Volatile variables and CSharp threads

seagull The volatile keyword is a convenience keyword for those who need to access member variables of a class or structure in multi-threaded conditions.

Again, since this is an advanced CSharp concept this is probably something that most of you will not need to worry about using, especially in ASP.NET.  However, there have been times when I’ve used multi-threading in an ASP.NET application (for screen scraping performance) so it is not completely out of the realm of possibility for you to need to know something about how to do multi-threaded programming.  If you do, you’ll be glad you learned about the volatile keyword.

The purpose of the volatile keyword is to tell the compiler that the variable you are marking as volatile may be accessed by multiple threads.  There are certain optimizations that the CSharp compiler makes when it compiles our code and unless the variable is marked as volatile, the compiler will make optimizations assuming that the variable will only be accessed by one thread at a time.

Note that the volatile keyword can only be used against the following types of data:

  • Reference types.
  • Pointer types (in an unsafe context). Note that although the pointer itself can be volatile, the object that it points to cannot. In other words, you cannot declare a “pointer to volatile.”
  • Integral types such as sbyte, byte, short, ushort, int, uint, char, float, and bool.
  • An enum type with an integral base type.
  • Generic type parameters known to be reference types.
  • IntPtr and UIntPtr.
  • Variables that are objects or structures will need to use other locking mechanisms.

    Only variables that are members of classes or structures can be declared as volatile.

    To declare a variable volatile, just add volatile as one of the variable modifiers.

    class Demo
        private volatile int m_multiThreadVar;

    Other places talking about Volatile

    • Charlie Calvert’s Community Blog : CSharp Language Specification … – re: CSharp Language Specification, Version 3.0 Available for Review. One week to review 500 pages, I’ll try my best… Section 10.5.3. isn’t very clear. It mentions “acquire semantics” and “release semantics for volatile fields; …
    • delegates and threading, how volatile! – net and specifically c-sharp. for so long i have wondered about threading, particularly as it became clear that intel and amd were moving to multi-core systems, and pushing them on consumers whether the latter wanted them or not. …
    • Double Check Locking and Other Premature Optimizations Can Shoot … – Note that we use the volatile keyword for the _singletonInstance static member. Why? Long story made short, this has to do with how different memory models can reorder reads and writes. For the current CLR you can ignore the volatile …

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