using ThreadStatic variables with async/await

Question

With the new async/await keywords in C#, there are now impacts to the way (and when) you use ThreadStatic data, because the callback delegate is executed on a different thre

Answer 1

Basically, I would emphasize: don't do that. [ThreadStatic] is never going to play nicely with code that jumps between threads.

But you don't have to. A Task already carries state - in fact, it can do it 2 different ways:

• there's an explicit state object, which can hold everything you need
• lambdas/anon-methods can form closures over state

Additionally, the compiler does everything you need here anyway:

private static async Task Start()
{
    string secret = "moo moo";
    Console.WriteLine("Started on thread [{0}]",
        Thread.CurrentThread.ManagedThreadId);
    Console.WriteLine("Secret is [{0}]", secret);

    await Sleepy();

    Console.WriteLine("Finished on thread [{0}]",
        Thread.CurrentThread.ManagedThreadId);
    Console.WriteLine("Secret is [{0}]", secret);
}

No static state; no issues with threads or multiple tasks. It just works. Note that secret is not just a "local" here; the compiler has worked some voodoo, like it does with iterator blocks and captured variables. Checking reflector, I get:

[CompilerGenerated]
private struct d__0 : IAsyncStateMachine
{
    // ... lots more here not shown
    public string 5__1;
}