C# 3.0 Cookbook, 3rd Edition

18.10. Performing Atomic Operations Among Threads

Problem

You are operating on data from multiple threads and want to insure that each operation is carried out fully before performing the next operation from a different thread.

Solution

Use the Interlocked family of functions to insure atomic access. Interlocked has methods to increment and decrement values, add a specific amount to a given value, exchange an original value for a new value, compare the current value to the original value, and exchange the original value for a new value if it is equal to the current value.

To increment or decrement an integer value, use the Increment or Decrement methods, respectively:

	int i = 0;
	long l = 0;
	Interlocked.Increment(ref i); // i = 1
	Interlocked.Decrement(ref i); // i = 0
	Interlocked.Increment(ref l); // l = 1
	Interlocked.Decrement(ref i); // l = 0

To add a specific amount to a given integer value, use the Add method:

	Interlocked.Add(ref i, 10); // i = 10;
	Interlocked.Add(ref l, 100); // l = 100;

To replace an existing value, use the Exchange method:

	string name = "Mr. Ed";
	Interlocked.Exchange(ref name, "Barney");

To check if another thread has changed a value out from under the existing code before replacing the existing value, use the CompareExchange method:

 int i = 0; double runningTotal = 0.0; double startingTotal = 0.0; double calc = 0.0; for (i = 0; i < 10; i++) { do { // Store of the original total startingTotal = runningTotal; // Do an intense calculation. calc = runningTotal + i * ...