如果你想稍微了解一下volatile关键字的功能，可以考虑以下程序(我使用的是DevStudio 2005):

#include <iostream>
void main()
{
  int j = 0;
  for (int i = 0 ; i < 100 ; ++i)
  {
    j += i;
  }
  for (volatile int i = 0 ; i < 100 ; ++i)
  {
    j += i;
  }
  std::cout << j;
}

使用标准的优化(发布)编译器设置，编译器创建以下汇编器(IA32):

void main()
{
00401000  push        ecx  
  int j = 0;
00401001  xor         ecx,ecx 
  for (int i = 0 ; i < 100 ; ++i)
00401003  xor         eax,eax 
00401005  mov         edx,1 
0040100A  lea         ebx,[ebx] 
  {
    j += i;
00401010  add         ecx,eax 
00401012  add         eax,edx 
00401014  cmp         eax,64h 
00401017  jl          main+10h (401010h) 
  }
  for (volatile int i = 0 ; i < 100 ; ++i)
00401019  mov         dword ptr [esp],0 
00401020  mov         eax,dword ptr [esp] 
00401023  cmp         eax,64h 
00401026  jge         main+3Eh (40103Eh) 
00401028  jmp         main+30h (401030h) 
0040102A  lea         ebx,[ebx] 
  {
    j += i;
00401030  add         ecx,dword ptr [esp] 
00401033  add         dword ptr [esp],edx 
00401036  mov         eax,dword ptr [esp] 
00401039  cmp         eax,64h 
0040103C  jl          main+30h (401030h) 
  }
  std::cout << j;
0040103E  push        ecx  
0040103F  mov         ecx,dword ptr [__imp_std::cout (40203Ch)] 
00401045  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (402038h)] 
}
0040104B  xor         eax,eax 
0040104D  pop         ecx  
0040104E  ret              

Looking at the output, the compiler has decided to use the ecx register to store the value of the j variable. For the non-volatile loop (the first) the compiler has assigned i to the eax register. Fairly straightforward. There are a couple of interesting bits though - the lea ebx,[ebx] instruction is effectively a multibyte nop instruction so that the loop jumps to a 16 byte aligned memory address. The other is the use of edx to increment the loop counter instead of using an inc eax instruction. The add reg,reg instruction has lower latency on a few IA32 cores compared to the inc reg instruction, but never has higher latency.

Now for the loop with the volatile loop counter. The counter is stored at [esp] and the volatile keyword tells the compiler the value should always be read from/written to memory and never assigned to a register. The compiler even goes so far as to not do a load/increment/store as three distinct steps (load eax, inc eax, save eax) when updating the counter value, instead the memory is directly modified in a single instruction (an add mem,reg). The way the code has been created ensures the value of the loop counter is always up-to-date within the context of a single CPU core. No operation on the data can result in corruption or data loss (hence not using the load/inc/store since the value can change during the inc thus being lost on the store). Since interrupts can only be serviced once the current instruction has completed, the data can never be corrupted, even with unaligned memory.

Once you introduce a second CPU to the system, the volatile keyword won't guard against the data being updated by another CPU at the same time. In the above example, you would need the data to be unaligned to get a potential corruption. The volatile keyword won't prevent potential corruption if the data cannot be handled atomically, for example, if the loop counter was of type long long (64 bits) then it would require two 32 bit operations to update the value, in the middle of which an interrupt can occur and change the data.

因此，volatile关键字只适用于小于或等于本机寄存器大小的对齐数据，这样操作总是原子的。

volatile关键字被设想用于IO操作，其中IO将不断变化，但有一个恒定的地址，例如内存映射的UART设备，编译器不应该一直重用从地址中读取的第一个值。

如果要处理大数据或有多个cpu，则需要更高级别(OS)的锁定系统来正确处理数据访问。

编译器有时会改变代码中语句的顺序来优化它。通常这在单线程环境中不是问题，但在多线程环境中可能是问题。请看下面的例子:

 private static int _flag = 0;
 private static int _value = 0;

 var t1 = Task.Run(() =>
 {
     _value = 10; /* compiler could switch these lines */
     _flag = 5;
 });

 var t2 = Task.Run(() =>
 {
     if (_flag == 5)
     {
         Console.WriteLine("Value: {0}", _value);
     }
 });

如果运行t1和t2，您将期望没有输出或结果为“Value: 10”。可能是编译器在t1函数内部切换了行。如果t2执行，可能是_flag值为5，而_value值为0。因此，预期的逻辑可能会被打破。

为了解决这个问题，你可以使用volatile关键字，你可以应用到字段。此语句禁用编译器优化，以便您可以强制代码中的正确顺序。

private static volatile int _flag = 0;

只有在真正需要时才应该使用volatile，因为它会禁用某些编译器优化，这会影响性能。它也不是所有。net语言都支持(Visual Basic不支持)，因此它阻碍了语言互操作性。

如果你想稍微了解一下volatile关键字的功能，可以考虑以下程序(我使用的是DevStudio 2005):

#include <iostream>
void main()
{
  int j = 0;
  for (int i = 0 ; i < 100 ; ++i)
  {
    j += i;
  }
  for (volatile int i = 0 ; i < 100 ; ++i)
  {
    j += i;
  }
  std::cout << j;
}

使用标准的优化(发布)编译器设置，编译器创建以下汇编器(IA32):

void main()
{
00401000  push        ecx  
  int j = 0;
00401001  xor         ecx,ecx 
  for (int i = 0 ; i < 100 ; ++i)
00401003  xor         eax,eax 
00401005  mov         edx,1 
0040100A  lea         ebx,[ebx] 
  {
    j += i;
00401010  add         ecx,eax 
00401012  add         eax,edx 
00401014  cmp         eax,64h 
00401017  jl          main+10h (401010h) 
  }
  for (volatile int i = 0 ; i < 100 ; ++i)
00401019  mov         dword ptr [esp],0 
00401020  mov         eax,dword ptr [esp] 
00401023  cmp         eax,64h 
00401026  jge         main+3Eh (40103Eh) 
00401028  jmp         main+30h (401030h) 
0040102A  lea         ebx,[ebx] 
  {
    j += i;
00401030  add         ecx,dword ptr [esp] 
00401033  add         dword ptr [esp],edx 
00401036  mov         eax,dword ptr [esp] 
00401039  cmp         eax,64h 
0040103C  jl          main+30h (401030h) 
  }
  std::cout << j;
0040103E  push        ecx  
0040103F  mov         ecx,dword ptr [__imp_std::cout (40203Ch)] 
00401045  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (402038h)] 
}
0040104B  xor         eax,eax 
0040104D  pop         ecx  
0040104E  ret              

Looking at the output, the compiler has decided to use the ecx register to store the value of the j variable. For the non-volatile loop (the first) the compiler has assigned i to the eax register. Fairly straightforward. There are a couple of interesting bits though - the lea ebx,[ebx] instruction is effectively a multibyte nop instruction so that the loop jumps to a 16 byte aligned memory address. The other is the use of edx to increment the loop counter instead of using an inc eax instruction. The add reg,reg instruction has lower latency on a few IA32 cores compared to the inc reg instruction, but never has higher latency.

Now for the loop with the volatile loop counter. The counter is stored at [esp] and the volatile keyword tells the compiler the value should always be read from/written to memory and never assigned to a register. The compiler even goes so far as to not do a load/increment/store as three distinct steps (load eax, inc eax, save eax) when updating the counter value, instead the memory is directly modified in a single instruction (an add mem,reg). The way the code has been created ensures the value of the loop counter is always up-to-date within the context of a single CPU core. No operation on the data can result in corruption or data loss (hence not using the load/inc/store since the value can change during the inc thus being lost on the store). Since interrupts can only be serviced once the current instruction has completed, the data can never be corrupted, even with unaligned memory.

Once you introduce a second CPU to the system, the volatile keyword won't guard against the data being updated by another CPU at the same time. In the above example, you would need the data to be unaligned to get a potential corruption. The volatile keyword won't prevent potential corruption if the data cannot be handled atomically, for example, if the loop counter was of type long long (64 bits) then it would require two 32 bit operations to update the value, in the middle of which an interrupt can occur and change the data.

因此，volatile关键字只适用于小于或等于本机寄存器大小的对齐数据，这样操作总是原子的。

volatile关键字被设想用于IO操作，其中IO将不断变化，但有一个恒定的地址，例如内存映射的UART设备，编译器不应该一直重用从地址中读取的第一个值。

如果要处理大数据或有多个cpu，则需要更高级别(OS)的锁定系统来正确处理数据访问。

有时候，编译器会优化一个字段并使用寄存器来存储它。如果线程1写了字段，而另一个线程访问了它，因为更新存储在寄存器(而不是内存)中，第二个线程将得到陈旧的数据。

你可以把volatile关键字看作是对编译器说“我想让你把这个值存储在内存中”。这保证了第二个线程检索到最新的值。

从MSDN: volatile修饰符通常用于由多个线程访问而不使用lock语句序列化访问的字段。使用volatile修饰符可确保一个线程检索到另一个线程写入的最新值。

多个线程可以访问一个变量。 最新的更新将在变量上