Multiple ways to define C# Enums with [Flags] attribute?

问题

I understand how Enums work in C#, and I get what the Flags attribute brings to the table.

I saw this question, here. Which recommends the first flavor, but doesn't provide any reason/justification for it.

Is there a difference in the way in which these two are defined, is one better than the other? What are the advantages to using the first synax as instead of the second? I've always used the second flavor when defining Flags type Enums... have I been doing it wrong all this time?

[Serializable]
[Flags]
public enum SiteRoles
{
    User = 1 << 0,
    Admin = 1 << 1,
    Helpdesk = 1 << 2
}

Is that not the same as

[Serializable]
[Flags]
public enum SiteRoles
{
    User = 1,
    Admin = 2,
    Helpdesk = 4
}

回答1:

The main advantage with the first one is that you don't need to calculate the correct values for each flag since the compiler will do it for you. Apart from that they are the same.

回答2:

Consider more complex samples:

[Flags]
public enum SiteRoles
{
    User = 1 << 12,
    Admin = 1 << 13,
    Helpdesk = 1 << 15,
    AdvancedUser = User | Helpdesk, //or (1<<12)|(1<<13)
}

[Flags]
public enum SiteRoles
{
    User = 4096, //not so obvious!
    Admin = 8192,
    Helpdesk = 16384,
    AdvancedUser = 12288, //!
}

[Flags]
public enum SiteRoles
{
    User = 0x1000, //we can use hexademical digits
    Admin = 0x2000,
    Helpdesk = 0x4000,
    AdvancedUser = 0x3000, //it much simpler calculate binary operator OR with hexademicals
}

This samples shows that in this case first version is MUCH MORE readable. Decimal literals is not the best way to represent flag constants. And for more information about bitwise operations (that also can be used to represent flag constants) see http://en.wikipedia.org/wiki/Bitwise_operation

回答3:

AFAIK its a readability debate. Some would say the first is more readable because you have the actual index of the flag on the right hand side of the '<<'.

回答4:

There is another way to do this which is rather elegant and so I thought I'd share something I recently wrote. It has the benefit of requiring very little math and so I think it's less error prone. It is very readable, IMHO.

[Flags][Serializable]
public enum ScopeType : int
{
    Unknown = 0,
    Global = 1,
    Namespace = Global << 1,
    Class = Namespace << 1,
    Struct = Class << 1,
    Interface = Struct << 1,
    Enum = Interface << 1,
    Function = Enum << 1,
    Property = Function << 1,
    PropertyGetter = Property << 1,
    PropertySetter = PropertyGetter << 1,
    Using = PropertySetter << 1,
    If = Using << 1,
    ElseIf = If << 1,
    Else = ElseIf << 1,
    Switch = Else << 1,
    Case = Switch << 1,
    For = Case << 1,
    While = For << 1,
    DoWhile = While << 1,
    Lambda = DoWhile << 1,
    Try = Lambda << 1,
    Catch = Try << 1,
    Finally = Catch << 1,
    Initializer = Finally << 1,
    Checked = Initializer << 1,
    Unchecked = Checked << 1,
    Unsafe = Unchecked << 1,
    Lock = Unsafe << 1,
    Fixed = Lock << 1,

    // I can also group flags together using bitwise-OR.
    PropertyAccessor = PropertyGetter | PropertySetter,
    TypeDefinition = Class | Struct | Interface | Enum,
    TryCatchFinally = Try | Catch | Finally,
    Conditional = If | ElseIf | Else,
    Branch = Conditional | Case | TryCatchFinally,
    Loop = For | While | DoWhile
}

NOTE: Since the enumeration inherits from System.Int32, I can only define 32 flags. If you need more you will have to use a larger integer (System.Int64), create more than one enumeration and chain them together, or just create a class with a bunch of boolean values.

来源：https://stackoverflow.com/questions/2134204/multiple-ways-to-define-c-sharp-enums-with-flags-attribute

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