(Post 02/10/2007) C# supports parallel execution of code through multithreading. A thread is an independent execution path, able to run simultaneously with other threads.

A C# program starts in a single thread created automatically by the CLR and operating system (the "main" thread), and is made multi-threaded by creating additional threads. Here's a simple example and its output:

All examples assume the following namespaces are imported, unless otherwise specified:

using System;
using System.Threading;

class ThreadTest {
static void Main() {
Thread t = new Thread (WriteY);
t.Start(); // Run WriteY on the new thread
while (true) Console.Write ("x"); // Write 'x' forever
}

static void WriteY() {
while (true) Console.Write ("y"); // Write 'y' forever
}
}

The main thread creates a new thread t on which it runs a method that repeatedly prints the character y. Simultaneously, the main thread repeatedly prints the character x.

The CLR assigns each thread its own memory stack so that local variables are kept separate. In the next example, we define a method with a local variable, then call the method simultaneously on the main thread and a newly created thread:

static void Main() {
new Thread (Go).Start(); // Call Go() on a new thread
Go(); // Call Go() on the main thread
}

static void Go() {
// Declare and use a local variable - 'cycles'
for (int cycles = 0; cycles < 5; cycles++) Console.Write ('?');
}

A separate copy of the cycles variable is created on each thread's memory stack, and so the output is, predictably, ten question marks.

Threads share data if they have a common reference to the same object instance. Here's an example:

class ThreadTest {
bool done;

static void Main() {
ThreadTest tt = new ThreadTest(); // Create a common instance
new Thread (tt.Go).Start();
tt.Go();
}

// Note that Go is now an instance method
void Go() {
if (!done) { done = true; Console.WriteLine ("Done"); }
}
}

Because both threads call Go() on the same ThreadTest instance, they share the done field. This results in "Done" being printed once instead of twice:

Static fields offer another way to share data between threads. Here's the same example with done as a static field:

class ThreadTest {
static bool done; // Static fields are shared between all threads

static void Main() {
new Thread (Go).Start();
Go();
}

static void Go() {
if (!done) { done = true; Console.WriteLine ("Done"); }
}
}

Both of these examples illustrate another key concept – that of thread safety (or, rather, lack of it!) The output is actually indeterminate: it's possible (although unlikely) that "Done" could be printed twice. If, however, we swap the order of statements in the Go method, then the odds of "Done" being printed twice go up dramatically:

static void Go() {
if (!done) { Console.WriteLine ("Done"); done = true; }
}

The problem is that one thread can be evaluating the if statement right as the other thread is executing the WriteLine statement – before it's had a chance to set done to true.

The remedy is to obtain an exclusive lock while reading and writing to the common field. C# provides the lock statement for just this purpose:

class ThreadSafe {
static bool done;
static object locker = new object();

static void Main() {
new Thread (Go).Start();
Go();
}

static void Go() {
lock (locker) {
if (!done) { Console.WriteLine ("Done"); done = true; }
}
}
}

When two threads simultaneously contend a lock (in this case, locker), one thread waits, or blocks, until the lock becomes available. In this case, it ensures only one thread can enter the critical section of code at a time, and "Done" will be printed just once. Code that's protected in such a manner – from indeterminacy in a multithreading context – is called thread-safe.

Temporarily pausing, or blocking, is an essential feature in coordinating, or synchronizing the activities of threads. Waiting for an exclusive lock is one reason for which a thread can block. Another is if a thread wants to pause, or Sleep for a period of time:

Thread.Sleep (TimeSpan.FromSeconds (30)); // Block for 30 seconds

A thread can also wait for another thread to end, by calling its Join method:

Thread t = new Thread (Go); // Assume Go is some static method
t.Start();
t.Join(); // Wait (block) until thread t ends

A thread, while blocked, doesn't consume CPU resources.

How Threading Works

Multithreading is managed internally by a thread scheduler, a function the CLR typically delegates to the operating system. A thread scheduler ensures all active threads are allocated appropriate execution time, and that threads that are waiting or blocked – for instance – on an exclusive lock, or on user input – do not consume CPU time.

On a single-processor computer, a thread scheduler performs time-slicing – rapidly switching execution between each of the active threads. This results in "choppy" behavior, such as in the very first example, where each block of a repeating X or Y character corresponds to a time-slice allocated to the thread. Under Windows XP, a time-slice is typically in the tens-of-milliseconds region – chosen such as to be much larger than the CPU overhead in actually switching context between one thread and another (which is typically in the few-microseconds region).

On a multi-processor computer, multithreading is implemented with a mixture of time-slicing and genuine concurrency – where different threads run code simultaneously on different CPUs. It's almost certain there will still be some time-slicing, because of the operating system's need to service its own threads – as well as those of other applications.

A thread is said to be preempted when its execution is interrupted due to an external factor such as time-slicing. In most situations, a thread has no control over when and where it's preempted.

Threads vs. Processes

All threads within a single application are logically contained within a process – the operating system unit in which an application runs.

Threads have certain similarities to processes – for instance, processes are typically time-sliced with other processes running on the computer in much the same way as threads within a single C# application. The key difference is that processes are fully isolated from each other; threads share (heap) memory with other threads running in the same application. This is what makes threads useful: one thread can be fetching data in the background, while another thread is displaying the data as it arrives.

When to Use Threads

A common application for multithreading is performing time-consuming tasks in the background. The main thread keeps running, while the worker thread does its background job. With Windows Forms applications, if the main thread is tied up performing a lengthy operation, keyboard and mouse messages cannot be processed, and the application becomes unresponsive. For this reason, it’s worth running time-consuming tasks on worker threads even if the main thread has the user stuck on a “Processing… please wait” modal dialog in cases where the program can’t proceed until a particular task is complete. This ensures the application doesn’t get tagged as “Not Responding” by the operating system, enticing the user to forcibly end the process in frustration! The modal dialog approach also allows for implementing a "Cancel" button, since the modal form will continue to receive events while the actual task is performed on the worker thread. The BackgroundWorker class assists in just this pattern of use.

In the case of non-UI applications, such as a Windows Service, multithreading makes particular sense when a task is potentially time-consuming because it’s awaiting a response from another computer (such as an application server, database server, or client). Having a worker thread perform the task means the instigating thread is immediately free to do other things.

Another use for multithreading is in methods that perform intensive calculations. Such methods can execute faster on a multi-processor computer if the workload is divided amongst multiple threads. (One can test for the number of processors via the Environment.ProcessorCount property).

A C# application can become multi-threaded in two ways: either by explicitly creating and running additional threads, or using a feature of the .NET framework that implicitly creates threads – such as BackgroundWorker, thread pooling, a threading timer, a Remoting server, or a Web Services or ASP.NET application. In these latter cases, one has no choice but to embrace multithreading. A single-threaded ASP.NET web server would not be cool – even if such a thing were possible! Fortunately, with stateless application servers, multithreading is usually fairly simple; one's only concern perhaps being in providing appropriate locking mechanisms around data cached in static variables.

When Not to Use Threads

Multithreading also comes with disadvantages. The biggest is that it can lead to vastly more complex programs. Having multiple threads does not in itself create complexity; it's the interaction between the threads that creates complexity. This applies whether or not the interaction is intentional, and can result long development cycles, as well as an ongoing susceptibility to intermittent and non-reproducable bugs. For this reason, it pays to keep such interaction in a multi-threaded design simple – or not use multithreading at all – unless you have a peculiar penchant for re-writing and debugging!

Multithreading also comes with a resource and CPU cost in allocating and switching threads if used excessively. In particular, when heavy disk I/O is involved, it can be faster to have just one or two workers thread performing tasks in sequence, rather than having a multitude of threads each executing a task at the same time. Later we describe how to implement a Producer/Consumer queue, which provides just this functionality.

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