[Flags] 枚举属性在 C# 中是什么意思? [英] What does the [Flags] Enum Attribute mean in C#?

问题描述

我时不时会看到一个类似如下的枚举:

[标志]公共枚举选项{无 = 0,选项 1 = 1,选项 2 = 2，选项 3 = 4，选项 4 = 8}

我不明白 [Flags] 属性到底是做什么的.

任何人都可以发布一个很好的解释或示例?

解决方案

[Flags] 属性应该在可枚举表示可能值的集合而不是单个值时使用.此类集合通常与按位运算符一起使用，例如:

var allowedColors = MyColor.Red |MyColor.Green |MyColor.Blue;

请注意，[Flags] 属性不会自己启用此功能 - 它所做的只是允许 .ToString() 方法:

enum Suits { Spades = 1, Clubs = 2, Diamonds = 4, Hearts = 8 }[Flags] enum SuitsFlags { Spades = 1, Clubs = 2, Diamonds = 4, Hearts = 8 }...var str1 = (Suits.Spades | Suits.Diamonds).ToString();//"5"var str2 = (SuitsFlags.Spades | SuitsFlags.Diamonds).ToString();//"黑桃，钻石"

还需要注意的是，[Flags] 不会自动使枚举值成为 2 的幂.如果省略数值，则枚举将不会像按位运算预期的那样工作，因为默认情况下，这些值从 0 开始并递增.

不正确的声明:

[标志]公共枚举 MyColors{黄色,//0绿色，//1红色，//2蓝色//3}

如果以这种方式声明，这些值将是 Yellow = 0、Green = 1、Red = 2、Blue = 3.这将使其无法用作标志.

以下是正确声明的示例:

[标志]公共枚举 MyColors{黄色 = 1,绿色 = 2,红色 = 4,蓝色 = 8}

要检索属性中的不同值，可以这样做:

if (myProperties.AllowedColors.HasFlag(MyColor.Yellow)){//黄色是允许的...}

或在 .NET 4 之前:

if((myProperties.AllowedColors & MyColor.Yellow) == MyColor.Yellow){//黄色是允许的...}if((myProperties.AllowedColors & MyColor.Green) == MyColor.Green){//允许绿色...}

在幕后

这是有效的，因为您在枚举中使用了 2 的幂.在幕后，您的枚举值在二进制 1 和 0 中如下所示:

 黄色:00000001绿色:00000010红色:00000100蓝色:00001000

同样，在您使用二进制按位 OR | 运算符将属性 AllowedColors 设置为红色、绿色和蓝色后，AllowedColors 看起来像这样:

myProperties.AllowedColors: 00001110

因此，当您检索该值时，您实际上是在对这些值执行按位 AND &:

myProperties.AllowedColors: 00001110MyColor.Green:00000010-----------------------00000010//嘿，这和 MyColor.Green 一样！

None = 0 值

关于在枚举中使用 0 ，引用自 MSDN:

[标志]公共枚举 MyColors{无 = 0,....}

<块引用>

使用 None 作为值为零的标志枚举常量的名称.您不能在按位 AND 运算中使用 None 枚举常量来测试标志，因为结果始终为零.但是，您可以在数值和None 枚举常量以确定是否设置了数值中的任何位.

您可以在 msdn 中找到有关 flags 属性及其用法的更多信息 和 在 msdn 设计标志

From time to time I see an enum like the following:

[Flags]
public enum Options 
{
    None    = 0,
    Option1 = 1,
    Option2 = 2,
    Option3 = 4,
    Option4 = 8
}

I don't understand what exactly the [Flags] attribute does.

Anyone have a good explanation or example they could post?

解决方案

The [Flags] attribute should be used whenever the enumerable represents a collection of possible values, rather than a single value. Such collections are often used with bitwise operators, for example:

var allowedColors = MyColor.Red | MyColor.Green | MyColor.Blue;

Note that the [Flags] attribute doesn't enable this by itself - all it does is allow a nice representation by the .ToString() method:

enum Suits { Spades = 1, Clubs = 2, Diamonds = 4, Hearts = 8 }
[Flags] enum SuitsFlags { Spades = 1, Clubs = 2, Diamonds = 4, Hearts = 8 }

...

var str1 = (Suits.Spades | Suits.Diamonds).ToString();
           // "5"
var str2 = (SuitsFlags.Spades | SuitsFlags.Diamonds).ToString();
           // "Spades, Diamonds"

It is also important to note that [Flags] does not automatically make the enum values powers of two. If you omit the numeric values, the enum will not work as one might expect in bitwise operations, because by default the values start with 0 and increment.

Incorrect declaration:

[Flags]
public enum MyColors
{
    Yellow,  // 0
    Green,   // 1
    Red,     // 2
    Blue     // 3
}

The values, if declared this way, will be Yellow = 0, Green = 1, Red = 2, Blue = 3. This will render it useless as flags.

Here's an example of a correct declaration:

[Flags]
public enum MyColors
{
    Yellow = 1,
    Green = 2,
    Red = 4,
    Blue = 8
}

To retrieve the distinct values in your property, one can do this:

if (myProperties.AllowedColors.HasFlag(MyColor.Yellow))
{
    // Yellow is allowed...
}

or prior to .NET 4:

if((myProperties.AllowedColors & MyColor.Yellow) == MyColor.Yellow)
{
    // Yellow is allowed...
}

if((myProperties.AllowedColors & MyColor.Green) == MyColor.Green)
{
    // Green is allowed...
}    

Under the covers

This works because you used powers of two in your enumeration. Under the covers, your enumeration values look like this in binary ones and zeros:

 Yellow: 00000001
 Green:  00000010
 Red:    00000100
 Blue:   00001000

Similarly, after you've set your property AllowedColors to Red, Green and Blue using the binary bitwise OR | operator, AllowedColors looks like this:

myProperties.AllowedColors: 00001110

So when you retrieve the value you are actually performing bitwise AND & on the values:

myProperties.AllowedColors: 00001110
             MyColor.Green: 00000010
             -----------------------
                            00000010 // Hey, this is the same as MyColor.Green!

The None = 0 value

And regarding the use of 0 in your enumeration, quoting from MSDN:

[Flags]
public enum MyColors
{
    None = 0,
    ....
}

Use None as the name of the flag enumerated constant whose value is zero. You cannot use the None enumerated constant in a bitwise AND operation to test for a flag because the result is always zero. However, you can perform a logical, not a bitwise, comparison between the numeric value and the None enumerated constant to determine whether any bits in the numeric value are set.

You can find more info about the flags attribute and its usage at msdn and designing flags at msdn

这篇关于[Flags] 枚举属性在 C# 中是什么意思?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！