原文：
http://www.blogcn.com/User8/flier_lu/index.html?id=3236734 为了在一个标志字段中保存多种类型的标志，C 语言中定常见模式之一，是先定义一个 XXX_MASK，再定义一个 XXX_SHIFT，然后通过移位操作定义这段位上的标志，如 WinCrypt.h 中定义证书存储位置标志时，将位置标志位放在高16位中：
 // Includes flags and location
#define CERT_SYSTEM_STORE_MASK 0xFFFF0000
// Location of the system store:
#define CERT_SYSTEM_STORE_LOCATION_MASK 0x00FF0000
#define CERT_SYSTEM_STORE_LOCATION_SHIFT 16
// Registry: HKEY_CURRENT_USER or HKEY_LOCAL_MACHINE
#define CERT_SYSTEM_STORE_CURRENT_USER_ID 1
#define CERT_SYSTEM_STORE_LOCAL_MACHINE_ID 2
//
#define CERT_SYSTEM_STORE_CURRENT_USER
(CERT_SYSTEM_STORE_CURRENT_USER_ID << CERT_SYSTEM_STORE_LOCATION_SHIFT)
#define CERT_SYSTEM_STORE_LOCAL_MACHINE
(CERT_SYSTEM_STORE_LOCAL_MACHINE_ID << CERT_SYSTEM_STORE_LOCATION_SHIFT)
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而在 C# 中一般使用以 FlagsAttribute 标记后的 Enum 来模拟类似语义，如
[Flags]
public enum CertSystemStoreFlag : uint
  {
 // Registry: HKEY_CURRENT_USER or HKEY_LOCAL_MACHINE
CERT_SYSTEM_STORE_CURRENT_USER_ID = 1,
CERT_SYSTEM_STORE_LOCAL_MACHINE_ID = 2,
// Includes flags and location
CERT_SYSTEM_STORE_MASK = 0xFFFF0000,
// Set if pvPara points to a CERT_SYSTEM_STORE_RELOCATE_PARA structure
CERT_SYSTEM_STORE_RELOCATE_FLAG = 0x80000000,
CERT_SYSTEM_STORE_LOCATION_MASK = 0x00FF0000,
CERT_SYSTEM_STORE_LOCATION_SHIFT = 16,
CERT_SYSTEM_STORE_CURRENT_USER =
 }
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FlagsAttribute 标记使得此 Enum 能够以位域(bit field)形式进行操作，既有 Flags 的类型安全特性，又有进行位操作的灵活性。同时还能定义此 Enum 的基本类型，如上面指定的 uint，以最大限度兼容现有 C 代码。
不过这样组合使用 Enum 的多个特性时有一个小小的语法陷阱，不能将 C# 完全等同于 C++ 的语法，例如要这样照搬 C++ 定义语法：
[Flags]
public enum CertSystemStoreFlag : uint
{
CERT_SYSTEM_STORE_CURRENT_USER_ID = 1,
CERT_SYSTEM_STORE_LOCATION_SHIFT = 16,
CERT_SYSTEM_STORE_CURRENT_USER = CERT_SYSTEM_STORE_CURRENT_USER_ID << CERT_SYSTEM_STORE_LOCATION_SHIFT,
}
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编译时就会获得一个让人困惑的警告消息：
以下为引用：
CS0019: Operator '<<' cannot be applied to operands of type 'uint' and 'uint'
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以一个 C++ 背景的程序员角度来看，实在是无法想像为什么移位操作竟然不能对 uint 进行处理，不过在 C# 中这恰恰是语法所要求的。ECMA-334 C# Language Specification 的
第 14.8 节 Shift operators 是这样定义 C# 的移位操作符的：
以下为引用：
The << and >> operators are used to perform bit shifting operations.
shift-expression :
additive-expression
shift-expression << additive-expression
shift-expression >> additive-expression
...
The predefined shift operators are listed below.
2 Shift left:
int operator <<(int x, int count);
uint operator <<(uint x, int count);
long operator <<(long x, int count);
ulong operator <<(ulong x, int count);
3 The << operator shifts x left by a number of bits computed as described below. 4 The high-order bits outside the range of the result type of x are discarded, the remaining bits are shifted left, and the low-order empty bit positions are set to zero.
5 Shift right:
int operator >>(int x, int count);
uint operator >>(uint x, int count);
long operator >>(long x, int count);
ulong operator >>(ulong x, int count);
6 The >> operator shifts x right by a number of bits computed as described below. 7 When x is of type int or long, the low-order bits of x are discarded, the remaining bits are shifted right, and the high-order empty bit positions are set to zero if x is non-negative and set to one if x is negative. 8 When x is of type uint or ulong, the low-order bits of x are discarded, the remaining bits are shifted right, and the high-order empty bit positions are set to zero.
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可以看到，双目移位操作符的两个参数实际上是不一样的，操作符左的参数可以是有/无符号的int和long，但操作符右边的则都是 int。因此上面那个错误的表达式中，移位操作符右边数字应该被显式转换为 int 来符合 C# 的语法要求。
[Flags]
public enum CertSystemStoreFlag : uint
{
CERT_SYSTEM_STORE_CURRENT_USER_ID = 1,
CERT_SYSTEM_STORE_LOCATION_SHIFT = 16,
CERT_SYSTEM_STORE_CURRENT_USER = CERT_SYSTEM_STORE_CURRENT_USER_ID << (int)CERT_SYSTEM_STORE_LOCATION_SHIFT,
}
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呵呵，虽然比较别扭，但没办法，谁让 C# 定义得这么严谨呢 :P
btw: 感谢 Junfeng Zhang 帮忙指出问题所在
