C# 0 The Complete Reference

const and volatile

The

const

 modifier is used to declare fields or local variables that cannot be changed. These 

variables must be given initial values when they are declared. Thus, a 

const

 variable is 

essentially a constant. For example,

const int i = 10;

creates a 

const

 variable called 

i

 that has the value 10. Although a 

const

 field is similar to a 

readonly

 field, the two are not the same. A 

const

 field cannot be set within a constructor, 

but a 

readonly

 field can.

The

volatile

 modifier tells the compiler that a field’s value may be changed by two or 

more concurrently executing threads. In this situation, one thread may not know when the 

field has been changed by another thread. This is important because the C# compiler will 

automatically perform certain optimizations that work only when a field is accessed by a 

single thread of execution. To prevent these optimizations from being applied to a shared 

field, declare it 

volatile

. This tells the compiler that it must obtain the value of this field each 

time it is accessed.