Multithreading requires careful programming. For most tasks, you can reduce complexity by queuing requests for execution by thread pool threads. This topic addresses more difficult situations, such as coordinating the work of multiple threads, or handling threads that block.
Deadlocks and Race Conditions
Multithreading solves problems with throughput and responsiveness, but in doing so it introduces new problems: deadlocks and race conditions.
Deadlocks
A deadlock occurs when each of two threads tries to lock a resource the other has already locked. Neither thread can make any further progress.
Many methods of the managed threading classes provide time-outs to help you detect deadlocks. For example, the following code attempts to acquire a lock on the current instance. If the lock is not obtained in 300 milliseconds, Monitor.TryEnter returns false.
if (Monitor.TryEnter(this, 300)) {
try {
// Place code protected by the Monitor here.
}
finally {
Monitor.Exit(this);
}
}
else {
// Code to execute if the attempt times out.
}
...
Consider the following guidelines when using multiple threads:
Don't use Thread.Abort to terminate other threads. Calling Abort on another thread is akin to throwing an exception on that thread, without knowing what point that thread has reached in its processing.
Don't use Thread.Suspend and Thread.Resume to synchronize the activities of multiple threads. Do use Mutex, ManualResetEvent, AutoResetEvent, and Monitor.
Don't control the execution of worker threads from your main program (using events, for example). Instead, design your program so that worker threads are responsible for waiting until work is available, executing it, and notifying other parts of your program when finished. If your worker threads do not block, consider using thread pool threads. Monitor.PulseAll is useful in situations where worker threads block.
Don't use types as lock objects. That is, avoid code such as lock(typeof(X)) in C# or SyncLock(GetType(X)) in Visual Basic, or the use of Monitor.Enter with Type objects. For a given type, there is only one instance of System.Type per application domain. If the type you take a lock on is public, code other than your own can take locks on it, leading to deadlocks. For additional issues, see Reliability Best Practices.
Use caution when locking on instances, for example lock(this) in C# or SyncLock(Me) in Visual Basic. If other code in your application, external to the type, takes a lock on the object, deadlocks could occur.
Do ensure that a thread that has entered a monitor always leaves that monitor, even if an exception occurs while the thread is in the monitor. The C# lock statement and the Visual Basic SyncLock statement provide this behavior automatically, employing a finally block to ensure that Monitor.Exit is called. If you cannot ensure that Exit will be called, consider changing your design to use Mutex. A mutex is automatically released when the thread that currently owns it terminates.
Do use multiple threads for tasks that require different resources, and avoid assigning multiple threads to a single resource. For example, any task involving I/O benefits from having its own thread, because that thread will block during I/O operations and thus allow other threads to execute. User input is another resource that benefits from a dedicated thread. On a single-processor computer, a task that involves intensive computation coexists with user input and with tasks that involve I/O, but multiple computation-intensive tasks contend with each other.
Read more: MSDN