c# - Will every 'await' operator result in a state machine?

Question

Please consider the following code:

public async Task<string> GetString()
{
    //Some code here...
    var data = await A();
    //Some more code...
    return data;
}
private async Task<string> A()
{
    //Some code here..
    var data = await B();
    //manipulating data...
    return data;
}
private async Task<string> B()
{
    //Some code here..
    var data = await C();
    //manipulating data...
    return data;
}
private async Task<string> C()
{
    //Some code here..
    var data = await FetchFromDB();
    //manipulating data...
    return data;
}
private async Task<string> FetchFromDB()
{
    return await SOME_HTTP_REQUEST;
}

This code demonstrate a most basic functionality - nested async methods. Will every method get translate into a state machine? Or is the compiler sophisticated enough to generate a more efficient structure? In some of my projects, there are ~20 methods between the UI/WebAPI and the I/O call - does that affect the trade-off between the async-await overhead (such as the state machine) and the non-blocking thread benefits? I mean, if, for example, the overhead of 4 state machines (4 nested async methods) equals to 50ms of blocking I/O (in terms of trade-off), will 20 state machine be equal to longer I/O's delay (250ms)?

Answer 1

await doesn't matter in this case. Each async method will generate a state machine (even if it has no awaits at all).

You can see that with this TryRoslyn example.

If you have cases where a state machine isn't needed, where the method doesn't really need to be async like this one for example:

private async Task<string> D()
{
    var data = await FetchFromDB();
    return data;
}

You can remove the async keyword and the state machine that comes with it:

private Task<string> D()
{
    return FetchFromDB();
}

But otherwise, you actually need the state machine and an async method can't operation without it.

You should note that the overhead is quite small and is usually negligible compared to the resources saved by using async-await. If you realize that's not the case (by testing) you should probably just make that operation synchronous.