Indicates that an enumeration can be treated as a bit field; that is, a set of flags.
Assembly: mscorlib (in mscorlib.dll)
This API supports the product infrastructure and is not intended to be used directly from your code. Returns a value that indicates whether this instance is equal to a specified object.(Inherited from Attribute.)
Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection.(Inherited from Object.)
Returns the hash code for this instance.(Inherited from Attribute.)
When overridden in a derived class, indicates whether the value of this instance is the default value for the derived class.(Inherited from Attribute.)
When overridden in a derived class, returns a value that indicates whether this instance equals a specified object.(Inherited from Attribute.)
Returns a string that represents the current object.(Inherited from Object.)
|_Attribute.GetIDsOfNames(Guid, IntPtr, UInt32, UInt32, IntPtr)|
Maps a set of names to a corresponding set of dispatch identifiers.(Inherited from Attribute.)
|_Attribute.GetTypeInfo(UInt32, UInt32, IntPtr)|
Retrieves the type information for an object, which can be used to get the type information for an interface.(Inherited from Attribute.)
Retrieves the number of type information interfaces that an object provides (either 0 or 1).(Inherited from Attribute.)
|_Attribute.Invoke(UInt32, Guid, UInt32, Int16, IntPtr, IntPtr, IntPtr, IntPtr)|
Provides access to properties and methods exposed by an object.(Inherited from Attribute.)
Bit fields are generally used for lists of elements that might occur in combination, whereas enumeration constants are generally used for lists of mutually exclusive elements. Therefore, bit fields are designed to be combined with a bitwise OR operation to generate unnamed values, whereas enumerated constants are not. Languages vary in their use of bit fields compared to enumeration constants.
Use thecustom attribute for an enumeration only if a bitwise operation (AND, OR, EXCLUSIVE OR) is to be performed on a numeric value.
Define enumeration constants in powers of two, that is, 1, 2, 4, 8, and so on. This means the individual flags in combined enumeration constants do not overlap.
Consider creating an enumerated constant for commonly used flag combinations. For example, if you have an enumeration used for file I/O operations that contains the enumerated constants Read = 1 and Write = 2, consider creating the enumerated constant ReadWrite = Read OR Write, which combines the Read and Write flags. In addition, the bitwise OR operation used to combine the flags might be considered an advanced concept in some circumstances that should not be required for simple tasks.
Use caution if you define a negative number as a flag enumerated constant because many flag positions might be set to 1, which might make your code confusing and encourage coding errors.
A convenient way to test whether a flag is set in a numeric value is to perform a bitwise AND operation between the numeric value and the flag enumerated constant, which sets all bits in the numeric value to zero that do not correspond to the flag, then test whether the result of that operation is equal to the flag enumerated constant.
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.
If you create a value enumeration instead of a flags enumeration, it is still worthwhile to create a None enumerated constant. The reason is that by default the memory used for the enumeration is initialized to zero by the common language runtime. Consequently, if you do not define a constant whose value is zero, the enumeration will contain an illegal value when it is created.
If there is an obvious default case your application needs to represent, consider using an enumerated constant whose value is zero to represent the default. If there is no default case, consider using an enumerated constant whose value is zero that means the case that is not represented by any of the other enumerated constants.
Do not define an enumeration value solely to mirror the state of the enumeration itself. For example, do not define an enumerated constant that merely marks the end of the enumeration. If you need to determine the last value of the enumeration, check for that value explicitly. In addition, you can perform a range check for the first and last enumerated constant if all values within the range are valid.
Do not specify enumerated constants that are reserved for future use.
When you define a method or property that takes an enumerated constant as a value, consider validating the value. The reason is that you can cast a numeric value to the enumeration type even if that numeric value is not defined in the enumeration.
The following example defines two color-related enumerations, SingleHue and MultiHue. The latter has the FlagsAttribute attribute; the former does not. The example shows the difference in behavior when a range of integers, including integers that do not represent underlying values of the enumeration type, are cast to the enumeration type and their string representations displayed. For example, note that 3 cannot be represented as a SingleHue value because 3 is not the underlying value of any SingleHue member, whereas the FlagsAttribute attribute makes it possible to represent 3 as a MultiHue value of Black, Red.
Available since 4.5
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Available since 8.1
Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.