AsyncLocal

Conveying Context with AsyncLocal

AsyncLocal

Represents ambient data that is local to a given asynchronous control flow, such as an asynchronous method.

It’s a static class that allows you to access a variable without having to pass it through the call stack.
It’s similar to CurrentPrincipal and CurrentCulture, and not quite the same as ThreadLocal.
It’s thread-safe.
It works well with the async/await paradigm.

For example, a middleware could set something into AsyncLocal, eg a correlation id or a user id, and then later in the request, something could read it. It’s accessed statically, so there’s no tight-coupling.

The magic happens in the property Value, which looks like a simple property, but is actually a method doing the magic.

You would use it in a static class, like this:

public static class Foo
{
    private static readonly AsyncLocal<string> _bar = new AsyncLocal<string>();

    public static string Bar
    {
        get => _bar.Value;
        set => _bar.Value = value;
    }
}

And then you can access it wherever you like, eg your middleware might do:

Foo.Bar = "baz";

and at some random point later you can read Foo.Bar

As I said, the magic is that, although it looks like a static, it’s actually scoped to the current request (asynchronous control flow)