Is there some way to handle async/await behind an ASMX service?

Question

I have a web app serving a WCF REST API for JSON and an ASMX web service. The application has been around for a few years. It's based on ASP.NET 2.0, but upgraded to .NET 4.0 a couple of years ago, and I just upgraded to .NET 4.5 to be able to use the new async framework.

Behind the application are some legacy services, and I realized that there is a big potential for increasing the performance by going async. I have implemented async all the way through the application, and everything is working perfectly through the WCF REST API.

Too late I discovered that the ASMX API fails, I wanted methods like this:

[WebMethod(Description = "Takes an internal trip ID as parameter.")]
async public Task<Trip> GetTrip(int tripid)
{
    var t = await Trip.GetTrip(tripid);
    return t;
}

I then learned that async/await isn't supported in ASMX at all, and everybody advises to migrate to WCF. I am not too joyful about this. The ASMX (actually three of them) are stuffed with different methods, and there are loads of API consumers that we want to keep serving from the old API.

But we need the increased performance! Does anybody know about a workaround so I can keep using async/await behind the ASMX, but expose the ASMX as before?

Solution

It may be possible to do this, but it would be a bit awkward. ASMX supports APM-style asynchronous methods, and you can convert TAP to APM (however, note that the MSDN example on that page does not propagate exceptions correctly).

I have an example on my blog that shows how to wrap TAP implementations in APM (with exception propagation that keeps the correct exception type but loses the stack; see ExceptionDispatchInfo for fully correct exception propagation). I used this for a while when WCF only supported APM. A very similar approach should work for ASMX.

However, note that you will have to target 4.5 (i.e., httpRuntime.targetFramework) for async/await to work as expected.

Comments

[usr]

You understand that async is only faster under special circumstances? Might well be slower. Research this first.

[Paddy]

possible duplicate of Calling Task-based methods from ASMX

[Robert Taylor]

using .Wait() will cause the WebMethod to block, negating any benefit of being async in the first place.

[Haris Munawar]

Hi Stephen, Will wrapping an EAP into Task will be a fake asynchrony? e.g. as you suggested here

[Stephen Cleary]

@HarisMunawar: No. "Fake asynchrony" is blocking a thread pool thread in order to provide an asynchronous API. EAP is asynchronous, so there's no thread pool thread blocked.