Choosing the default value of an Enum type without having to change values

Question

In C   is it possible to decorate an Enum type with an attribute or do something else to specify what the default value should be  without having the change the values  The numbers required might be set in stone for whatever reason  and it d be handy to still have control over the default   enum Orientation       None   -1      North   0      East   1      South   2      West   3    Orientation o     Is  North  by default

User · Answer

DefaultValue None   public enum Orientation       None   -1      North   0      East   1      South   2      West   3     Then in the code you can use   public Orientation GetDefaultOrientation        return default Orientation

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An enum s default is whatever enumeration equates to zero   I don t believe this is changeable by attribute or other means    MSDN says   The default value of an enum E is the value produced by the expression  E 0

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Actually an enum s default is the first element in the enum whose value is 0   So for example   public enum Animals       Cat      Dog      Pony   0      its value will actually be Cat not Pony unless you assign a non zero value to Cat  Animals animal

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The default for an enum  in fact  any value type  is 0 -- even if that is not a valid value for that enum   It cannot be changed

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If zero doesn t work as the proper default value  you can use the component model to define a workaround for the enum    DefaultValue None   public enum Orientation        None   -1       North   0       East   1       South   2       West   3     public static class Utilities       public static TEnum GetDefaultValue lt TEnum gt    where TEnum   struct               Type t   typeof TEnum           DefaultValueAttribute   attributes    DefaultValueAttribute   t GetCustomAttributes typeof DefaultValueAttribute   false           if  attributes    null  amp  amp              attributes Length  gt  0                        return  TEnum attributes 0  Value                    else                       return default TEnum                       and then you can call   Orientation o   Utilities GetDefaultValue lt Orientation gt     System Diagnostics Debug Print o ToString       Note  you will need to include the following line at the top of the file   using System ComponentModel    This does not change the actual C  language default value of the enum  but gives a way to indicate  and get  the desired default value

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The default value of any enum is zero  So if you want to set one enumerator to be the default value  then set that one to zero and all other enumerators to non-zero  the first enumerator to have the value zero will be the default value for that enum if there are several enumerators with the value zero    enum Orientation       None   0    default value since it has the value  0      North   1      East   2      South   3      West   4    Orientation o     initialized to  None    If your enumerators don t need explicit values  then just make sure the first enumerator is the one you want to be the default enumerator since  By default  the first enumerator has the value 0  and the value of each successive enumerator is increased by 1    C  reference   enum Orientation       None    default value since it is the first enumerator     North      East      South      West    Orientation o     initialized to  None

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If you define the Default enum as the enum with the smallest value you can use this   public enum MyEnum   His   -1  Hers   -2  Mine   -4  Theirs   -3    var firstEnum     MyEnum   Enum GetValues typeof MyEnum    0    firstEnum    Mine    This doesn t assume that the enum has  a zero value

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The default is the first one in the definition  For example   public enum MyEnum His Hers Mine Theirs   Enum GetValues typeOf MyEnum   GetValue 0     This will return His

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One more way that might be helpful - to use some kind of  alias   For example   public enum Status       New   10      Old   20      Actual   30          Use Status Default to specify default status in your code       Default   New

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enum Orientations       None  North  East  South  West   private Orientations   orientation   get  set     public Orientations  Orientation       get               return  orientation    Orientations None            set                orientation   value            If you set the property to null will return Orientations None on get  The property  orientation  is null by default

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The default value for an enumeration type is 0     By default  the first enumerator has the value 0  and the value of each successive enumerator is increased by 1     The value type enum has the value produced by the expression  E 0  where E is the enum identifier     You can check the documentation for C  enum here  and the documentation for C  default values table here

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You can t  but if you want  you can do some trick           public struct Orientation                           public static Orientation None   -1          public static Orientation North   0          public static Orientation East   1          public static Orientation South   2          public static Orientation West   3          usage of this struct as simple enum  where you can create p a    Orientation East  or any value that you want  by default to use the trick itself  you need to convert from int by code  there the implementation            region ConvertingToEnum         private int val          static Dictionary lt int  string gt  dict   null           public Orientation int val                        this val   val                     public static implicit operator Orientation int value                        return new Orientation value - 1                      public static bool operator    Orientation a  Orientation b                        return a val    b val                     public static bool operator    Orientation a  Orientation b                        return a val    b val                     public override string ToString                         if  dict    null                  InitializeDict                if  dict ContainsKey val                   return dict val               return val ToString                       private void InitializeDict                         dict   new Dictionary lt int  string gt                 foreach  var fields in GetType   GetFields BindingFlags Public   BindingFlags Static                                 dict Add   Orientation fields GetValue null   val  fields Name                                     endregion

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The default value of enum is the enummember equal to 0 or the first element if value is not specified      But i have faced critical problems using enum in my projects and overcome by doing something below     How ever my need was related to class level          class CDDtype               public int Id   get  set            public DDType DDType   get  set             public CDDtype                         DDType   DDType None                             DefaultValue None       public enum DDType                      None   -1                 ON   0                 FC   1                 NC   2                 CC   3         and get expected result      CDDtype d1  new CDDtype        CDDtype d2   new CDDtype   Id   50         Console Write d1 DDType    None     Console Write d2 DDType    None   Now what if value is coming from DB      Ok in this scenario    pass value in below function and it will convertthe  value to enum    below function handled various scenario and it s generic     and believe me it is very fast                public static T ToEnum lt T gt  this object value                  Checking value is null or DBNull         if   value IsNull                          return  T Enum Parse typeof T   value ToStringX                          Returanable object         object ValueToReturn   null             First checking whether any  DefaultValueAttribute  is present or not         var DefaultAtt    from a in typeof T  CustomAttributes                           where a AttributeType    typeof DefaultValueAttribute                            select a  FirstOrNull               If DefaultAttributeValue is present         if    DefaultAtt IsNull     amp  amp   DefaultAtt ConstructorArguments Count    1                         ValueToReturn   DefaultAtt ConstructorArguments 0  Value                       If still no value found         if  ValueToReturn IsNull                            Trying to get the very first property of that enum             Array Values   Enum GetValues typeof T                   getting very first member of this enum             if  Values Length  gt  0                                ValueToReturn   Values GetValue 0                                      If any result found         if   ValueToReturn IsNull                          return  T Enum Parse typeof T   ValueToReturn ToStringX                        return default T

[c#] Choosing the default value of an Enum type without having to change values

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