C# Thread object lifetime

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Word “thread” could mean several things here:

  • System.Threading.Thread object (created by new Thread()),
  • CLR thread (managed thread),
  • OS thread (un-managed thread).

Thread object will be candidate for GC as soon as Start() method completes, because there are no more references to it.

Managed thread will stay alive while doSomeLengthyOperation() runs.

Quoting the article by James Kovacs, Microsoft MVP:

A managed thread’s lifetime is
independent of the Thread object that
creates it, a very good thing given
that you wouldn’t want the GC to
terminate a thread that was still
doing work simply because you lost all
references to the associated Thread
object. So the GC is collecting the
Thread object, but not the actual
managed thread.

The article also contains some helpful code samples if you want to experiment yourself.

Operating system thread, theoretically, has no one-to-one relationship with managed threads. From MSDN:

…a sophisticated host can use the CLR Hosting API to schedule many managed threads against the same operating system thread, or to move a managed thread between different operating system threads.

In practice, however, CLR thread maps directly to a Windows thread today.

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