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?
CatAndDog = Cat | Dog
(the logical OR instead of the Conditional), I assume?
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
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
Third = Second << 1
rather than Third = 1 << 2
-- see more complete description below
1 << 31 == -2147483648
, 1 << 32 == 1
, 1 << 33 == 2
, and so on. By contrast, if you say ThirtySecond = 2147483648
for an int type enum, the compiler throws an error.
0b1
, 0b10
etc. (or 0b00000001
if you like everything lining up properly!)
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
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.
0b_0100
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");
}
}
}
}
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.
HasFlag
method to enumerations, so you can do opt.HasFlag( PossibleOptions.OptionOne )
without having to write your own extensions
HasFlag
is much slower than doing bitwise operations.
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
}
0b_1111
@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)));
To add Mode.Write
:
Mode = Mode | Mode.Write;
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 enums 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();
}
}
}
enum
, we'd need to check for != 0
.
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
}
Flags
.
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 [] {});
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
Flags
requires that your enum values have unique bits set. And there should be at least one set (unless you a pseudo value like None
). Your enum should like ` public enum Sides { Left=1, Right=2, Top=4, Bottom=8 } `
Success story sharing
ToString
implementation of your enum uses Flags, and so doesEnum.IsDefined
,Enum.Parse
, etc. Try to remove Flags and look at the result of MyColor.Yellow | MyColor.Red; without it you get "5", with Flags you get "Yellow, Red". Some other parts of the framework also use [Flags] (e.g., XML Serialization).myProperties.AllowedColors = MyColor.Red | MyColor.Green | MyColor.Blue;
. You can do:myProperties.AllowedColors = myProperties.AllowedColors ^ MyColor.Blue //myProperties.AllowedColors == MyColor.Red | Mycolor.Green