I Was reading random questions and answers here on SO and came across this question:
C#, IAsyncResult and the thread pool
The question is asked is X method using the thread pool or using normal threads.
What's the difference between using the thread pool and a normal thread?
A threadpool creates threads and assigsn work to a free thread. This way creating and disposing threads for every work item is preventend, since creating and disposing threads is a relativly costly operation.
The ThreadPool holds a number of threads that are ready for you to use, which may eliminate the cost of creating new threads, which is what happens when you create a normal thread.
The threadpool is typically suitable for short running tasks. It has the limitation that it is a application-wide limited resource (25 per CPU), and there are a lot of internal classes that use the threadpoool, so if you perform a lot of long running tasks you will use up all the threads.
For long running tasks, it's better to use a manually created thread, with its background property set to true.
Note: In the .NET Framework version 2.0, the Thread.CurrentPrincipal property value is propagated to worker threads queued using the QueueUserWorkItem method. In earlier versions, the principal information is not propagated.
When Not to Use Thread Pool Threads
There are several scenarios in which it is appropriate to create and manage your own threads instead of using thread pool threads:
You require a foreground thread(!).
You require a thread to have a particular priority.
You have tasks that cause the thread to block for long periods of time. The thread pool has a maximum number of threads, so a large number of blocked thread pool threads might prevent tasks from starting.
You need to place threads into a single-threaded apartment. All ThreadPool threads are in the multithreaded apartment.
You need to have a stable identity associated with the thread, or to dedicate a thread to a task.
One big difference is that Unhandled exceptions on thread pool threads terminate the process; with these three exceptions:
A ThreadAbortException is thrown in a thread pool thread, because Abort was called.
An AppDomainUnloadedException is thrown in a thread pool thread, because the application domain is being unloaded.
The common language runtime or a host process terminates the thread.
Some good references are:
Programming the Thread Pool in the .NET Framework
Update: in response to comments. You can use the GetAvailableThreads method to determine the actual number of threads in the thread pool at any given time. ThreadPool.GetMaxThreads is a different quantity. Its the max allowed in the pool before requests are queued, not the actual number of threads currently in the pool.
The difference is not between the threads themselves as they will behave the same, the difference is in who manages the lifetime of a thread, and how you use them.
The threadpool in .net is a pool of threads which will grow or shrink, when you queue somethign up for the thread pool to process, it will determine if it needs to start a new thread or will reuse an existing one. You don't have to explicitly create threads.
This is nice but the downside of this there is a limited number of threads available in the threadpool, so if you queue up a long running task this might have a negative impact on your application, since you could be using up a threadpool thread that something else might want to use.
See this article about the thread pool