Programming Visual Basic .NET, Second Edition

Race Conditions and Deadlocks

The .NET library provides sufficient thread support that you will rarely find yourself creating your own threads and managing synchronization manually.

Thread synchronization can be tricky, especially in complex programs. If you do decide to create your own threads, you must confront and solve all the traditional problems of thread synchronization, such as race conditions and deadlock.

Race Conditions

A race condition exists when the success of your program depends on the uncontrolled order of completion of two independent threads.

Suppose, for example, that you have two threads—one is responsible for opening a file and the other is responsible for writing to the file. It is important that you control the second thread so that it’s assured that the first thread has opened the file. If not, under some conditions the first thread will open the file, and the second thread will work fine; under other unpredictable conditions, the first thread won’t finish opening the file before the second thread tries to write to it, and you’ll throw an exception (or worse, your program will simply seize up and die). This is a race condition, and race conditions can be very difficult to debug.

You cannot leave these two threads to operate independently; you must ensure that Thread1 will have completed before Thread2 begins. To accomplish this, you might Join( ) Thread2 on Thread1. As an alternative, you can use a Monitor and Wait( ) for the appropriate conditions before ...