how does ASP.net “HttpResponse.IsClientConnected”

if HTTP is connection-less, how does ASP.net response property, HttpResponse.IsClientConnected detect client is connected or not?

回答1:

HTTP is not "connection-less" - you still need a connection to receive data from the server; more correctly, HTTP is stateless. Applications running on-top of HTTP will most likely actually be stateful, but HTTP itself is not.

"Connectionless" can also refer to a system using UDP as the transport instead of TCP. HTTP primarily runs over TCP and pretty much every real webserver expects, and returns, TCP messages instead of UDP. You might see HTTP-like traffic in UDP-based protocols like UPnP, but because you want your webpage to be delivered reliably, TCP will always be used instead of UDP.

As for IsClientConnected, when you access that property it calls into the current HttpWorkerRequest which is an abstract class implemented by the current host environment.

IIS7+ implements it such that if it previously received a TCP disconnect message (that sets a field) the method would now return false.
The ISAPI implementation (IIS 6) instead calls into a function within IIS that informs the caller if the TCP client on the current request/response context is still connected, though presumably it works on the same basis: when the webserver receives a TCP timeout, disconnect or connection-reset message it sets a flag and lets execution continue instead of terminating the response-generator thread.

Here's the relevant source code:

HttpResponse.IsClientConnected: http://referencesource.microsoft.com/#System.Web/HttpResponse.cs,80335a4fb70ac25f
IIS7WorkerRequest.IsClientConnected: http://referencesource.microsoft.com/#System.Web/Hosting/IIS7WorkerRequest.cs,1aed87249b1e3ac9
ISAPIWorkerRequest.IsClientConnected: http://referencesource.microsoft.com/#System.Web/Hosting/ISAPIWorkerRequest.cs,f3e25666672e90e8

回答2:

It all starts with an HTTP request. Inside it, you can, for example, spawn worker threads, that can outlive the request itself. Here is where IsClientConnected comes in handy, so that the worker thread knows that the client has already received the response and disconnected or not.