Enum Class

Definition

Provides the base class for enumerations.

[System.Runtime.InteropServices.ComVisible(true)]
public abstract class Enum : ValueType, IComparable, IConvertible, IFormattable
Inheritance
Derived
Attributes
Implements

Inherited Members

System.Object

Examples

The following example demonstrates using an enumeration to represent named values and another enumeration to represent named bit fields.

using namespace System;
enum class Days
{
   Saturday, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday
};

enum class BoilingPoints
{
   Celsius = 100,
   Fahrenheit = 212
};

[FlagsAttribute]

enum class Colors
{
   Red = 1,
   Green = 2,
   Blue = 4,
   Yellow = 8
};

int main()
{
   Type^ weekdays = Days::typeid;
   Type^ boiling = BoilingPoints::typeid;
   Console::WriteLine(  "The days of the week, and their corresponding values in the Days Enum are:" );
   Array^ a = Enum::GetNames( weekdays );
   Int32 i = 0;
   do
   {
      Object^ o = a->GetValue( i );
      Console::WriteLine(  "{0,-11}= {1}", o->ToString(), Enum::Format( weekdays, Enum::Parse( weekdays, o->ToString() ),  "d" ) );
   }
   while ( ++i < a->Length );

   Console::WriteLine();
   Console::WriteLine(  "Enums can also be created which have values that represent some meaningful amount." );
   Console::WriteLine(  "The BoilingPoints Enum defines the following items, and corresponding values:" );
   i = 0;
   Array^ b = Enum::GetNames( boiling );
   do
   {
      Object^ o = b->GetValue( i );
      Console::WriteLine(  "{0,-11}= {1}", o->ToString(), Enum::Format( boiling, Enum::Parse( boiling, o->ToString() ),  "d" ) );
   }
   while ( ++i < b->Length );

   Array^ c = Enum::GetNames( Colors::typeid );
   Colors myColors = Colors::Red | Colors::Blue | Colors::Yellow;
   Console::WriteLine();
   Console::Write(  "myColors holds a combination of colors. Namely:" );
   for ( i = 0; i < 3; i++ )
      Console::Write(  " {0}", c->GetValue( i ) );
}
using System;

public class EnumTest {
    enum Days { Saturday, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday };
    enum BoilingPoints { Celsius = 100, Fahrenheit = 212 };
    [FlagsAttribute]
    enum Colors { Red = 1, Green = 2, Blue = 4, Yellow = 8 };

    public static void Main() {

        Type weekdays = typeof(Days);
        Type boiling = typeof(BoilingPoints);

        Console.WriteLine("The days of the week, and their corresponding values in the Days Enum are:");

        foreach ( string s in Enum.GetNames(weekdays) )
            Console.WriteLine( "{0,-11}= {1}", s, Enum.Format( weekdays, Enum.Parse(weekdays, s), "d"));

        Console.WriteLine();
        Console.WriteLine("Enums can also be created which have values that represent some meaningful amount.");
        Console.WriteLine("The BoilingPoints Enum defines the following items, and corresponding values:");

        foreach ( string s in Enum.GetNames(boiling) )
            Console.WriteLine( "{0,-11}= {1}", s, Enum.Format(boiling, Enum.Parse(boiling, s), "d"));

        Colors myColors = Colors.Red | Colors.Blue | Colors.Yellow;
        Console.WriteLine();
        Console.WriteLine("myColors holds a combination of colors. Namely: {0}", myColors);
    }
}
Public Class EnumTest
    Enum Days
        Saturday
        Sunday
        Monday
        Tuesday
        Wednesday
        Thursday
        Friday
    End Enum 
    
    Enum BoilingPoints
        Celsius = 100
        Fahrenheit = 212
    End Enum 
    
    <FlagsAttribute()> _
    Enum Colors
        Red = 1
        Green = 2
        Blue = 4
        Yellow = 8
    End Enum 

    Public Shared Sub Main()
        Dim weekdays As Type = GetType(Days)
        Dim boiling As Type = GetType(BoilingPoints)

        Console.WriteLine("The days of the week, and their corresponding values in the Days Enum are:")

        Dim s As String
        For Each s In  [Enum].GetNames(weekdays)
            Console.WriteLine("{0,-11} = {1}", s, [Enum].Format(weekdays, [Enum].Parse(weekdays, s), "d"))
        
        Next s
        Console.WriteLine()
        Console.WriteLine("Enums can also be created which have values that represent some meaningful amount.")
        Console.WriteLine("The BoilingPoints Enum defines the following items, and corresponding values:")

        For Each s In  [Enum].GetNames(boiling)
            Console.WriteLine("{0,-11} = {1}", s, [Enum].Format(boiling, [Enum].Parse(boiling, s), "d"))
        Next s

        Dim myColors As Colors = Colors.Red Or Colors.Blue Or Colors.Yellow
        Console.WriteLine()
        Console.WriteLine("myColors holds a combination of colors. Namely: {0}", myColors)
    End Sub 
End Class 

Remarks

An enumeration is a set of named constants whose underlying type is any integral type. If no underlying type is explicitly declared, Int32 is used. Enum is the base class for all enumerations in the .NET Framework. Enumeration types are defined by the enum keyword in C# and the Enum...End Enum construct in Visual Basic.

Enum provides methods for comparing instances of this class, converting the value of an instance to its string representation, converting the string representation of a number to an instance of this class, and creating an instance of a specified enumeration and value.

You can also treat an enumeration as a bit field. For more information, see the Non-Exclusive Members and the Flags Attribute section and the FlagsAttribute topic.

In this topic:

Creating an enumeration type
Instantiating an enumeration type
Enumeration best practices
Performing operations with enumerations
Performing conversions
Parsing enumeration values
Formatting enumeration values
Iterating enumeration members
Non-exclusive members and the Flags attribute
Adding enumeration methods

Creating an enumeration type

Programming languages typically provide syntax to declare an enumeration that consists of a set of named constants and their values. The following example illustrates the syntax used by C# and Visual Basic to define an enumeration. It creates an enumeration named ArrivalStatus that has three members: ArrivalStatus.Early, ArrivalStatus.OnTime, and ArrivalStatus.Late. Note that in both cases, the enumeration does not explicitly inherit from Enum; the inheritance relationship is handled implicitly by the compiler.

public enum ArrivalStatus { Late=-1, OnTime=0, Early=1 };
Public Enum ArrivalStatus As Integer
   Late = -1
   OnTime = 0
   Early = 1
End Enum
Warning

You should never create an enumeration type whose underlying type is non-integral or Char. Although you can create such an enumeration type by using reflection, method calls that use the resulting type are unreliable and may also throw additional exceptions.

Instantiating an enumeration type

You can instantiate an enumeration type just as you instantiate any other value type: by declaring a variable and assigning one of the enumeration's constants to it. The following example instantiates an ArrivalStatus whose value is ArrivalStatus.OnTime.

public class Example
{
   public static void Main()
   {
      ArrivalStatus status = ArrivalStatus.OnTime;
      Console.WriteLine("Arrival Status: {0} ({0:D})", status);
   }
}
// The example displays the following output:
//       Arrival Status: OnTime (0)
Public Module Example
   Public Sub Main()
      Dim status As ArrivalStatus = ArrivalStatus.OnTime
      Console.WriteLine("Arrival Status: {0} ({0:D})", status)
   End Sub
End Module
' The example displays the following output:
'        Arrival Status: OnTime (0)

You can also instantiate an enumeration value in the following ways:

  • By using a particular programming language's features to cast (as in C#) or convert (as in Visual Basic) an integer value to an enumeration value. The following example creates an ArrivalStatus object whose value is ArrivalStatus.Early in this way.

    ArrivalStatus status2 = (ArrivalStatus) 1;
    Console.WriteLine("Arrival Status: {0} ({0:D})", status2);
    // The example displays the following output:
    //       Arrival Status: Early (1)
    
    Dim status2 As ArrivalStatus = CType(1, ArrivalStatus)
    Console.WriteLine("Arrival Status: {0} ({0:D})", status2)
    ' The example displays the following output:
    '       Arrival Status: Early (1)
    
  • By calling its implicit default constructor. As the following example shows, in this case the underlying value of the enumeration instance is 0. However, this is not necessarily the value of a valid constant in the enumeration.

    ArrivalStatus status1 = new ArrivalStatus();
    Console.WriteLine("Arrival Status: {0} ({0:D})", status1);
    // The example displays the following output:
    //       Arrival Status: OnTime (0)
    
    Dim status1 As New ArrivalStatus()
    Console.WriteLine("Arrival Status: {0} ({0:D})", status1)
    ' The example displays the following output:
    '        Arrival Status: OnTime (0)
    
  • By calling the Parse or TryParse method to parse a string that contains the name of a constant in the enumeration. For more information, see the Parsing Enumeration Values section.

  • By calling the ToObject method to convert an integral value to an enumeration type. For more information, see the Performing Conversions section.

Enumeration best practices

We recommend that you use the following best practices when you define enumeration types:

  • If you have not defined an enumeration member whose value is 0, consider creating a None enumerated constant. 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 that your application has to represent, consider using an enumerated constant whose value is zero to represent it. If there is no default case, consider using an enumerated constant whose value is zero to specify the case that is not represented by any of the other enumerated constants.

  • 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.

Additional best practices for enumeration types whose constants are bit fields are listed in the Non-Exclusive Members and the Flags Attribute section.

Performing operations with enumerations

You cannot define new methods when you are creating an enumeration. However, an enumeration type inherits a complete set of static and instance methods from the Enum class. The following sections survey most of these methods, in addition to several other methods that are commonly used when working with enumeration values.

Performing conversions

You can convert between an enumeration member and its underlying type by using a casting (in C#) or conversion (in Visual Basic) operator. The following example uses casting or conversion operators to perform conversions both from an integer to an enumeration value and from an enumeration value to an integer.

int value3 = 2;
ArrivalStatus status3 = (ArrivalStatus) value3;

int value4 = (int) status3;
Dim value3 As Integer = 2
Dim status3 As ArrivalStatus = CType(value3, ArrivalStatus)

Dim value4 As Integer = CInt(status3)

The Enum class also includes a ToObject method that converts a value of any integral type to an enumeration value. The following example uses the ToObject(Type, Int32) method to convert an Int32 to an ArrivalStatus value. Note that, because the ToObject returns a value of type Object, the use of a casting or conversion operator may still be necessary to cast the object to the enumeration type.

int number = -1;
ArrivalStatus arrived = (ArrivalStatus) ArrivalStatus.ToObject(typeof(ArrivalStatus), number);
Dim number As Integer = -1
Dim arrived As ArrivalStatus = CType(ArrivalStatus.ToObject(GetType(ArrivalStatus), number), ArrivalStatus)

When converting an integer to an enumeration value, it is possible to assign a value that is not actually a member of the enumeration. To prevent this, you can pass the integer to the IsDefined method before performing the conversion. The following example uses this method to determine whether the elements in an array of integer values can be converted to ArrivalStatus values.

using System;

public enum ArrivalStatus { Unknown=-3, Late=-1, OnTime=0, Early=1 };

public class Example
{
   public static void Main()
   {
      int[] values = { -3, -1, 0, 1, 5, Int32.MaxValue };
      foreach (var value in values)
      {
         ArrivalStatus status;
         if (Enum.IsDefined(typeof(ArrivalStatus), value))
            status = (ArrivalStatus) value;
         else
            status = ArrivalStatus.Unknown;
         Console.WriteLine("Converted {0:N0} to {1}", value, status);
      }
   }
}
// The example displays the following output:
//       Converted -3 to Unknown
//       Converted -1 to Late
//       Converted 0 to OnTime
//       Converted 1 to Early
//       Converted 5 to Unknown
//       Converted 2,147,483,647 to Unknown
Public Enum ArrivalStatus As Integer
   Unknown = -3
   Late = -1
   OnTime = 0
   Early = 1
End Enum

Module Example
   Public Sub Main()
      Dim values() As Integer = { -3, -1, 0, 1, 5, Int32.MaxValue }
      For Each value In values
         Dim status As ArrivalStatus
         If [Enum].IsDefined(GetType(ArrivalStatus), value)
            status = CType(value, ArrivalStatus) 
         Else
            status = ArrivalStatus.Unknown
         End If
         Console.WriteLine("Converted {0:N0} to {1}", value, status)
      Next   
   End Sub
End Module
' The example displays the following output:
'       Converted -3 to Unknown
'       Converted -1 to Late
'       Converted 0 to OnTime
'       Converted 1 to Early
'       Converted 5 to Unknown
'       Converted 2,147,483,647 to Unknown

Although the Enum class provides explicit interface implementations of the IConvertible interface for converting from an enumeration value to an integral type, you should use the methods of the Convert class, such as ToInt32, to perform these conversions. The following example illustrates how you can use the GetUnderlyingType method along with the Convert.ChangeType method to convert an enumeration value to its underlying type. Note that this example does not require the underlying type of the enumeration to be known at compile time.

ArrivalStatus status = ArrivalStatus.Early;
var number = Convert.ChangeType(status, Enum.GetUnderlyingType(typeof(ArrivalStatus)));
Console.WriteLine("Converted {0} to {1}", status, number);
// The example displays the following output:
//       Converted Early to 1
Dim status As ArrivalStatus = ArrivalStatus.Early
Dim number = Convert.ChangeType(status, [Enum].GetUnderlyingType(GetType(ArrivalStatus)))
Console.WriteLine("Converted {0} to {1}", status, number)
' The example displays the following output:
'       Converted Early to 1

Parsing enumeration values

The Parse and TryParse methods allow you to convert the string representation of an enumeration value to that value. The string representation can be either the name or the underlying value of an enumeration constant. Note that the parsing methods will successfully convert string representations of numbers that are not members of a particular enumeration if the strings can be converted to a value of the enumeration's underlying type. To prevent this, the IsDefined method can be called to ensure that the result of the parsing method is a valid enumeration value. The example illustrates this approach and demonstrates calls to both the Parse(Type, String) and Enum.TryParse<TEnum>(String, TEnum) methods. Note that the non-generic parsing method returns an object that you may have to cast (in C#) or convert (in Visual Basic) to the appropriate enumeration type.

string number = "-1";
string name = "Early";

try {
   ArrivalStatus status1 = (ArrivalStatus) Enum.Parse(typeof(ArrivalStatus), number);
   if (!(Enum.IsDefined(typeof(ArrivalStatus), status1)))
      status1 = ArrivalStatus.Unknown;
   Console.WriteLine("Converted '{0}' to {1}", number, status1);
}
catch (FormatException) {
   Console.WriteLine("Unable to convert '{0}' to an ArrivalStatus value.", 
                     number);
}   
   
ArrivalStatus status2;
if (Enum.TryParse<ArrivalStatus>(name, out status2)) {
   if (!(Enum.IsDefined(typeof(ArrivalStatus), status2)))
      status2 = ArrivalStatus.Unknown;
   Console.WriteLine("Converted '{0}' to {1}", name, status2);
}
else {
   Console.WriteLine("Unable to convert '{0}' to an ArrivalStatus value.", 
                     number);
}
// The example displays the following output:
//       Converted '-1' to Late
//       Converted 'Early' to Early
Dim number As String = "-1"
Dim name As String = "Early"
Dim invalid As String = "32"

Try 
   Dim status1 As ArrivalStatus = CType([Enum].Parse(GetType(ArrivalStatus), number), ArrivalStatus)
   If Not [Enum].IsDefined(GetType(ArrivalStatus), status1) Then status1 = ArrivalStatus.Unknown
   Console.WriteLine("Converted '{0}' to {1}", number, status1)
Catch e As FormatException
   Console.WriteLine("Unable to convert '{0}' to an ArrivalStatus value.", 
                     number)
End Try   
   
Dim status2 As ArrivalStatus
If [Enum].TryParse(Of ArrivalStatus)(name, status2) Then
   If Not [Enum].IsDefined(GetType(ArrivalStatus), status2) Then status2 = ArrivalStatus.Unknown
   Console.WriteLine("Converted '{0}' to {1}", name, status2)
Else
   Console.WriteLine("Unable to convert '{0}' to an ArrivalStatus value.", 
                     number)
End If
' The example displays the following output:
'       Converted '-1' to Late
'       Converted 'Early' to Early

Formatting enumeration values

You can convert enumeration values to their string representations by calling the static Format method, as well as the overloads of the instance ToString method. You can use a format string to control the precise way in which an enumeration value is represented as a string. For more information, see Enumeration Format Strings. The following example uses each of the supported enumeration format strings ("G" or "g", "D" or "d", "X" or "x", and "F" or "f" ) to convert a member of the ArrivalStatus enumeration to its string representations.

string[] formats= { "G", "F", "D", "X"};
ArrivalStatus status = ArrivalStatus.Late;
foreach (var fmt in formats)
   Console.WriteLine(status.ToString(fmt));

// The example displays the following output:
//       Late
//       Late
//       -1
//       FFFFFFFF
Dim formats() As String = { "G", "F", "D", "X"}
Dim status As ArrivalStatus = ArrivalStatus.Late
For Each fmt As String In formats
   Console.WriteLine(status.ToString(fmt))
Next
' The example displays the following output:
'       Late
'       Late
'       -1
'       FFFFFFFF

Iterating enumeration members

The Enum type does not implement the IEnumerable or IEnumerable<T> interface, which would enable you to iterate members of a collection by using a foreach (in C#) or For Each (in Visual Basic) construct. However, you can enumerate members in either of two ways.

  • You can call the GetNames method to retrieve a string array containing the names of the enumeration members. Next, for each element of the string array, you can call the Parse method to convert the string to its equivalent enumeration value. The following example illustrates this approach.

    string[] names = Enum.GetNames(typeof(ArrivalStatus));
    Console.WriteLine("Members of {0}:", typeof(ArrivalStatus).Name);
    Array.Sort(names);
    foreach (var name in names) {
       ArrivalStatus status = (ArrivalStatus) Enum.Parse(typeof(ArrivalStatus), name);
       Console.WriteLine("   {0} ({0:D})", status);
    }
    // The example displays the following output:
    //       Members of ArrivalStatus:
    //          Early (1)
    //          Late (-1)
    //          OnTime (0)
    //          Unknown (-3)      
    
    Dim names() As String = [Enum].GetNames(GetType(ArrivalStatus))
    Console.WriteLine("Members of {0}:", GetType(ArrivalStatus).Name)
    Array.Sort(names)
    For Each name In names
       Dim status As ArrivalStatus = CType([Enum].Parse(GetType(ArrivalStatus), name),
                                     ArrivalStatus)
       Console.WriteLine("   {0} ({0:D})", status)
    Next
    ' The example displays the following output:
    '       Members of ArrivalStatus:
    '          Early (1)
    '          Late (-1)
    '          OnTime (0)
    '          Unknown (-3)      
    
  • You can call the GetValues method to retrieve an array that contains the underlying values in the enumeration. Next, for each element of the array, you can call the ToObject method to convert the integer to its equivalent enumeration value. The following example illustrates this approach.

    var values = Enum.GetValues(typeof(ArrivalStatus));
    Console.WriteLine("Members of {0}:", typeof(ArrivalStatus).Name);
    foreach (var value in values) {
       ArrivalStatus status = (ArrivalStatus) Enum.ToObject(typeof(ArrivalStatus), value);
       Console.WriteLine("   {0} ({0:D})", status);
    }                                       
    // The example displays the following output:
    //       Members of ArrivalStatus:
    //          OnTime (0)
    //          Early (1)
    //          Unknown (-3)
    //          Late (-1)
    
    Dim values = [Enum].GetValues(GetType(ArrivalStatus))
    Console.WriteLine("Members of {0}:", GetType(ArrivalStatus).Name)
    For Each value In values
       Dim status As ArrivalStatus = CType([Enum].ToObject(GetType(ArrivalStatus), value),
                                           ArrivalStatus)
       Console.WriteLine("   {0} ({0:D})", status)
    Next                                       
    ' The example displays the following output:
    '       Members of ArrivalStatus:
    '          OnTime (0)
    '          Early (1)
    '          Unknown (-3)
    '          Late (-1)
    

Non-exclusive members and the Flags attribute

One common use of an enumeration is to represent a set of mutually exclusive values. For example, an ArrivalStatus instance can have a value of Early, OnTime, or Late. It makes no sense for the value of an ArrivalStatus instance to reflect more than one enumeration constant.

In other cases, however, the value of an enumeration object can include multiple enumeration members, and each member represents a bit field in the enumeration value. The FlagsAttribute attribute can be used to indicate that the enumeration consists of bit fields. For example, an enumeration named Pets might be used to indicate the kinds of pets in a household. It can be defined as follows.

[Flags] public enum Pets { None=0, Dog=1, Cat=2, Bird=4, Rodent=8,
                           Reptile=16, Other=32 };

<Flags> Public Enum Pets As Integer
   None = 0
   Dog = 1
   Cat = 2
   Bird = 4
   Rodent = 8
   Reptile = 16
   Other = 32
End Enum

The Pets enumeration can then be used as shown in the following example.

Pets familyPets = Pets.Dog | Pets.Cat;
Console.WriteLine("Pets: {0:G} ({0:D})", familyPets);
// The example displays the following output:
//       Pets: Dog, Cat (3)      
Dim familyPets As Pets = Pets.Dog Or Pets.Cat
Console.WriteLine("Pets: {0:G} ({0:D})", familyPets)
' The example displays the following output:
'       Pets: Dog, Cat (3)      

The following best practices should be used when defining a bitwise enumeration and applying the FlagsAttribute attribute.

  • 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 call the instance HasFlag method, as shown in the following example.

    Pets familyPets = Pets.Dog | Pets.Cat;
    if (familyPets.HasFlag(Pets.Dog))
       Console.WriteLine("The family has a dog.");
    // The example displays the following output:
    //       The family has a dog.      
    
    Dim familyPets As Pets = Pets.Dog Or Pets.Cat
    If familyPets.HasFlag(Pets.Dog) Then
       Console.WriteLine("The family has a dog.")
    End If
    ' The example displays the following output:
    '       The family has a dog.      
    

    It is equivalent to performing 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, and then testing whether the result of that operation is equal to the flag enumerated constant. This is illustrated in the following example.

    Pets familyPets = Pets.Dog | Pets.Cat;
    if ((familyPets & Pets.Dog) == Pets.Dog)
       Console.WriteLine("The family has a dog.");
    // The example displays the following output:
    //       The family has a dog.      
    
    Dim familyPets As Pets = Pets.Dog Or Pets.Cat
    If familyPets And Pets.Dog = Pets.Dog Then
       Console.WriteLine("The family has a dog.")
    End If   
    ' The example displays the following output:
    '       The family has a dog.      
    
  • 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. This is illustrated in the following example.

    Pets familyPets = Pets.Dog | Pets.Cat;
    if (familyPets == Pets.None)
       Console.WriteLine("The family has no pets.");
    else
       Console.WriteLine("The family has pets.");   
    // The example displays the following output:
    //       The family has pets.      
    
    Dim familyPets As Pets = Pets.Dog Or Pets.Cat
    If familyPets = Pets.None Then
       Console.WriteLine("The family has no pets.")
    Else
       Console.WriteLine("The family has pets.")   
    End If
    ' The example displays the following output:
    '       The family has pets.      
    
  • 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.

Adding enumeration methods

Because enumeration types are defined by language structures, such as enum (C#) and Enum (Visual Basic), you cannot define custom methods for an enumeration type other than those methods inherited from the Enum class. However, you can use extension methods to add functionality to a particular enumeration type.

In the following example, the Grades enumeration represents the possible letter grades that a student may receive in a class. An extension method named Passing is added to the Grades type so that each instance of that type now "knows" whether it represents a passing grade or not. The Extensions class also contains a static read-write variable that defines the minimum passing grade. The return value of the Passing extension method reflects the current value of that variable.

using System;

// Define an enumeration to represent student grades.
public enum Grades { F = 0, D = 1, C = 2, B = 3, A = 4 };

// Define an extension method for the Grades enumeration.
public static class Extensions
{        
  public static Grades minPassing = Grades.D;

  public static bool Passing(this Grades grade)
  {
      return grade >= minPassing;
  }
}

class Example
{       
  static void Main()
  {
      Grades g1 = Grades.D;
      Grades g2 = Grades.F;
      Console.WriteLine("{0} {1} a passing grade.", g1, g1.Passing() ? "is" : "is not");
      Console.WriteLine("{0} {1} a passing grade.", g2, g2.Passing() ? "is" : "is not");

      Extensions.minPassing = Grades.C;
      Console.WriteLine("\nRaising the bar!\n");
      Console.WriteLine("{0} {1} a passing grade.", g1, g1.Passing() ? "is" : "is not");
      Console.WriteLine("{0} {1} a passing grade.", g2, g2.Passing() ? "is" : "is not");
  }
}
// The exmaple displays the following output:
//       D is a passing grade.
//       F is not a passing grade.
//       
//       Raising the bar!
//       
//       D is not a passing grade.
//       F is not a passing grade.
Imports System.Runtime.CompilerServices

' Define an enumeration to represent student grades.
Public Enum Grades As Integer
   F = 0
   D = 1
   C = 2
   B = 3
   A = 4
End Enum   

' Define an extension method for the Grades enumeration.
Public Module Extensions
  Public minPassing As Grades = Grades.D
 
  <Extension>
  Public Function Passing(grade As Grades) As Boolean
     Return grade >= minPassing
  End Function
End Module

Public Module Example
  Public Sub Main()
      Dim g1 As Grades = Grades.D
      Dim g2 As Grades = Grades.F
      Console.WriteLine("{0} {1} a passing grade.", 
                        g1, If(g1.Passing(), "is", "is not"))
      Console.WriteLine("{0} {1} a passing grade.", 
                        g2, If(g2.Passing(), "is", "is not"))
      Console.WriteLine()
      
      Extensions.minPassing = Grades.C
      Console.WriteLine("Raising the bar!")
      Console.WriteLine()
      Console.WriteLine("{0} {1} a passing grade.", 
                        g1, If(g1.Passing(), "is", "is not"))
      Console.WriteLine("{0} {1} a passing grade.", 
                        g2, If(g2.Passing(), "is", "is not"))
  End Sub
End Module
' The exmaple displays the following output:
'       D is a passing grade.
'       F is not a passing grade.
'       
'       Raising the bar!
'       
'       D is not a passing grade.
'       F is not a passing grade.

Constructors

Enum()

Initializes a new instance of the Enum class.

Methods

CompareTo(Object)

Compares this instance to a specified object and returns an indication of their relative values.

Equals(Object)

Returns a value indicating whether this instance is equal to a specified object.

Format(Type, Object, String)

Converts the specified value of a specified enumerated type to its equivalent string representation according to the specified format.

GetHashCode()

Returns the hash code for the value of this instance.

GetName(Type, Object)

Retrieves the name of the constant in the specified enumeration that has the specified value.

GetNames(Type)

Retrieves an array of the names of the constants in a specified enumeration.

GetTypeCode()

Returns the type code of the underlying type of this enumeration member.

GetUnderlyingType(Type)

Returns the underlying type of the specified enumeration.

GetValues(Type)

Retrieves an array of the values of the constants in a specified enumeration.

HasFlag(Enum)

Determines whether one or more bit fields are set in the current instance.

IsDefined(Type, Object)

Returns an indication whether a constant with a specified value exists in a specified enumeration.

Parse(Type, String)

Converts the string representation of the name or numeric value of one or more enumerated constants to an equivalent enumerated object.

Parse(Type, String, Boolean)

Converts the string representation of the name or numeric value of one or more enumerated constants to an equivalent enumerated object. A parameter specifies whether the operation is case-insensitive.

Parse<TEnum>(String, Boolean)
Parse<TEnum>(String)
ToObject(Type, Int16)

Converts the specified 16-bit signed integer to an enumeration member.

ToObject(Type, UInt64)

Converts the specified 64-bit unsigned integer value to an enumeration member.

ToObject(Type, UInt32)

Converts the specified 32-bit unsigned integer value to an enumeration member.

ToObject(Type, UInt16)

Converts the specified 16-bit unsigned integer value to an enumeration member.

ToObject(Type, SByte)

Converts the specified 8-bit signed integer value to an enumeration member.

ToObject(Type, Object)

Converts the specified object with an integer value to an enumeration member.

ToObject(Type, Int64)

Converts the specified 64-bit signed integer to an enumeration member.

ToObject(Type, Int32)

Converts the specified 32-bit signed integer to an enumeration member.

ToObject(Type, Byte)

Converts the specified 8-bit unsigned integer to an enumeration member.

ToString(String, IFormatProvider)

This method overload is obsolete; use ToString(String).

ToString(String)

Converts the value of this instance to its equivalent string representation using the specified format.

ToString(IFormatProvider)

This method overload is obsolete; use ToString().

ToString()

Converts the value of this instance to its equivalent string representation.

TryParse(Type, String, Boolean, Object)
TryParse(Type, String, Object)
TryParse<TEnum>(String, Boolean, TEnum)

Converts the string representation of the name or numeric value of one or more enumerated constants to an equivalent enumerated object. A parameter specifies whether the operation is case-sensitive. The return value indicates whether the conversion succeeded.

TryParse<TEnum>(String, TEnum)

Converts the string representation of the name or numeric value of one or more enumerated constants to an equivalent enumerated object. The return value indicates whether the conversion succeeded.

Explicit Interface Implementations

IConvertible.GetTypeCode()
IConvertible.ToBoolean(IFormatProvider)

Converts the current value to a Boolean value based on the underlying type.

IConvertible.ToByte(IFormatProvider)

Converts the current value to an 8-bit unsigned integer based on the underlying type.

IConvertible.ToChar(IFormatProvider)

Converts the current value to a Unicode character based on the underlying type.

IConvertible.ToDateTime(IFormatProvider)

Converts the current value to a DateTime based on the underlying type.

IConvertible.ToDecimal(IFormatProvider)

Converts the current value to a Decimal based on the underlying type.

IConvertible.ToDouble(IFormatProvider)

Converts the current value to a double-precision floating point number based on the underlying type.

IConvertible.ToInt16(IFormatProvider)

Converts the current value to a 16-bit signed integer based on the underlying type.

IConvertible.ToInt32(IFormatProvider)

Converts the current value to a 32-bit signed integer based on the underlying type.

IConvertible.ToInt64(IFormatProvider)

Converts the current value to a 64-bit signed integer based on the underlying type.

IConvertible.ToSByte(IFormatProvider)

Converts the current value to an 8-bit signed integer based on the underlying type.

IConvertible.ToSingle(IFormatProvider)

Converts the current value to a single-precision floating-point number based on the underlying type.

IConvertible.ToString(IFormatProvider)
IConvertible.ToType(Type, IFormatProvider)

Converts the current value to a specified type based on the underlying type.

IConvertible.ToUInt16(IFormatProvider)

Converts the current value to a 16-bit unsigned integer based on the underlying type.

IConvertible.ToUInt32(IFormatProvider)

Converts the current value to a 32-bit unsigned integer based on the underlying type.

IConvertible.ToUInt64(IFormatProvider)

Converts the current value to a 64-bit unsigned integer based on the underlying type.

IFormattable.ToString(String, IFormatProvider)

Thread Safety

This type is thread safe.