In the previous lessons, we've covered the basic ground work for threading both as a background thread (in lesson 2) and as a new foreground thread (in lesson 1). In this lesson, I'll begin discussing one of my favorite ways of threading available to developers in the .NET framework, namely, the ThreadPool.
Essentially, the ThreadPool is what its name says it is a pool of Threads. But the ThreadPool is really a smart object; As soon as one of the ThreadPool [TP] threads completes its work/tasks, the thread expires and is returned to the TP where it is recycled back into the pool as a prospective new thread available to do your background bidding. The number of threads for the process is usually fixed depending on the amount of memory available, and the needs of the application. However, it might be possible to increase the number of available threads. There is always one and only one TP available to any process.
If you're a Win32 veteran you will realize that there is quite a difference between Multi-Threaded applications there and ones in the CLR. In Win32 [Multi-Threaded] applications, each process was made up of a group of threads and each thread consisted of TLS (Thread Local Storage) and Call Stacks to provide time slices for single processor computers. Single processor machines allot time slices for each thread to execute based on the thread priority. When the time slice for a particular thread has reached completion, the thread is suspended and another thread is allowed to begin its processing. CLR Multi-Threading is, again, different. For each task queued in the thread pool (work items), the CLR assigns a thread from the pool (a worker thread) and then recycles the thread back into the pool once the thread's works has been completed.
The TP has a predefined size of 25 threads. Although this can be changed, I've found through practical experience that this number is usually more than sufficient for the majority of applications. In fact, I personally have never needed something larger, though I can envision Server class applications that would need threads in the thousands.
Getting the TP to start a new thread processing is simple really. You first need to define a WaitCallBack, which in the TP's sense really means a delegate that will be the entry point for the worker thread's allotted tasks.
The QueueUserWorkItem method has two prototypes:
- Public Shared Function QueueUserWorkItem (callBack As WaitCallback, state As Object) As Boolean
- Public Shared Function QueueUserWorkItem (callBack As WaitCallback ) As Boolean
The first method accepts an Object that contains any data that might pertain to your worker thread's needs. The example I have provided uses this overload.
[Side Note: Don't forget you can box anything in an Object type… you simply need to cast it back to its original typeJ]
The example I'm providing is quite simple really. It spawns several threads from the TP and emulates a process by looping through a sleep and wake up mechanism. For the newbies, please note that the main thread effectively finished processing, but is 'stuck' at the Console.ReadLine() before the module exits. This is important because if you remove it (try it) the program will exit even though there are other threads (TP threads) still running. The reason is because TP threads are background threads.
Salam
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Welcome’s receive a look at how the software works.
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