How to know who kills my threads

Various people (including myself, here) pointed out that hosting a long-running thread in IIS is a bad idea. Your thread will being running inside an IIS 'worker process'. These processes are periodically terminated (recycled) by IIS, which will cause your thread to die.

I suggest that you try turning-off IIS worker process recycling to see if that makes a difference. You can find more information here.

Your thread probably just threw an exception. Try putting a try/catch block around DoSomethingForALongLongTime and see what it picks up.

Update: I didn't notice before that you were starting this from a web server. That can be a very bad idea. In particular, is the separate thread using any information derived from HttpContext.Current? That would include Request, Response, Session, etc., as well as any information from the page.

This is bad because these things only last as long as the request lasts. Once the request is over, they become invalid, to say the very least.

If you need to kick off a long-running thread from within a web application or web service, then you should create a simple Windows Service and host a WCF service within it. Have the web page then send all the information needed to perform the task to the service. The service can even use MSMQ as a transport, which will ensure that no messages are lost, even if the service gets busy.

A potential way to get more information: attach a debugger and break on thread termination. Depending on how your thread is being terminated, this might not work.

1. Download Debugging Tools for Windows if you don't already have it
2. Run windbg.exe, attach to your process
3. Break into windbg, type sxe et to enable breaking on thread exit
4. When the debugger breaks, inspect the state of the system, other threads, etc.
5. To get the managed stack, load sos.dll (.loadby sos mscorsvr, .loadby sos mscorwks, or .loadby sos clr should work), then run !clrstack (see !help for other sos commands)

If you get a lot of noise from other threads exiting, script windbg to continue after breaking if it's not the thread ID you care about.

Edit: If you think the thread is being terminated from within your process, you can also set a breakpoint on TerminateThread (bp kernel32!TerminateThread) and ExitThread (bp kernel32!ExitThread) to catch the stack of the killer.