What's the best design in .NET to use for sending data over an unreliable (3G) network connection?

Question

I'm re-designing an app I inherited that sends digital photos from a laptop to a web server. The idea is to take photos "out on the field" and have them instantly published on a web page (with some more fancy features).

Typical scenario
1. Photos are transferred from the camera to the laptop using standard USB.
2. The photos are processed in various ways. (Not important)
3. Each photo is POSTed in small pieces (~64 kb each) using a webrequest to a standard Apache web server where it's merged together again.

The problem with the current design is that it often hangs when the network connection is unreliable. As we're using a mobile network (3G) and often end up out of coverage, I need a way to handle this properly.

My question is whether there's a better solution for doing this that won't make the app hang when the connection drops every now and then.

(Bonus question is how this could be properly unit tested without having to take a hike with the laptop.)

EDIT 2008-11-24: I've now managed to set up a proper test environment for this using a combination of NetLimiter and TMnetsim (freeware). I tried setting 5 kb/sec and dropping 1% of all packets - my app still works well with the new design.

EDIT 2008-12-11: Just to update how I did this. I created one background worker (as suggested below) that is started whenever a camera is detected to copy the photos from the camera to PC. Then another background worker i started when files arrive on PC to upload using asynchronous HTTP transfer. It sure was a pain to get everything right, especially since the operation should be "cancellable" at any time... But anyhow, now it works. A big THANKS to everyone who helped me!

Answer 1

I'd avoid using HTTP at all from any thread that has UI unless you actually want to block until the response is received. You can try using the same logic from a background thread which will run as long as it needs to. Just be sure to have logic which will detect when the connection is lost (probably from a timeout) and will retry at a regular (but not frequent) interval until connecting again.

Your best bet would be creating some sort of background worker process which will upload the photos once they are saved to a dropbox directory on the device. I will say though that creating a .NET based background process is not trivial.

Answer 2

First find out why it's hanging - are the requests just sitting there? Do they time out? What happens if you lower the timeout setting?

Are you doing the POST from the UI thread? (Don't do that :)

You could potentially detect the connection dropping by making heartbeat requests with very short timeouts, too.