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.

Attributes of the FlagsAttribute

AttributeUsageAttribute is applied to this class, and its AttributeUsageAttribute.Inherited property specifies false. This attribute can only be applied to enumerations.

Guidelines for FlagsAttribute and Enum

• Use the FlagsAttribute custom 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 demonstrates the use of FlagsAttribute on the formatting of a Enum. With this attribute, the Position enumeration is used as a bit-field, and the value 3 (Top | Left) is considered a valid value for the enumeration when formatting. Without this attribute, the enumeration Color is not used as a bit-field, and the value 3 (Red | Blue) is not considered a valid value for the enumeration when formatting.

using System;
[FlagsAttribute()] 
public enum Position { 

  Top = 0x1, 
  Left = 0x2, 
  Bottom = 0x4, 
  Right = 0x8 
} 

//enum Color declared without FlagsAttribute 
public enum Color { 

  Red = 0x1, 
  Blue = 0x2, 
  Yellow = 0x4 
} 

public class enumFormat { 

  public static void Main() { 

    Position p = Position.Top | Position.Left; 
    Console.WriteLine("Position: {0}", p); 
    Color c = Color.Red | Color.Blue; 
    Console.WriteLine("Color: {0}", c); 
  } 
} 
      

The output is