Cover by Alex Davies

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Chapter 14. The Async Compiler Transform—in Depth

Async is implemented in the C# compiler with some help from the .NET framework base class libraries. The runtime itself didn’t need any changes to support async. That means await is implemented by a transformation to something that we could have written ourselves in earlier versions of C#. We can use a decompiler like .NET Reflector to take a look at the generated code.

As well as being interesting, understanding the generated code is helpful for debugging, performance analysis, and other diagnostics on async code.

The stub Method

The async method is replaced by a stub method. The first thing that happens when you call an async method is that the stub method runs. Let’s look at this simple async method as an example:

public async Task<int> AlexsMethod()
{
    int foo = 3;
    await Task.Delay(500);
    return foo;
}

The stub method generated by the compiler looks like this:

public Task<int> AlexsMethod()
{
    <AlexsMethod>d__0 stateMachine = new <AlexsMethod>d__0();
    stateMachine.<>4__this = this;
    stateMachine.<>t__builder = AsyncTaskMethodBuilder<int>.Create();
    stateMachine.<>1__state = -1;
    stateMachine.<>t__builder.Start<<AlexsMethod>d__0>(ref stateMachine);
    return stateMachine.<>t__builder.Task;
}

I’ve manually improved the names of the variables to make it easier to understand.

As we saw in Async, Method Signatures, and Interfaces, the async keyword has no effect on how the method is used from the outside. That becomes obvious when you see that the ...

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