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c#asp.net-mvc-4asynchronousasync-await
Herman Kan
Herman Kan

Reputation: 2292

How to make HttpContext available in Task called synchronously?

In my ASP.NET MVC application, I need to implement/override some intrinsic entities like IModelBinder.BindModel(), IActionFilter.OnActionExecuting(), etc. Those methods are calling an async method, but obviously I cannot turn them into async Task<> to be able to use the await keyword.

So I wrote the following "adapter":

public static T GetResult<T>(Func<Task<T>> func)
{
    var httpContext = HttpContext.Current;

    var proxyTask = Task.Run(() =>
    {
        HttpContext.Current = httpContext;
        return func();
    });

    return proxyTask.Result;
}

which lets me call my async GetData()in a synchronous manner:

public static async Task<Data> GetData()
{
    if (isCached)
        return GetCachedData();

    var data = await GetOriginalData();
    SetCachedData(data);
    return data;
}

...

var data = GetResult(() => GetData());

Okay, it works, so I would call it a day, however as you can see, most of the time, GetData() runs synchronously, therefore spawning a new thread unconditionally is not good performance-wise. That was bugging me, so I have ended up with a different solution:

public static T GetResult<T>(Func<Task<T>> func)
{
    var syncContext = SynchronizationContext.Current;
    SynchronizationContext.SetSynchronizationContext(null);

    var task = func();

    SynchronizationContext.SetSynchronizationContext(syncContext);

    return task.Result;
}

It also works, however the issue is that since SynchronizationContext is no longer flowing in, there is no HttpContext available in the called method.

I could probably work that around by putting HttpContext.Current into logical CallContext but am still wondering if there is a better way to tackle the issue? Something like customizing SynchronizationContext.Current.CreateCopy().

Upvotes: 1

Views: 1151

Answers (1)

Stephen Cleary
Stephen Cleary

Reputation: 456322

spawning a new thread unconditionally

Actually, just borrowing a thread briefly from the thread pool. Not nearly as bad as starting a new thread, but still not the best performance-wise.

It also works, however the issue is that since SynchronizationContext is no longer flowing in, there is no HttpContext available in the called method.

Are you sure? I would expect HttpContext.Current to be set at the beginning of GetData for both ways to call it. I would also expect HttpContext.Current to be null after the await for both ways to call it. If it's not null after await in your testing, it's probably because the continuation just happened to end up on the same thread.

This is one of the main problems with trying to use HttpContext.Current outside of the request context (SynchronizationContext). When you place it on a bare thread (Task.Run), it will just stay there until that thread enters another request context.

I could probably work that around by putting HttpContext.Current into logical CallContext

I would recommend pulling out whatever data you need from Current while on the original request context, and then passing that data in explicitly to the methods that need it.

Regarding the original problem (avoiding the extra thread in the synchronous case), I'd recommend just adding a synchronous TryGetData method.

Upvotes: 3

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