07 November 2012

Cooperative Thread Abort in .NET

Did you know that .NET uses a cooperative thread abort mechanism?

As someone coming from a C++ background I always thought that killing a thread is bad behavior and should be prevented at all costs. When you terminate a Win32 thread it can interrupt the thread in any state at every machine instruction so it may leave corrupted data structures behind.
A .NET thread cannot be terminated. Instead you can abort it. This is not just a naming issue, it is indeed a different behavior. If you call Abort() on a thread it will throw a ThreadAbortException in the aborted thread. All active catch and finally blocks will be executed before the thread gets terminated eventually. In theory, this allows the thread to do a proper cleanup. In reality this works only if every line of code is programmed in a way that it can handle a ThreadAbortException
in a proper way. And the first time you call 3rd-party code not under your control you are doomed.

Too make the situation more complex there are situations where throwing the ThreadAbortException is delayed. In versions prior to .NET 4.5 this was poorly documented but in the brand new documentation of the Thread.Abort method is very explicit about this. A thread cannot be aborted inside a catch or finally block or a static constructor (and probably not the initialization of static fields also) or any other constrained execution region.

Why is this important to you?

Well, if you are working in an ASP.NET/IIS environment the framework itself will call Abort() on threads which are executing too long. In this way the IIS can heal itself if some requests hit bad blocking code like endless loops, deadlocks or waiting on external requests. But if you were unlucky enough to implement your blocking code inside static constructors, catch or finally blocks your requests will hang forever in your worker process. It will look like the httpRuntime executionTimeout is not working and only a iisreset will cure the situation.

Download the CooperativeThreadAbortDemo sample application.