multithreading - What is the purpose of 'volatile' keyword in C#

Question

What is the purpose of volatile keyword in C#?

Where would I need to use this keyword?

I saw the following statement, but I am unable to understand why volatile is required here?

internal volatile string UserName; 

Answer 1

I refer you to section 10.5.3 of the specification, which states:

For non-volatile fields, optimization techniques that reorder instructions can lead to unexpected and unpredictable results in multi-threaded programs that access fields without synchronization such as that provided by the lock-statement (§8.12). These optimizations can be performed by the compiler, by the run-time system, or by hardware. For volatile fields, such reordering optimizations are restricted:

A read of a volatile field is called a volatile read. A volatile read has “acquire semantics”; that is, it is guaranteed to occur prior to any references to memory that occur after it in the instruction sequence.

A write of a volatile field is called a volatile write. A volatile write has “release semantics”; that is, it is guaranteed to happen after any memory references prior to the write instruction in the instruction sequence.

These restrictions ensure that all threads will observe volatile writes performed by any other thread in the order in which they were performed. A conforming implementation is not required to provide a single total ordering of volatile writes as seen from all threads of execution.

Read that extremely carefully if you have any intention of ever making a volatile field. If you do not completely and thoroughly understand all the implications of volatile semantics then do not attempt to use them. It is usually far better to use a lock, which automatically gives you sufficient memory barriers to ensure the necessary acquire and release semantics. Remember, locks are only really expensive when they are contended.