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C# Volatile Example

Volatile, a modifier, reduces concurrency issues. Compilers reorder instructions in blocks. This improves performance but causes problems for concurrent threads. Optimizations such as code motion can cause these problems.

Lock

Example. Before we begin, please note that this example isn't ideal in that it would function correctly without the volatile modifier. It serves only to illustrate the concept of the volatile keyword, not to provide a real-world example.

Next: In the example, there are two static fields. One is an object reference of string type. The other is a volatile bool value type.

Static FieldStatic String

Then: In the Main method, a new thread is created, and the SetVolatile method is invoked. In SetVolatile, both the fields are set.

ThreadStart
C# program that uses volatile field

using System;
using System.Threading;

class Program
{
    static string _result;
    static volatile bool _done;

    static void SetVolatile()
    {
	// Set the string.
	_result = "Dot Net Perls";
	// The volatile field must be set at the end of this method.
	_done = true;
    }

    static void Main()
    {
	// Run the above method on a new thread.
	new Thread(new ThreadStart(SetVolatile)).Start();

	// Wait a while.
	Thread.Sleep(200);

	// Read the volatile field.
	if (_done)
	{
	    Console.WriteLine(_result);
	}
    }
}

Output

Dot Net Perls

Why is the bool volatile? First, the logic in this program is correct even without the volatile modifier on the bool. But compilers introduce extra, hidden complexity. They use a lot of optimizations.

And: Some of these optimizations reorder loads and stores from variables. So conceptually the program could be incorrect.

The order of assignments could change. When multiple threads execute at once, this can cause serious problems. Please keep in mind that the volatile modifier does not force synchronization of loads and stores.

Instead, it simply tells the compiler not to change the order of accesses to the field. By eliminating reordering optimizations, the code becomes more predictable from a programmer's perspective.


Flags. The example, adapted from the C# specification, demonstrates the concept of a volatile field used as a flag. The bool type is commonly used as a flag. So when the flag is set to true, you can take other action on that variable.

Notice: Thanks to Robert Paveza for writing in with a correction. It originally contained an incorrect statement.


Performance. Can volatile fields cause a performance problem if they are used too much? In some cases, this may happen. It is probably a better idea not to use volatile in your programs at all.

However, if you are using a volatile field that is accessed in a tight loop millions of times, you can copy it into a regular local variable and then use that local variable. This will provide a performance boost.

Local Variable Field

Summary. The volatile modifier provides a way for you to restrict the optimizations the compiler makes regarding loads and stores into the field. You will typically use volatile in multithreaded programs and with flag variables.