What does 'context' exactly mean in C# async/await code?

Question

Lets looks at some simple C# async/await code where I have an object reference (obj) before and after an await with ConfigureAwait(false)

private async Task<SomeObject> AnAsyncLibraryMethod(SomeObject obj)
{
    Console.WriteLine(Thread.CurrentThread.ManagedThreadId);
    obj.Name = "Harry"; // <-- obj here

    // MAIN POINT
    var newSubObj = await FetchOverHttpAsync().ConfigureAwait(false);

    // Continuation here
    Console.WriteLine(Thread.CurrentThread.ManagedThreadId);
    obj.Name = "Sally"; // <-- same obj here
    return obj;
}

public class SomeObject { public string Name; }

ConfigureAwait(false) seems to means NOT marshal the continuation back to the original context captured - ok, but what does that really mean? I've tried the code above and obj IS correctly referenced back (even when it resumes on a different thread).

So the "context" doesn't appear to be the thread's working memory (i.e. thread local storage). So what does "context" contain? Therefore, what does it really mean to

marshal the continuation back to the original context captured

Answer 1

If I'm not totally wrong, ConfigureAwait(false); only means that the code which runs after the code you are awaiting, is not required to use the SynchronizationContext from before the await.

SynchronizationContext can be different things, as Stephen pointed out. Imagine you are in a web environment and your code after the await relies on HttpContext.Current.Items: this might not work anymore if you set ConfigureAwait(false);.

The following code in an MVC controller would throw an exception, for example:

public async Task<ActionResult> Index()
{
    System.Web.HttpContext.Current.Items["test"] = "test";

    var result = await SomethingAsync();
            
    return View();
}

private async Task<object> SomethingAsync()
{
    await Task.Delay(1000).ConfigureAwait(false);

    // this will throw a nullpointer if ConfigureAwait is set to false
    return System.Web.HttpContext.Current.Items["test"];
}

Your variable though is simply in the scope of the method and therefor it will be available, basically the method closure/scope, if this makes sense?