volatile 关键字指示一个字段可以由多个同时执行的线程修改。声明为 volatile 的字段不受编译器优化（假定由单个线程访问）的限制。这样可以确保该字段在任何时间呈现的都是最新的值。
volatile 修饰符通常用于由多个线程访问但不使用 lock 语句对访问进行序列化的字段。
volatile 关键字可应用于以下类型的字段：
引用类型。
指针类型（在不安全的上下文中）。请注意，虽然指针本身可以是可变的，但是它指向的对象不能是可变的。换句话说，您无法声明“指向可变对象的指针”。
整型，如 sbyte、byte、short、ushort、int、uint、char、float 和 bool。
具有整数基类型的枚举类型。
已知为引用类型的泛型类型参数。
IntPtr 和 UIntPtr。
The volatile keyword indicates that a field might be modified by multiple threads that are executing at the same time. Fields that are declared volatile are not subject to compiler optimizations that assume access by a single thread. This ensures that the most up-to-date value is present in the field at all times.
The volatile modifier is usually used for a field that is accessed by multiple threads without using the lock statement to serialize access.
The volatile keyword can be applied to fields of these types:
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."
Types such as sbyte, byte, short, ushort, int, uint, char, float, and bool.
An enum type with one of the following base types: byte, sbyte, short, ushort, int, or uint.
Generic type parameters known to be reference types.
The volatile keyword can only be applied to fields of a class or struct. Local variables cannot be declared volatile.
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