Threading & .NET

Multitasking in the .NET Framework

The Thread class in the .NET Framework provides support for running threads in applications. The class contains methods that seem to answer the need for pausing, resuming, and terminating threads. These methods should be used sparingly as they can have unforeseen consequences. To understand why, it is important to understand the two types of multitasking:

• • Cooperative multitasking requires that each thread periodically check its state to determine whether execution should continue or not. It is "cooperative" because each thread must cooperate by checking its state; otherwise, the multitasking will start to degrade. Early versions of Microsoft Windows used this form of multitasking.
• • Preemptive multitasking does not give a thread any choice in determining how to execute. The thread subsystem is responsible for determining when and how long to execute a thread. Preemptive multitasking, used in all current versions of Windows, is more reliable, but requires that you deal with multitasking issues like accessing shared resources.

Thread.Suspend follows the preemptive model. This method simply suspends the thread regardless of what it is doing at the time. Because the thread is not notified of the pause, it may be in an unstable state. Thread.Suspend should not be used to pause threads. Instead, a cooperative model should be used. Thread.Suspend has been deprecated in .NET 2.0.

Thread.Resume is the complement of Thread.Suspend. Since it has no effect without Thread.Suspend, it should not be called. It has been deprecated in .NET 2.0.

Finally, Thread.Abort terminates a thread using the preemptive model. Like Thread.Suspend, this can cause problems. The ThreadAbortException is raised on the thread when Thread.Abort is called. Provided the thread handles this exception, it can ensure that it cleans up any resources. However, this is like hitting the power switch to turn off your computer. It is harsh and can be avoided through proper cooperative multitasking.