Start a task with clean AsyncLocal state - asp.net-core

Question

In ASP.NET Core, I am using IHttpContextAccessor to access the current HttpContext. HttpContextAccessor uses AsyncLocal<T>.
In one situation, I am trying to start a Task from within a request, that should continue running after the request has ended (long running, background task), and I am trying to detach it from the current execution context so that IHttpContextAccessor returns null (otherwise I am accessing an invalid HttpContext).
I tried Task.Run, Thread.Start, but every time, the context seems to carry over and IHttpContextAccessor returns the old context whatever I try (sometimes even contexts from other requests 🤔).
How can I start a task that will have a clean AsyncLocal state so that IHttpContextAccessor returns null?

Answer 1

Late answer but might be useful for someone else. You can do this by suppressing the ExecutionContext flow as you submit your task:
ExecutionContext.SuppressFlow();
var task = Task.Run(async () => {
await LongRunningMethodAsync();
});
ExecutionContext.RestoreFlow();
or better:
using (ExecutionContext.SuppressFlow()) {
var task = Task.Run(async () => {
await LongRunningMethodAsync();
});
}
This will prevent the ExecutionContext being captured in the submitted task. Therefore it will have a 'clean' AsyncLocal state.
As above though I wouldn't do this in ASP.net if it's CPU intensive background work.

Answer 2

You could await the long running task and set HttpContext to null inside await. It would be safe for all code outside the task.
[Fact]
public async Task AsyncLocalTest()
{
_accessor = new HttpContextAccessor();
_accessor.HttpContext = new DefaultHttpContext();
await LongRunningTaskAsync();
Assert.True(_accessor.HttpContext != null);
}
private async Task LongRunningTaskAsync()
{
_accessor.HttpContext = null;
}
You could run a separate thread, it doesn't matter.
Task.Run(async () => await LongRunningTaskAsync());
Just make the task awaitable to cleanup HttpContext for the task's async context only.