C# 中的 [Flags] 枚举属性是什么意思？

Question

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?

Answer 1

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

Answer 2

You can also do this

[Flags]
public enum MyEnum
{
    None   = 0,
    First  = 1 << 0,
    Second = 1 << 1,
    Third  = 1 << 2,
    Fourth = 1 << 3
}

I find the bit-shifting easier than typing 4,8,16,32 and so on. It has no impact on your code because it's all done at compile time

Answer 3

Combining answers https://stackoverflow.com/a/8462/1037948 (declaration via bit-shifting) and https://stackoverflow.com/a/9117/1037948 (using combinations in declaration) you can bit-shift previous values rather than using numbers. Not necessarily recommending it, but just pointing out you can.

Rather than:

[Flags]
public enum Options : byte
{
    None    = 0,
    One     = 1 << 0,   // 1
    Two     = 1 << 1,   // 2
    Three   = 1 << 2,   // 4
    Four    = 1 << 3,   // 8

    // combinations
    OneAndTwo = One | Two,
    OneTwoAndThree = One | Two | Three,
}

You can declare

[Flags]
public enum Options : byte
{
    None    = 0,
    One     = 1 << 0,       // 1
    // now that value 1 is available, start shifting from there
    Two     = One << 1,     // 2
    Three   = Two << 1,     // 4
    Four    = Three << 1,   // 8

    // same combinations
    OneAndTwo = One | Two,
    OneTwoAndThree = One | Two | Three,
}

---

Confirming with LinqPad:

foreach(var e in Enum.GetValues(typeof(Options))) {
    string.Format("{0} = {1}", e.ToString(), (byte)e).Dump();
}

Results in:

None = 0
One = 1
Two = 2
OneAndTwo = 3
Three = 4
OneTwoAndThree = 7
Four = 8

Answer 4

Please see the following for an example which shows the declaration and potential usage:

namespace Flags
{
    class Program
    {
        [Flags]
        public enum MyFlags : short
        {
            Foo = 0x1,
            Bar = 0x2,
            Baz = 0x4
        }

        static void Main(string[] args)
        {
            MyFlags fooBar = MyFlags.Foo | MyFlags.Bar;

            if ((fooBar & MyFlags.Foo) == MyFlags.Foo)
            {
                Console.WriteLine("Item has Foo flag set");
            }
        }
    }
}

Answer 5

In extension to the accepted answer, in C#7 the enum flags can be written using binary literals:

[Flags]
public enum MyColors
{
    None   = 0b0000,
    Yellow = 0b0001,
    Green  = 0b0010,
    Red    = 0b0100,
    Blue   = 0b1000
}

I think this representation makes it clear how the flags work under the covers .

Answer 6

I asked recently about something similar.

If you use flags you can add an extension method to enums to make checking the contained flags easier (see post for detail)

This allows you to do:

[Flags]
public enum PossibleOptions : byte
{
    None = 0,
    OptionOne = 1,
    OptionTwo = 2,
    OptionThree = 4,
    OptionFour = 8,

    //combinations can be in the enum too
    OptionOneAndTwo = OptionOne | OptionTwo,
    OptionOneTwoAndThree = OptionOne | OptionTwo | OptionThree,
    ...
}

Then you can do:

PossibleOptions opt = PossibleOptions.OptionOneTwoAndThree 

if( opt.IsSet( PossibleOptions.OptionOne ) ) {
    //optionOne is one of those set
}

I find this easier to read than the most ways of checking the included flags.

Answer 7

When working with flags I often declare additional None and All items. These are helpful to check whether all flags are set or no flag is set.

[Flags] 
enum SuitsFlags { 

    None =     0,

    Spades =   1 << 0, 
    Clubs =    1 << 1, 
    Diamonds = 1 << 2, 
    Hearts =   1 << 3,

    All =      ~(~0 << 4)

}

Usage:

Spades | Clubs | Diamonds | Hearts == All  // true
Spades & Clubs == None  // true

Update 2019-10:

Since C# 7.0 you can use binary literals, which are probably more intuitive to read:

[Flags] 
enum SuitsFlags { 

    None =     0b0000,

    Spades =   0b0001, 
    Clubs =    0b0010, 
    Diamonds = 0b0100, 
    Hearts =   0b1000,

    All =      0b1111

}

Answer 8

@Nidonocu

To add another flag to an existing set of values, use the OR assignment operator.

Mode = Mode.Read;
//Add Mode.Write
Mode |= Mode.Write;
Assert.True(((Mode & Mode.Write) == Mode.Write)
  && ((Mode & Mode.Read) == Mode.Read)));

Answer 9

添加Mode.Write ：

Mode = Mode | Mode.Write;

Answer 10

There's something overly verbose to me about the if ((x & y) == y)... construct, especially if x AND y are both compound sets of flags and you only want to know if there's any overlap.

In this case, all you really need to know is if there's a non-zero value[1] after you've bitmasked .

[1] See Jaime's comment. If we were authentically bitmasking , we'd only need to check that the result was positive. But since enum s can be negative, even, strangely, when combined with the [Flags] attribute , it's defensive to code for != 0 rather than > 0 .

Building off of @andnil's setup...

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace BitFlagPlay
{
    class Program
    {
        [Flags]
        public enum MyColor
        {
            Yellow = 0x01,
            Green = 0x02,
            Red = 0x04,
            Blue = 0x08
        }

        static void Main(string[] args)
        {
            var myColor = MyColor.Yellow | MyColor.Blue;
            var acceptableColors = MyColor.Yellow | MyColor.Red;

            Console.WriteLine((myColor & MyColor.Blue) != 0);     // True
            Console.WriteLine((myColor & MyColor.Red) != 0);      // False                
            Console.WriteLine((myColor & acceptableColors) != 0); // True
            // ... though only Yellow is shared.

            Console.WriteLine((myColor & MyColor.Green) != 0);    // Wait a minute... ;^D

            Console.Read();
        }
    }
}

Answer 11

Flags allow you to use bitmasking inside your enumeration. This allows you to combine enumeration values, while retaining which ones are specified.

[Flags]
public enum DashboardItemPresentationProperties : long
{
    None = 0,
    HideCollapse = 1,
    HideDelete = 2,
    HideEdit = 4,
    HideOpenInNewWindow = 8,
    HideResetSource = 16,
    HideMenu = 32
}

Answer 12

Apologies if someone already noticed this scenario. A perfect example of flags we can see in reflection. Yes Binding Flags ENUM .

[System.Flags]
[System.Runtime.InteropServices.ComVisible(true)]
[System.Serializable]
public enum BindingFlags

Usage

// BindingFlags.InvokeMethod
// Call a static method.
Type t = typeof (TestClass);

Console.WriteLine();
Console.WriteLine("Invoking a static method.");
Console.WriteLine("-------------------------");
t.InvokeMember ("SayHello", BindingFlags.InvokeMethod | BindingFlags.Public | 
    BindingFlags.Static, null, null, new object [] {});

Answer 13

Define the Problem

Let's define an enum that represents the types of users:

public enum UserType
{
    Customer = 1,
    Driver = 2,  
    Admin = 3,
}

We define the UserType enum that contains three values: Customer, Driver, and Admin.

But what if we need to represent a collection of values?

For example, at a delivery company, we know that both the Admin and the Driver are employees. So let's add a new enumeration item Employee . Later on, we will show you how we can represent both the admin and the driver with it:

public enum UserType
{   
    Customer = 1,
    Driver = 2,  
    Admin = 3,
    Employee = 4
}

Define and Declare a Flags Attribute

A Flags is an attribute that allows us to represent an enum as a collection of values rather than a single value. So, let's see how we can implement the Flags attribute on enumeration:

[Flags]
public enum UserType
{   
    Customer = 1,
    Driver = 2,  
    Admin = 4,
}

We add the Flags attribute and number the values with powers of 2. Without both, this won't work.

Now going back to our previous problem, we can represent Employee using the | operator:

var employee = UserType.Driver | UserType.Admin;

Also, we can define it as a constant inside the enum to use it directly:

[Flags]
public enum UserType
{                
    Customer = 1,             
    Driver = 2,               
    Admin = 4,                
    Employee = Driver | Admin
}

Behind the Scenes

To understand the Flags attribute better, we must go back to the binary representation of the number. For example, we can represent 1 as binary 0b_0001 and 2 as 0b_0010 :

[Flags]
public enum UserType
{
    Customer = 0b_0001,
    Driver = 0b_0010,
    Admin = 0b_0100,
    Employee = Driver | Admin, //0b_0110
}

We can see that each value is represented in an active bit. And this is where the idea of numbering the values with the power of 2 came from. We can also note that Employee contains two active bits, that is, it is a composite of two values Driver and Admin.

Operations on Flags Attribute

We can use the bitwise operators to work with Flags .

Initialize a Value

For the initialization, we should use the value 0 named None, which means the collection is empty:

[Flags]
public enum UserType
{
    None = 0,
    Customer = 1,
    Driver = 2,
    Admin = 4,
    Employee = Driver | Admin
}

Now, we can define a variable:

var flags = UserType.None;

Add a Value

We can add value by using |operator:

flags |= UserType.Driver;

Now, the flags variable equals Driver.

Remove a Value

We can remove value by use &, ~ operators:

flags &= ~UserType.Driver;

Now, flagsvariable equals None.

We can check if the value exists by using & operator:

Console.WriteLine((flags & UserType.Driver) == UserType.Driver);

The result is False .

Also, we can do this by using the HasFlag method:

Console.WriteLine(flags.HasFlag(UserType.Driver));

Also, the result will be False.

As we can see, both ways, using the & operator and the HasFlag method, give the same result, but which one should we use? To find out, we will test the performance on several frameworks.

Measure the Performance

First, we will create a Console App, and in the .csproj file we will replace the TargetFramwork tag with the TargetFramworks tag:

<TargetFrameworks>net48;netcoreapp3.1;net6.0</TargetFrameworks>
We use the TargetFramworks tag to support multiple frameworks: .NET Framework 4.8, .Net Core 3.1, and .Net 6.0.

Secondly, let's introduce the BenchmarkDotNet library to get the benchmark results:

[Benchmark]
public bool HasFlag()
{
    var result = false;
    for (int i = 0; i < 100000; i++)
    {
        result = UserType.Employee.HasFlag(UserType.Driver);
    }
    return result;
}
[Benchmark]
public bool BitOperator()
{
    var result = false;
    for (int i = 0; i < 100000; i++)
    {
        result = (UserType.Employee & UserType.Driver) == UserType.Driver;
    }
    return result;
}

We add [SimpleJob(RuntimeMoniker.Net48)] , [SimpleJob(RuntimeMoniker.NetCoreApp31)] , and [SimpleJob(RuntimeMoniker.Net60)] attributes to the HasFlagBenchmarker class to see the performance differences between different versions of .NET Framework / .NET Core :

Method	Job	Runtime	Mean	Error	StdDev	Median
HasFlag	.NET 6.0	.NET 6.0	37.79 us	3.781 us	11.15 us	30.30 us
BitOperator	.NET 6.0	.NET 6.0	38.17 us	3.853 us	11.36 us	30.38 us
HasFlag	.NET Core 3.1	.NET Core 3.1	38.31 us	3.939 us	11.61 us	30.37 us
BitOperator	.NET Core 3.1	.NET Core 3.1	38.07 us	3.819 us	11.26 us	30.33 us
HasFlag	.NET Framework 4.8	.NET Framework 4.8	2,893.10 us	342.563 us	1,010.06 us	2,318.93 us
BitOperator	.NET Framework 4.8	.NET Framework 4.8	38.04 us	3.920 us	11.56 us	30.17 us

So, in .NET Framework 4.8 a HasFlag method was much slower than the BitOperator . But, the performance improves in .Net Core 3.1 and .Net 6.0 . So in newer versions, we can use both ways.

Answer 14

• Flags are used when an enumerable value represents a collection of enum members.

• here we use bitwise operators, | and &

• Example

   [Flags] public enum Sides { Left=0, Right=1, Top=2, Bottom=3 } Sides leftRight = Sides.Left | Sides.Right; Console.WriteLine (leftRight);//Left, Right string stringValue = leftRight.ToString(); Console.WriteLine (stringValue);//Left, Right Sides s = Sides.Left; s |= Sides.Right; Console.WriteLine (s);//Left, Right s ^= Sides.Right; // Toggles Sides.Right Console.WriteLine (s); //Left