Artículo
11/17/2011

Enum Class

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Updated: June 2010

Provides the base class for enumerations.

Inheritance Hierarchy

System.Object
System.ValueType
System.Enum

Namespace: System
Assembly: mscorlib (in mscorlib.dll)

Syntax

'Declaration
<ComVisibleAttribute(True)> _
Public MustInherit Class Enum _
    Inherits ValueType _
    Implements IComparable, IFormattable, IConvertible

[ComVisibleAttribute(true)]
public abstract class Enum : ValueType, 
    IComparable, IFormattable, IConvertible

The Enum type exposes the following members.

Constructors

	Name	Description
	Enum	Initializes a new instance of the Enum class.

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Methods

	Name	Description
	CompareTo	Compares this instance to a specified object and returns an indication of their relative values.
	Equals	Returns a value indicating whether this instance is equal to a specified object. (Overrides ValueType.Equals(Object).)
	Finalize	Allows an object to try to free resources and perform other cleanup operations before the Object is reclaimed by garbage collection. (Inherited from Object.)
	GetHashCode	Returns the hash code for the value of this instance. (Overrides ValueType.GetHashCode().)
	GetName	Retrieves the name of the constant in the specified enumeration that has the specified value.
	GetNames	Retrieves an array of the names of the constants in a specified enumeration.
	GetType	Gets the Type of the current instance. (Inherited from Object.)
	GetTypeCode	Returns the underlying TypeCode for this instance.
	GetUnderlyingType	Returns the underlying type of the specified enumeration.
	GetValues	Retrieves an array of the values of the constants in a specified enumeration.
	HasFlag	Determines whether one or more bit fields are set in the current instance.
	IsDefined	Returns an indication whether a constant with a specified value exists in a specified enumeration.
	MemberwiseClone	Creates a shallow copy of the current Object. (Inherited from Object.)
	Parse	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.
	ToObject	Returns an instance of the specified enumeration set to the specified value.
	ToString()	Converts the value of this instance to its equivalent string representation. (Overrides ValueType.ToString().)
	ToString(IFormatProvider)	Obsolete. This method overload is obsolete; use Enum.ToString().
	ToString(String)	Converts the value of this instance to its equivalent string representation using the specified format.
	ToString(String, IFormatProvider)	Obsolete. This method overload is obsolete; use Enum.ToString(String).
	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.
	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.

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Explicit Interface Implementations

	Name	Description
	IConvertible.ToBoolean	Infrastructure. Converts the current value to a Boolean value based on the underlying type.
	IConvertible.ToByte	Infrastructure. Converts the current value to an 8-bit unsigned integer based on the underlying type.
	IConvertible.ToChar	Infrastructure. Converts the current value to a Unicode character based on the underlying type.
	IConvertible.ToDateTime	Infrastructure. Converts the current value to a DateTime based on the underlying type.
	IConvertible.ToDecimal	Infrastructure. Converts the current value to a Decimal based on the underlying type.
	IConvertible.ToDouble	Infrastructure. Converts the current value to a double-precision floating point number based on the underlying type.
	IConvertible.ToInt16	Infrastructure. Converts the current value to a 16-bit signed integer based on the underlying type.
	IConvertible.ToInt32	Infrastructure. Converts the current value to a 32-bit signed integer based on the underlying type.
	IConvertible.ToInt64	Infrastructure. Converts the current value to a 64-bit signed integer based on the underlying type.
	IConvertible.ToSByte	Infrastructure. Converts the current value to an 8-bit signed integer based on the underlying type.
	IConvertible.ToSingle	Infrastructure. Converts the current value to a single-precision floating point number based on the underlying type.
	IConvertible.ToType	Infrastructure. Converts the current value to a specified type based on the underlying type.
	IConvertible.ToUInt16	Infrastructure. Converts the current value to a 16-bit unsigned integer based on the underlying type.
	IConvertible.ToUInt32	Infrastructure. Converts the current value to a 32-bit unsigned integer based on the underlying type.
	IConvertible.ToUInt64	Infrastructure. Converts the current value to a 64-bit unsigned integer based on the underlying type.

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Remarks

An enumeration is a named constant whose underlying type is any integral type except Char. If no underlying type is explicitly declared, Int32 is used. Programming languages typically provide syntax to declare an enumeration that consists of a set of named constants and their values.

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

Enum provides methods to compare instances of this class, convert the value of an instance to its string representation, convert the string representation of a number to an instance of this class, and create an instance of a specified enumeration and value.

You can also treat an enumeration as a bit field. For more information, see FlagsAttribute.

Implemented Interfaces

This class inherits from ValueType, and implements the IComparable, IFormattable, and IConvertible interfaces. Use the Convert class for conversions instead of this class' explicit interface member implementation of IConvertible.

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.

Examples

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

Public Class Example
   Enum Days
      Sunday
      Monday
      Tuesday
      Wednesday
      Thursday
      Friday
      Saturday
   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 Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      Dim weekdays As Type = GetType(Days)

      outputBlock.Text &= String.Format("The day of the week today is {0}.", _
                          [Enum].Parse(weekdays, Date.Now.DayOfWeek.ToString(), False))
      outputBlock.Text &= vbCrLf

      outputBlock.Text &= "Enums can also represent some meaningful value. For example:" & vbCrLf
      outputBlock.Text &= String.Format("The BoilingPoints Enum defines the following items, and corresponding values:") + vbCrLf
      outputBlock.Text &= String.Format("   The boiling point in degrees {0:G} is {0:D}.", _ 
                                        BoilingPoints.Celsius) & vbCrLf
      outputBlock.Text &= String.Format("   The boiling point in degrees {0:G} is {0:D}.", _
                                        BoilingPoints.Fahrenheit) & vbCrLf

      Dim myColors As Colors = Colors.Red Or Colors.Blue Or Colors.Yellow
      outputBlock.Text &= String.Format("myColors holds a combination of colors. Namely: {0}", myColors) & vbCrLf
   End Sub
End Class

using System;

public class Example
{
   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 Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      Type weekdays = typeof(Days);

      outputBlock.Text += String.Format("The day of the week today is {0}.", 
                          Enum.Parse(weekdays, DateTime.Now.DayOfWeek.ToString(), false));
      outputBlock.Text += "\n";

      outputBlock.Text += "Enums can also represent some meaningful value. For example:" + "\n";
      outputBlock.Text += String.Format("The BoilingPoints Enum defines the following items, and corresponding values:") + "\n";
      outputBlock.Text += String.Format("   The boiling point in degrees {0:G} is {0:D}.", 
                                        BoilingPoints.Celsius) + "\n";
      outputBlock.Text += String.Format("   The boiling point in degrees {0:G} is {0:D}.", 
                                        BoilingPoints.Fahrenheit) + "\n";

      Colors myColors = Colors.Red | Colors.Blue | Colors.Yellow;
      outputBlock.Text += String.Format("myColors holds the following combination of colors: {0}", myColors) + "\n";
   }
}

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

Thread Safety

This type is thread safe.

Change History

Date	History	Reason
June 2010	Added warning about non-integer enumerations.	Customer feedback.

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