMachine)
	{
	}

	void IAsyncStateMachine.SetStateMachine(IAsyncStateMachine stateMachine)
	{
		//ILSpy generated this explicit interface implementation from .override directive in SetStateMachine
		this.SetStateMachine(stateMachine);
	}
}

• Fields

public int <>1__state;
public AsyncTaskMethodBuilder <>t__builder;
public int delay;
private TaskAwaiter <>u__1;

The next thing one would notice with the generated code is the presence of certains fields in the state machine. The fields could be broadly categorized into

• Current State : As discussed in earlier post, this could have any of the following values

-1 : Not Started
-2 : Completed
Any other Value : Paused

• Method Builder : Communicates with the async infrastructure and returns the task
• TaskAwaiter
• Parameters and local variables
• Temporary Stack Variables

TaskAwaiter and parameters are used to remember the values when the State Machine resumes after a Pause. If the state machine requires a variable which it doesn’t need to remember after resuming, then it remains as private variable.

Temporary stack variables are used as a part of larger expression, when the compiler needs to remember intermediate results. For example,

int result = x + y + await task;

The most important point to remember about the Fields and Variables is that the compiler ensures it uses minimum fields/variables as possible by reusing them.

If your code have multiple await that was supposed to return

• 3 Task<int>
• 2 Task<string>
• 1 Task

Then the compiler would most likely create just 3 awaiters, one each for the different types involved.

That is it about the general structure of the State Machine. We would now proceed to the most important part, which is of course the MoveNext() method. We will do it in the next post.