Chapter 20. Threads and Synchronization

Threads are relatively lightweight processes responsible for multitasking within a single application. The System.Threading namespace provides a wealth of classes and interfaces to manage multithreaded programming. The majority of programmers might never need to manage threads explicitly, however, because the Common Language Runtime (CLR) abstracts much of the threading support into classes that greatly simplify most threading tasks. For example, in Chapter 21 you will see how to create multithreaded reading and writing streams without resorting to managing the threads yourself.

The first part of this chapter shows you how to create, manage, and kill threads. Even if you don't create your own threads explicitly, you'll want to ensure that your code can handle multiple threads if it's run in a multithreading environment. This concern is especially important if you are creating components that might be used by other programmers in a program that supports multithreading. It is particularly significant to web services developers. Although web services (covered in Chapter 15) have many attributes of desktop applications, they are run on the server, generally lack a user interface, and force the developer to think about server-side issues such as efficiency and multithreading.

The second part of this chapter focuses on synchronization. When you have a limited resource, you may need to restrict access to that resource to one thread at a time. A classic analogy is to a restroom on an airplane. You want to allow access to the restroom for only one person at a time. This is done by putting a lock on the door. When passengers want to use the restroom, they try the door handle; if it is locked, they either go away and do something else, or they wait patiently in line with others who want access to the resource. When the resource becomes free, one person is taken off the line and given the resource, which is then locked again.

At times, various threads might want to access a resource in your program, such as a file. It might be important to ensure that only one thread has access to your resource at a time, and so you will lock the resource, allow a thread access, and then unlock the resource. Programming locks can be fairly sophisticated, ensuring a fair distribution of resources.