Create Generic method constraining T to an Enum

Question

I m building a function to extend the Enum Parse concept that   Allows a default value to be parsed in case that an Enum value is not found Is case insensitive   So I wrote the following   public static T GetEnumFromString lt T gt  string value  T defaultValue  where T   Enum       if  string IsNullOrEmpty value   return defaultValue      foreach  T item in Enum GetValues typeof T                  if  item ToString   ToLower   Equals value Trim   ToLower     return item            return defaultValue      I am getting a Error Constraint cannot be special class System Enum   Fair enough  but is there a workaround to allow a Generic Enum  or am I going to have to mimic the Parse function and pass a type as an attribute  which forces the ugly boxing requirement to your code   EDIT All suggestions below have been greatly appreciated  thanks   Have settled on  I ve left the loop to maintain case insensitivity - I am using this when parsing XML   public static class EnumUtils       public static T ParseEnum lt T gt  string value  T defaultValue  where T   struct  IConvertible               if   typeof T  IsEnum  throw new ArgumentException  T must be an enumerated type            if  string IsNullOrEmpty value   return defaultValue           foreach  T item in Enum GetValues typeof T                          if  item ToString   ToLower   Equals value Trim   ToLower     return item                    return defaultValue            EDIT   16th Feb 2015  Julien Lebosquain has recently posted a compiler enforced type-safe generic solution in MSIL or F  below  which is well worth a look  and an upvote  I will remove this edit if the solution bubbles further up the page

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You can define a static constructor for the class that will check that the type T is an enum and throw an exception if it is not. This is the method mentioned by Jeffery Richter in his book CLR via C#.

internal sealed class GenericTypeThatRequiresAnEnum<T> {
    static GenericTypeThatRequiresAnEnum() {
        if (!typeof(T).IsEnum) {
        throw new ArgumentException("T must be an enumerated type");
        }
    }
}

Then in the parse method, you can just use Enum.Parse(typeof(T), input, true) to convert from string to the enum. The last true parameter is for ignoring case of the input.

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I loved Christopher Currens s solution using IL but for those who don t want to deal with tricky business of including MSIL into their build process I wrote similar function in C    Please note though that you can t use generic restriction like where T   Enum because Enum is special type  Therefore I have to check if given generic type is really enum   My function is   public static T GetEnumFromString lt T gt  string strValue  T defaultValue           Check if it realy enum at runtime      if   typeof T  IsEnum          throw new ArgumentException  Method GetEnumFromString can be used with enums only         if   string IsNullOrEmpty strValue                 IEnumerator enumerator   Enum GetValues typeof T   GetEnumerator            while  enumerator MoveNext                          T temp    T enumerator Current              if  temp ToString   ToLower   Equals strValue Trim   ToLower                     return temp                       return defaultValue

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This is my implementation  Basically  you can setup any attribute and it works    public static class EnumExtensions               public static string GetDescription this Enum  enum                        Type type    enum GetType                FieldInfo fi   type GetField  enum ToString                 DescriptionAttribute   attrs                   fi GetCustomAttributes typeof DescriptionAttribute   false  as DescriptionAttribute                if  attrs Length  gt  0                                return attrs 0  Description                            return null

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I modified the sample by dimarzionist  This version will only work with Enums and not let structs get through    public static T ParseEnum lt T gt  string enumString      where T   struct    enum            if  String IsNullOrEmpty enumString      typeof T  IsEnum         throw new Exception  Type given must be an Enum        try               return  T Enum Parse typeof T   enumString  true             catch  Exception ex               return default T

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I created an extension Method to get integer value from enum  take look at method implementation   public static int ToInt lt T gt  this T soure  where T   IConvertible  enum       if  typeof T  IsEnum                return  int   IConvertible soure    the tricky part             else           throw new ArgumentException  T must be an enumerated type        return soure ToInt32 CultureInfo CurrentCulture       this is usage   MemberStatusEnum Activated ToInt     using extension Method  int  MemberStatusEnum Activated   the ordinary way

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This is my implementation  Basically  you can setup any attribute and it works    public static class EnumExtensions               public static string GetDescription this Enum  enum                        Type type    enum GetType                FieldInfo fi   type GetField  enum ToString                 DescriptionAttribute   attrs                   fi GetCustomAttributes typeof DescriptionAttribute   false  as DescriptionAttribute                if  attrs Length  gt  0                                return attrs 0  Description                            return null

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Hope this is helpful   public static TValue ParseEnum lt TValue gt  string value  TValue defaultValue                    where TValue   struct    enum          try                     if  String IsNullOrEmpty value                     return defaultValue              return  TValue Enum Parse typeof  TValue   value                 catch Exception ex                      return defaultValue

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I created an extension Method to get integer value from enum  take look at method implementation   public static int ToInt lt T gt  this T soure  where T   IConvertible  enum       if  typeof T  IsEnum                return  int   IConvertible soure    the tricky part             else           throw new ArgumentException  T must be an enumerated type        return soure ToInt32 CultureInfo CurrentCulture       this is usage   MemberStatusEnum Activated ToInt     using extension Method  int  MemberStatusEnum Activated   the ordinary way

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It should also be considered that since the release of C  7 3 using Enum constraints is supported out-of-the-box without having to do additional checking and stuff   So going forward and given you ve changed the language version of your project to C  7 3 the following code is going to work perfectly fine       private static T GetEnumFromString lt T gt  string value  T defaultValue  where T   Enum                  Your code goes here            In case you re don t know how to change the language version to C  7 3 see the following screenshot    EDIT 1 - Required Visual Studio Version and considering ReSharper  For Visual Studio to recognize the new syntax you need at least version 15 7  You can find that also mentioned in Microsoft s release notes  see Visual Studio 2017 15 7 Release Notes  Thanks  MohamedElshawaf for pointing out this valid question   Pls also note that in my case ReSharper 2018 1 as of writing this EDIT does not yet support C  7 3  Having ReSharper activated it highlights the Enum constraint as an error telling me Cannot use  System Array    System Delegate    System Enum    System ValueType    object  as type parameter constraint   ReSharper suggests as a quick fix to Remove  Enum  constraint of type paramter T of method   However  if you turn off ReSharper temporarily under Tools -  Options -  ReSharper Ultimate -  General you ll see that the syntax is perfectly fine given that you use VS 15 7 or higher and C  7 3 or higher

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I modified the sample by dimarzionist  This version will only work with Enums and not let structs get through    public static T ParseEnum lt T gt  string enumString      where T   struct    enum            if  String IsNullOrEmpty enumString      typeof T  IsEnum         throw new Exception  Type given must be an Enum        try               return  T Enum Parse typeof T   enumString  true             catch  Exception ex               return default T

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Since Enum Type implements IConvertible interface  a better implementation should be something like this   public T GetEnumFromString lt T gt  string value  where T   struct  IConvertible      if   typeof T  IsEnum              throw new ArgumentException  T must be an enumerated type                       This will still permit passing of value types implementing IConvertible  The chances are rare though

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Just for completeness  the following is a Java solution   I am certain the same could be done in C  as well   It avoids having to specify the type anywhere in code - instead  you specify it in the strings you are trying to parse   The problem is that there isn t any way to know which enumeration the String might match - so the answer is to solve that problem   Instead of accepting just the string value  accept a String that has both the enumeration and the value in the form  enumeration value    Working code is below - requires Java 1 8 or later   This would also make the XML more precise as in you would see something like color  Color red  instead of just color  red    You would call the acceptEnumeratedValue   method with a string containing the enum name dot value name     The method returns the formal enumerated value   import java util HashMap  import java util Map  import java util function Function    public class EnumFromString        enum NumberEnum  One  Two  Three       enum LetterEnum  A  B  C         Map lt String  Function lt String    extends Enum gt  gt  enumsByName   new HashMap lt  gt          public static void main String   args            EnumFromString efs   new EnumFromString             System out print   nFirst string is NumberEnum Two - enum is     efs acceptEnumeratedValue  NumberEnum Two   name             System out print   nSecond string is LetterEnum B - enum is     efs acceptEnumeratedValue  LetterEnum B   name                 public EnumFromString             enumsByName put  NumberEnum   s - gt   return NumberEnum valueOf s              enumsByName put  LetterEnum   s - gt   return LetterEnum valueOf s                 public Enum acceptEnumeratedValue String enumDotValue             int pos   enumDotValue indexOf                String enumName   enumDotValue substring 0  pos           String value   enumDotValue substring pos   1            Enum enumeratedValue   enumsByName get enumName  apply value            return enumeratedValue

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C    7 3  Starting with C  7 3  available with Visual Studio 2017   v15 7   this code is now completely valid   public static TEnum Parse lt TEnum gt  string value      where TEnum   struct  Enum              C    7 2  You can have a real compiler enforced enum constraint by abusing constraint inheritance  The following code specifies both a class and a struct constraints at the same time   public abstract class EnumClassUtils lt TClass gt  where TClass   class        public static TEnum Parse lt TEnum gt  string value      where TEnum   struct  TClass               return  TEnum  Enum Parse typeof TEnum   value             public class EnumUtils   EnumClassUtils lt Enum gt        Usage   EnumUtils Parse lt SomeEnum gt   value      Note  this is specifically stated in the C  5 0 language specification      If type parameter S depends on type parameter T then          It is valid for   S to have the value type constraint and T to have the reference type   constraint  Effectively this limits T to the types System Object    System ValueType  System Enum  and any interface type

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I tried to improve the code a bit   public T LoadEnum lt T gt  string value  T defaultValue   default T   where T   struct  IComparable  IFormattable  IConvertible       if  Enum IsDefined typeof T   value                 return  T Enum Parse typeof T   value  true             return defaultValue

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I do have specific requirement where I required to use enum with text associated with enum value  For example when I use enum to specify error type it required to describe error details   public static class XmlEnumExtension       public static string ReadXmlEnumAttribute this Enum value                if  value    null  throw new ArgumentNullException  value            var attribs    XmlEnumAttribute    value GetType   GetField value ToString    GetCustomAttributes typeof  XmlEnumAttribute   true           return attribs Length  gt  0   attribs 0  Name   value ToString               public static T ParseXmlEnumAttribute lt T gt  this string str                foreach  T item in Enum GetValues typeof T                          var attribs    XmlEnumAttribute   item GetType   GetField item ToString    GetCustomAttributes typeof XmlEnumAttribute   true               if attribs Length  gt  0  amp  amp  attribs 0  Name Equals str   return item                    return  T Enum Parse typeof T   str  true            public enum MyEnum        XmlEnum  First Value        One       XmlEnum  Second Value        Two      Three     static void Main             Parsing from XmlEnum attribute     var str    Second Value       var me   str ParseXmlEnumAttribute lt MyEnum gt         System Console WriteLine me ReadXmlEnumAttribute            Parsing without XmlEnum     str    Three       me   str ParseXmlEnumAttribute lt MyEnum gt         System Console WriteLine me ReadXmlEnumAttribute         me   MyEnum One      System Console WriteLine me ReadXmlEnumAttribute

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Hope this is helpful   public static TValue ParseEnum lt TValue gt  string value  TValue defaultValue                    where TValue   struct    enum          try                     if  String IsNullOrEmpty value                     return defaultValue              return  TValue Enum Parse typeof  TValue   value                 catch Exception ex                      return defaultValue

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Hope this is helpful   public static TValue ParseEnum lt TValue gt  string value  TValue defaultValue                    where TValue   struct    enum          try                     if  String IsNullOrEmpty value                     return defaultValue              return  TValue Enum Parse typeof  TValue   value                 catch Exception ex                      return defaultValue

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This feature is finally supported in C  7 3  The following snippet  from the dotnet samples  demonstrates how  public static Dictionary lt int  string gt  EnumNamedValues lt T gt    where T   System Enum       var result   new Dictionary lt int  string gt         var values   Enum GetValues typeof T         foreach  int item in values          result Add item  Enum GetName typeof T   item        return result     Be sure to set your language version in your C  project to version 7 3   Original Answer below  I m late to the game  but I took it as a challenge to see how it could be done   It s not possible in C   or VB NET  but scroll down for F    but is possible in MSIL   I wrote this little    thing    license  http   www apache org licenses LICENSE-2 0 html  assembly MyThing    class public abstract sealed MyThing Thing        extends  mscorlib System Object      method public static   T  GetEnumFromString lt valuetype  ctor   mscorlib System Enum  T gt  string strValue                                                                                              T defaultValue  cil managed          maxstack  2      locals init   0    T temp                     1    T return value                     2  class  mscorlib System Collections IEnumerator enumerator                     3  class  mscorlib System IDisposable disposer         if string IsNullOrEmpty strValue   return defaultValue      ldarg strValue     call bool  mscorlib System String  IsNullOrEmpty string      brfalse s HASVALUE     br RETURNDEF            return default it empty             foreach  T item in Enum GetValues typeof T      HASVALUE         Enum GetValues GetEnumerator       ldtoken   T     call class  mscorlib System Type  mscorlib System Type  GetTypeFromHandle valuetype  mscorlib System RuntimeTypeHandle      call class  mscorlib System Array  mscorlib System Enum  GetValues class  mscorlib System Type      callvirt instance class  mscorlib System Collections IEnumerator  mscorlib System Array  GetEnumerator        stloc enumerator      try             CONDITION          ldloc enumerator         callvirt instance bool  mscorlib System Collections IEnumerator  MoveNext           brfalse s LEAVE                STATEMENTS             T item    T Enumerator Current         ldloc enumerator         callvirt instance object  mscorlib System Collections IEnumerator  get Current           unbox any   T         stloc temp         ldloca s temp         constrained    T                     if  item ToString   ToLower   Equals value Trim   ToLower     return item          callvirt instance string  mscorlib System Object  ToString           callvirt instance string  mscorlib System String  ToLower           ldarg strValue         callvirt instance string  mscorlib System String  Trim           callvirt instance string  mscorlib System String  ToLower           callvirt instance bool  mscorlib System String  Equals string          brfalse s CONDITION         ldloc temp         stloc return value         leave s RETURNVAL                LEAVE          leave s RETURNDEF           finally                  ArrayList s Enumerator may or may not inherit from IDisposable         ldloc enumerator         isinst  mscorlib System IDisposable         stloc s disposer         ldloc s disposer         ldnull         ceq         brtrue s LEAVEFINALLY         ldloc s disposer         callvirt instance void  mscorlib System IDisposable  Dispose         LEAVEFINALLY          endfinally            RETURNDEF      ldarg defaultValue     stloc return value      RETURNVAL      ldloc return value     ret         Which generates a function that would look like this  if it were valid C   T GetEnumFromString lt T gt  string valueString  T defaultValue  where T   Enum  Then with the following C  code  using MyThing     stuff    private enum MyEnum   Yes  No  Okay   static void Main string   args        Thing GetEnumFromString  quot No quot   MyEnum Yes      returns MyEnum No     Thing GetEnumFromString  quot Invalid quot   MyEnum Okay       returns MyEnum Okay     Thing GetEnumFromString  quot AnotherInvalid quot   0      compiler error  not an Enum    Unfortunately  this means having this part of your code written in MSIL instead of C   with the only added benefit being that you re able to constrain this method by System Enum   It s also kind of a bummer  because it gets compiled into a separate assembly   However  it doesn t mean you have to deploy it that way  By removing the line  assembly MyThing   and invoking ilasm as follows  ilasm exe  DLL  OUTPUT MyThing netmodule  you get a netmodule instead of an assembly  Unfortunately  VS2010  and earlier  obviously  does not support adding netmodule references  which means you d have to leave it in 2 separate assemblies when you re debugging   The only way you can add them as part of your assembly would be to run csc exe yourself using the  addmodule  files  command line argument   It wouldn t be too painful in an MSBuild script   Of course  if you re brave or stupid  you can run csc yourself manually each time   And it certainly gets more complicated as multiple assemblies need access to it  So  it CAN be done in  Net   Is it worth the extra effort   Um  well  I guess I ll let you decide on that one   F  Solution as alternative Extra Credit   It turns out that a generic restriction on enum is possible in at least one other  NET language besides MSIL  F   type MyThing       static member GetEnumFromString lt  T when  T   gt  Enum gt  str defaultValue   T               protect for null  only required in interop with C           let str   if isNull str then String Empty else str          Enum GetValues typedefof lt  T gt             gt  Seq cast lt   gt            gt  Seq tryFind fun v - gt  String Compare v ToString    str Trim    true    0            gt  function Some x - gt  x   None - gt  defaultValue  This one is easier to maintain since it s a well-known language with full Visual Studio IDE support  but you still need a separate project in your solution for it  However  it naturally produces considerably different IL  the code is very different  and it relies on the FSharp Core library  which  just like any other external library  needs to become part of your distribution  Here s how you can use it  basically the same as the MSIL solution   and to show that it correctly fails on otherwise synonymous structs     works  result is inferred to have type StringComparison var result   MyThing GetEnumFromString  quot OrdinalIgnoreCase quot   StringComparison Ordinal      type restriction is recognized by C   this fails at compile time var result   MyThing GetEnumFromString  quot OrdinalIgnoreCase quot   42

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This feature is finally supported in C  7 3  The following snippet  from the dotnet samples  demonstrates how  public static Dictionary lt int  string gt  EnumNamedValues lt T gt    where T   System Enum       var result   new Dictionary lt int  string gt         var values   Enum GetValues typeof T         foreach  int item in values          result Add item  Enum GetName typeof T   item        return result     Be sure to set your language version in your C  project to version 7 3   Original Answer below  I m late to the game  but I took it as a challenge to see how it could be done   It s not possible in C   or VB NET  but scroll down for F    but is possible in MSIL   I wrote this little    thing    license  http   www apache org licenses LICENSE-2 0 html  assembly MyThing    class public abstract sealed MyThing Thing        extends  mscorlib System Object      method public static   T  GetEnumFromString lt valuetype  ctor   mscorlib System Enum  T gt  string strValue                                                                                              T defaultValue  cil managed          maxstack  2      locals init   0    T temp                     1    T return value                     2  class  mscorlib System Collections IEnumerator enumerator                     3  class  mscorlib System IDisposable disposer         if string IsNullOrEmpty strValue   return defaultValue      ldarg strValue     call bool  mscorlib System String  IsNullOrEmpty string      brfalse s HASVALUE     br RETURNDEF            return default it empty             foreach  T item in Enum GetValues typeof T      HASVALUE         Enum GetValues GetEnumerator       ldtoken   T     call class  mscorlib System Type  mscorlib System Type  GetTypeFromHandle valuetype  mscorlib System RuntimeTypeHandle      call class  mscorlib System Array  mscorlib System Enum  GetValues class  mscorlib System Type      callvirt instance class  mscorlib System Collections IEnumerator  mscorlib System Array  GetEnumerator        stloc enumerator      try             CONDITION          ldloc enumerator         callvirt instance bool  mscorlib System Collections IEnumerator  MoveNext           brfalse s LEAVE                STATEMENTS             T item    T Enumerator Current         ldloc enumerator         callvirt instance object  mscorlib System Collections IEnumerator  get Current           unbox any   T         stloc temp         ldloca s temp         constrained    T                     if  item ToString   ToLower   Equals value Trim   ToLower     return item          callvirt instance string  mscorlib System Object  ToString           callvirt instance string  mscorlib System String  ToLower           ldarg strValue         callvirt instance string  mscorlib System String  Trim           callvirt instance string  mscorlib System String  ToLower           callvirt instance bool  mscorlib System String  Equals string          brfalse s CONDITION         ldloc temp         stloc return value         leave s RETURNVAL                LEAVE          leave s RETURNDEF           finally                  ArrayList s Enumerator may or may not inherit from IDisposable         ldloc enumerator         isinst  mscorlib System IDisposable         stloc s disposer         ldloc s disposer         ldnull         ceq         brtrue s LEAVEFINALLY         ldloc s disposer         callvirt instance void  mscorlib System IDisposable  Dispose         LEAVEFINALLY          endfinally            RETURNDEF      ldarg defaultValue     stloc return value      RETURNVAL      ldloc return value     ret         Which generates a function that would look like this  if it were valid C   T GetEnumFromString lt T gt  string valueString  T defaultValue  where T   Enum  Then with the following C  code  using MyThing     stuff    private enum MyEnum   Yes  No  Okay   static void Main string   args        Thing GetEnumFromString  quot No quot   MyEnum Yes      returns MyEnum No     Thing GetEnumFromString  quot Invalid quot   MyEnum Okay       returns MyEnum Okay     Thing GetEnumFromString  quot AnotherInvalid quot   0      compiler error  not an Enum    Unfortunately  this means having this part of your code written in MSIL instead of C   with the only added benefit being that you re able to constrain this method by System Enum   It s also kind of a bummer  because it gets compiled into a separate assembly   However  it doesn t mean you have to deploy it that way  By removing the line  assembly MyThing   and invoking ilasm as follows  ilasm exe  DLL  OUTPUT MyThing netmodule  you get a netmodule instead of an assembly  Unfortunately  VS2010  and earlier  obviously  does not support adding netmodule references  which means you d have to leave it in 2 separate assemblies when you re debugging   The only way you can add them as part of your assembly would be to run csc exe yourself using the  addmodule  files  command line argument   It wouldn t be too painful in an MSBuild script   Of course  if you re brave or stupid  you can run csc yourself manually each time   And it certainly gets more complicated as multiple assemblies need access to it  So  it CAN be done in  Net   Is it worth the extra effort   Um  well  I guess I ll let you decide on that one   F  Solution as alternative Extra Credit   It turns out that a generic restriction on enum is possible in at least one other  NET language besides MSIL  F   type MyThing       static member GetEnumFromString lt  T when  T   gt  Enum gt  str defaultValue   T               protect for null  only required in interop with C           let str   if isNull str then String Empty else str          Enum GetValues typedefof lt  T gt             gt  Seq cast lt   gt            gt  Seq tryFind fun v - gt  String Compare v ToString    str Trim    true    0            gt  function Some x - gt  x   None - gt  defaultValue  This one is easier to maintain since it s a well-known language with full Visual Studio IDE support  but you still need a separate project in your solution for it  However  it naturally produces considerably different IL  the code is very different  and it relies on the FSharp Core library  which  just like any other external library  needs to become part of your distribution  Here s how you can use it  basically the same as the MSIL solution   and to show that it correctly fails on otherwise synonymous structs     works  result is inferred to have type StringComparison var result   MyThing GetEnumFromString  quot OrdinalIgnoreCase quot   StringComparison Ordinal      type restriction is recognized by C   this fails at compile time var result   MyThing GetEnumFromString  quot OrdinalIgnoreCase quot   42

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This is my take at it  Combined from the answers and MSDN  public static TEnum ParseToEnum lt TEnum gt  this string text  where TEnum   struct  IConvertible  IComparable  IFormattable       if  string IsNullOrEmpty text      typeof TEnum  IsEnum          throw new ArgumentException  TEnum must be an Enum type         try               var enumValue    TEnum Enum Parse typeof TEnum   text Trim    true           return enumValue            catch  Exception                throw new ArgumentException string Format   0  is not a member of the  1  enumeration    text  typeof TEnum  Name              MSDN Source

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I ve encapsulated Vivek s solution into a utility class that you can reuse  Please note that you still should define type constraints  where T   struct  IConvertible  on your type   using System   internal static class EnumEnforcer            lt summary gt          Makes sure that generic input parameter is of an enumerated type           lt  summary gt           lt typeparam name  T  gt Type that should be checked  lt  typeparam gt           lt param name  typeParameterName  gt Name of the type parameter  lt  param gt           lt param name  methodName  gt Name of the method which accepted the parameter  lt  param gt      public static void EnforceIsEnum lt T gt  string typeParameterName  string methodName          where T   struct  IConvertible               if   typeof T  IsEnum                        string message   string Format                   Generic parameter  0  in  1  method forces an enumerated type  Make sure your type parameter  0  is an enum                    typeParameterName                  methodName                throw new ArgumentException message                             lt summary gt          Makes sure that generic input parameter is of an enumerated type           lt  summary gt           lt typeparam name  T  gt Type that should be checked  lt  typeparam gt           lt param name  typeParameterName  gt Name of the type parameter  lt  param gt           lt param name  methodName  gt Name of the method which accepted the parameter  lt  param gt           lt param name  inputParameterName  gt Name of the input parameter of this page  lt  param gt      public static void EnforceIsEnum lt T gt  string typeParameterName  string methodName  string inputParameterName          where T   struct  IConvertible               if   typeof T  IsEnum                        string message   string Format                   Generic parameter  0  in  1  method forces an enumerated type  Make sure your input parameter  2  is of correct type                    typeParameterName                  methodName                  inputParameterName                throw new ArgumentException message                             lt summary gt          Makes sure that generic input parameter is of an enumerated type           lt  summary gt           lt typeparam name  T  gt Type that should be checked  lt  typeparam gt           lt param name  exceptionMessage  gt Message to show in case T is not an enum  lt  param gt      public static void EnforceIsEnum lt T gt  string exceptionMessage          where T   struct  IConvertible               if   typeof T  IsEnum                        throw new ArgumentException exceptionMessage

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C    7 3  Starting with C  7 3  available with Visual Studio 2017   v15 7   this code is now completely valid   public static TEnum Parse lt TEnum gt  string value      where TEnum   struct  Enum              C    7 2  You can have a real compiler enforced enum constraint by abusing constraint inheritance  The following code specifies both a class and a struct constraints at the same time   public abstract class EnumClassUtils lt TClass gt  where TClass   class        public static TEnum Parse lt TEnum gt  string value      where TEnum   struct  TClass               return  TEnum  Enum Parse typeof TEnum   value             public class EnumUtils   EnumClassUtils lt Enum gt        Usage   EnumUtils Parse lt SomeEnum gt   value      Note  this is specifically stated in the C  5 0 language specification      If type parameter S depends on type parameter T then          It is valid for   S to have the value type constraint and T to have the reference type   constraint  Effectively this limits T to the types System Object    System ValueType  System Enum  and any interface type

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As stated in other answers before  while this cannot be expressed in source-code it can actually be done on IL Level   Christopher Currens answer shows how the IL do to that   With Fodys Add-In ExtraConstraints Fody there s a very simple way  complete with build-tooling  to achieve this  Just add their nuget packages  Fody  ExtraConstraints Fody  to your project and add the constraints as follows  Excerpt from the Readme of ExtraConstraints    public void MethodWithEnumConstraint lt  EnumConstraint  T gt           public void MethodWithTypeEnumConstraint lt  EnumConstraint typeof ConsoleColor    T gt            and Fody will add the necessary IL for the constraint to be present  Also note the additional feature of constraining delegates   public void MethodWithDelegateConstraint lt  DelegateConstraint  T gt            public void MethodWithTypeDelegateConstraint lt  DelegateConstraint typeof Func lt int gt     T gt             Regarding Enums  you might also want to take note of the highly interesting Enums NET

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Hope this is helpful   public static TValue ParseEnum lt TValue gt  string value  TValue defaultValue                    where TValue   struct    enum          try                     if  String IsNullOrEmpty value                     return defaultValue              return  TValue Enum Parse typeof  TValue   value                 catch Exception ex                      return defaultValue

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You can define a static constructor for the class that will check that the type T is an enum and throw an exception if it is not. This is the method mentioned by Jeffery Richter in his book CLR via C#.

internal sealed class GenericTypeThatRequiresAnEnum<T> {
    static GenericTypeThatRequiresAnEnum() {
        if (!typeof(T).IsEnum) {
        throw new ArgumentException("T must be an enumerated type");
        }
    }
}

Then in the parse method, you can just use Enum.Parse(typeof(T), input, true) to convert from string to the enum. The last true parameter is for ignoring case of the input.

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The existing answers are true as of C   lt  7 2  However  there is a C  language feature request  tied to a corefx feature request  to allow the following   public class MyGeneric lt TEnum gt  where TEnum   System Enum       At time of writing  the feature is  In discussion  at the Language Development Meetings   EDIT  As per nawfal s info  this is being introduced in C  7 3   EDIT 2  This is now in C  7 3 forward  release notes   Sample   public static Dictionary lt int  string gt  EnumNamedValues lt T gt        where T   System Enum       var result   new Dictionary lt int  string gt         var values   Enum GetValues typeof T         foreach  int item in values          result Add item  Enum GetName typeof T   item        return result

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I do have specific requirement where I required to use enum with text associated with enum value  For example when I use enum to specify error type it required to describe error details   public static class XmlEnumExtension       public static string ReadXmlEnumAttribute this Enum value                if  value    null  throw new ArgumentNullException  value            var attribs    XmlEnumAttribute    value GetType   GetField value ToString    GetCustomAttributes typeof  XmlEnumAttribute   true           return attribs Length  gt  0   attribs 0  Name   value ToString               public static T ParseXmlEnumAttribute lt T gt  this string str                foreach  T item in Enum GetValues typeof T                          var attribs    XmlEnumAttribute   item GetType   GetField item ToString    GetCustomAttributes typeof XmlEnumAttribute   true               if attribs Length  gt  0  amp  amp  attribs 0  Name Equals str   return item                    return  T Enum Parse typeof T   str  true            public enum MyEnum        XmlEnum  First Value        One       XmlEnum  Second Value        Two      Three     static void Main             Parsing from XmlEnum attribute     var str    Second Value       var me   str ParseXmlEnumAttribute lt MyEnum gt         System Console WriteLine me ReadXmlEnumAttribute            Parsing without XmlEnum     str    Three       me   str ParseXmlEnumAttribute lt MyEnum gt         System Console WriteLine me ReadXmlEnumAttribute         me   MyEnum One      System Console WriteLine me ReadXmlEnumAttribute

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I modified the sample by dimarzionist  This version will only work with Enums and not let structs get through    public static T ParseEnum lt T gt  string enumString      where T   struct    enum            if  String IsNullOrEmpty enumString      typeof T  IsEnum         throw new Exception  Type given must be an Enum        try               return  T Enum Parse typeof T   enumString  true             catch  Exception ex               return default T

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It should also be considered that since the release of C  7 3 using Enum constraints is supported out-of-the-box without having to do additional checking and stuff   So going forward and given you ve changed the language version of your project to C  7 3 the following code is going to work perfectly fine       private static T GetEnumFromString lt T gt  string value  T defaultValue  where T   Enum                  Your code goes here            In case you re don t know how to change the language version to C  7 3 see the following screenshot    EDIT 1 - Required Visual Studio Version and considering ReSharper  For Visual Studio to recognize the new syntax you need at least version 15 7  You can find that also mentioned in Microsoft s release notes  see Visual Studio 2017 15 7 Release Notes  Thanks  MohamedElshawaf for pointing out this valid question   Pls also note that in my case ReSharper 2018 1 as of writing this EDIT does not yet support C  7 3  Having ReSharper activated it highlights the Enum constraint as an error telling me Cannot use  System Array    System Delegate    System Enum    System ValueType    object  as type parameter constraint   ReSharper suggests as a quick fix to Remove  Enum  constraint of type paramter T of method   However  if you turn off ReSharper temporarily under Tools -  Options -  ReSharper Ultimate -  General you ll see that the syntax is perfectly fine given that you use VS 15 7 or higher and C  7 3 or higher

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If it s ok to use direct casting afterwards  I guess you can use the System Enum base class in your method  wherever necessary  You just need to replace the type parameters carefully  So the method implementation would be like   public static class EnumUtils       public static Enum GetEnumFromString string value  Enum defaultValue                if  string IsNullOrEmpty value   return defaultValue          foreach  Enum item in Enum GetValues defaultValue GetType                           if  item ToString   ToLower   Equals value Trim   ToLower     return item                    return defaultValue            Then you can use it like   var parsedOutput    YourEnum EnumUtils GetEnumFromString someString  YourEnum DefaultValue

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Interestingly enough  apparently this is possible in other langauges  Managed C    IL directly      To Quote          Both constraints actually produce valid IL and can also be consumed by C  if written in another language  you can declare those constraints in managed C   or in IL     Who knows

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You can define a static constructor for the class that will check that the type T is an enum and throw an exception if it is not. This is the method mentioned by Jeffery Richter in his book CLR via C#.

internal sealed class GenericTypeThatRequiresAnEnum<T> {
    static GenericTypeThatRequiresAnEnum() {
        if (!typeof(T).IsEnum) {
        throw new ArgumentException("T must be an enumerated type");
        }
    }
}

Then in the parse method, you can just use Enum.Parse(typeof(T), input, true) to convert from string to the enum. The last true parameter is for ignoring case of the input.

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I always liked this  you could modify as appropriate    public static IEnumerable lt TEnum gt  GetEnumValues       Type enumType   typeof TEnum      if  enumType IsEnum      throw new ArgumentException  Type argument must be Enum type       Array enumValues   Enum GetValues enumType     return enumValues Cast lt TEnum gt

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I modified the sample by dimarzionist  This version will only work with Enums and not let structs get through    public static T ParseEnum lt T gt  string enumString      where T   struct    enum            if  String IsNullOrEmpty enumString      typeof T  IsEnum         throw new Exception  Type given must be an Enum        try               return  T Enum Parse typeof T   enumString  true             catch  Exception ex               return default T

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If it s ok to use direct casting afterwards  I guess you can use the System Enum base class in your method  wherever necessary  You just need to replace the type parameters carefully  So the method implementation would be like   public static class EnumUtils       public static Enum GetEnumFromString string value  Enum defaultValue                if  string IsNullOrEmpty value   return defaultValue          foreach  Enum item in Enum GetValues defaultValue GetType                           if  item ToString   ToLower   Equals value Trim   ToLower     return item                    return defaultValue            Then you can use it like   var parsedOutput    YourEnum EnumUtils GetEnumFromString someString  YourEnum DefaultValue

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Interestingly enough  apparently this is possible in other langauges  Managed C    IL directly      To Quote          Both constraints actually produce valid IL and can also be consumed by C  if written in another language  you can declare those constraints in managed C   or in IL     Who knows

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I tried to improve the code a bit   public T LoadEnum lt T gt  string value  T defaultValue   default T   where T   struct  IComparable  IFormattable  IConvertible       if  Enum IsDefined typeof T   value                 return  T Enum Parse typeof T   value  true             return defaultValue

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Edit  The question has now superbly been answered by Julien Lebosquain  I would also like to extend his answer with ignoreCase  defaultValue and optional arguments  while adding TryParse and ParseOrDefault   public abstract class ConstrainedEnumParser lt TClass gt  where TClass   class    value type constraint S   TEnum   depends on reference type T   TClass    and on struct           internal constructor  to prevent this class from being inherited outside this code     internal ConstrainedEnumParser             Parse using pragmatic adhoc hard cast          - struct   class   enum         -  guaranteed  call from derived  lt System Enum gt -constrained type EnumUtils     public static TEnum Parse lt TEnum gt  string value  bool ignoreCase   false  where TEnum   struct  TClass               return  TEnum Enum Parse typeof TEnum   value  ignoreCase             public static bool TryParse lt TEnum gt  string value  out TEnum result  bool ignoreCase   false  TEnum defaultValue   default TEnum   where TEnum   struct  TClass    value type constraint S depending on T               var didParse   Enum TryParse value  ignoreCase  out result           if  didParse    false                        result   defaultValue                    return didParse            public static TEnum ParseOrDefault lt TEnum gt  string value  bool ignoreCase   false  TEnum defaultValue   default TEnum   where TEnum   struct  TClass    value type constraint S depending on T               if  string IsNullOrEmpty value     return defaultValue            TEnum result          if  Enum TryParse value  ignoreCase  out result     return result            return defaultValue           public class EnumUtils  ConstrainedEnumParser lt System Enum gt     reference type constraint to any  lt System Enum gt           call to parse will then contain constraint to specific  lt System Enum gt -class     Examples of usage   WeekDay parsedDayOrArgumentException   EnumUtils Parse lt WeekDay gt   monday   ignoreCase true   WeekDay parsedDayOrDefault  bool didParse   EnumUtils TryParse lt WeekDay gt   clubs   out parsedDayOrDefault  ignoreCase true   parsedDayOrDefault   EnumUtils ParseOrDefault lt WeekDay gt   friday   ignoreCase true  defaultValue WeekDay Sunday       Old  My old improvements on Vivek s answer by using the comments and  new  developments    use TEnum for clarity for users add more interface-constraints for additional constraint-checking let TryParse handle ignoreCase with the existing parameter  introduced in VS2010  Net 4   optionally use the generic default value  introduced in VS2005  Net 2  use optional arguments introduced in VS2010  Net 4  with default values  for defaultValue and ignoreCase   resulting in   public static class EnumUtils       public static TEnum ParseEnum lt TEnum gt  this string value                                           bool ignoreCase   true                                           TEnum defaultValue   default TEnum           where TEnum   struct   IComparable  IFormattable  IConvertible               if     typeof TEnum  IsEnum    throw new ArgumentException  TEnum must be an enumerated type              if  string IsNullOrEmpty value     return defaultValue            TEnum lResult          if  Enum TryParse value  ignoreCase  out lResult     return lResult            return defaultValue

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Since Enum Type implements IConvertible interface  a better implementation should be something like this   public T GetEnumFromString lt T gt  string value  where T   struct  IConvertible      if   typeof T  IsEnum              throw new ArgumentException  T must be an enumerated type                       This will still permit passing of value types implementing IConvertible  The chances are rare though

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I always liked this  you could modify as appropriate    public static IEnumerable lt TEnum gt  GetEnumValues       Type enumType   typeof TEnum      if  enumType IsEnum      throw new ArgumentException  Type argument must be Enum type       Array enumValues   Enum GetValues enumType     return enumValues Cast lt TEnum gt

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Edit  The question has now superbly been answered by Julien Lebosquain  I would also like to extend his answer with ignoreCase  defaultValue and optional arguments  while adding TryParse and ParseOrDefault   public abstract class ConstrainedEnumParser lt TClass gt  where TClass   class    value type constraint S   TEnum   depends on reference type T   TClass    and on struct           internal constructor  to prevent this class from being inherited outside this code     internal ConstrainedEnumParser             Parse using pragmatic adhoc hard cast          - struct   class   enum         -  guaranteed  call from derived  lt System Enum gt -constrained type EnumUtils     public static TEnum Parse lt TEnum gt  string value  bool ignoreCase   false  where TEnum   struct  TClass               return  TEnum Enum Parse typeof TEnum   value  ignoreCase             public static bool TryParse lt TEnum gt  string value  out TEnum result  bool ignoreCase   false  TEnum defaultValue   default TEnum   where TEnum   struct  TClass    value type constraint S depending on T               var didParse   Enum TryParse value  ignoreCase  out result           if  didParse    false                        result   defaultValue                    return didParse            public static TEnum ParseOrDefault lt TEnum gt  string value  bool ignoreCase   false  TEnum defaultValue   default TEnum   where TEnum   struct  TClass    value type constraint S depending on T               if  string IsNullOrEmpty value     return defaultValue            TEnum result          if  Enum TryParse value  ignoreCase  out result     return result            return defaultValue           public class EnumUtils  ConstrainedEnumParser lt System Enum gt     reference type constraint to any  lt System Enum gt           call to parse will then contain constraint to specific  lt System Enum gt -class     Examples of usage   WeekDay parsedDayOrArgumentException   EnumUtils Parse lt WeekDay gt   monday   ignoreCase true   WeekDay parsedDayOrDefault  bool didParse   EnumUtils TryParse lt WeekDay gt   clubs   out parsedDayOrDefault  ignoreCase true   parsedDayOrDefault   EnumUtils ParseOrDefault lt WeekDay gt   friday   ignoreCase true  defaultValue WeekDay Sunday       Old  My old improvements on Vivek s answer by using the comments and  new  developments    use TEnum for clarity for users add more interface-constraints for additional constraint-checking let TryParse handle ignoreCase with the existing parameter  introduced in VS2010  Net 4   optionally use the generic default value  introduced in VS2005  Net 2  use optional arguments introduced in VS2010  Net 4  with default values  for defaultValue and ignoreCase   resulting in   public static class EnumUtils       public static TEnum ParseEnum lt TEnum gt  this string value                                           bool ignoreCase   true                                           TEnum defaultValue   default TEnum           where TEnum   struct   IComparable  IFormattable  IConvertible               if     typeof TEnum  IsEnum    throw new ArgumentException  TEnum must be an enumerated type              if  string IsNullOrEmpty value     return defaultValue            TEnum lResult          if  Enum TryParse value  ignoreCase  out lResult     return lResult            return defaultValue

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This is my take at it  Combined from the answers and MSDN  public static TEnum ParseToEnum lt TEnum gt  this string text  where TEnum   struct  IConvertible  IComparable  IFormattable       if  string IsNullOrEmpty text      typeof TEnum  IsEnum          throw new ArgumentException  TEnum must be an Enum type         try               var enumValue    TEnum Enum Parse typeof TEnum   text Trim    true           return enumValue            catch  Exception                throw new ArgumentException string Format   0  is not a member of the  1  enumeration    text  typeof TEnum  Name              MSDN Source

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Just for completeness  the following is a Java solution   I am certain the same could be done in C  as well   It avoids having to specify the type anywhere in code - instead  you specify it in the strings you are trying to parse   The problem is that there isn t any way to know which enumeration the String might match - so the answer is to solve that problem   Instead of accepting just the string value  accept a String that has both the enumeration and the value in the form  enumeration value    Working code is below - requires Java 1 8 or later   This would also make the XML more precise as in you would see something like color  Color red  instead of just color  red    You would call the acceptEnumeratedValue   method with a string containing the enum name dot value name     The method returns the formal enumerated value   import java util HashMap  import java util Map  import java util function Function    public class EnumFromString        enum NumberEnum  One  Two  Three       enum LetterEnum  A  B  C         Map lt String  Function lt String    extends Enum gt  gt  enumsByName   new HashMap lt  gt          public static void main String   args            EnumFromString efs   new EnumFromString             System out print   nFirst string is NumberEnum Two - enum is     efs acceptEnumeratedValue  NumberEnum Two   name             System out print   nSecond string is LetterEnum B - enum is     efs acceptEnumeratedValue  LetterEnum B   name                 public EnumFromString             enumsByName put  NumberEnum   s - gt   return NumberEnum valueOf s              enumsByName put  LetterEnum   s - gt   return LetterEnum valueOf s                 public Enum acceptEnumeratedValue String enumDotValue             int pos   enumDotValue indexOf                String enumName   enumDotValue substring 0  pos           String value   enumDotValue substring pos   1            Enum enumeratedValue   enumsByName get enumName  apply value            return enumeratedValue

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Since Enum Type implements IConvertible interface  a better implementation should be something like this   public T GetEnumFromString lt T gt  string value  where T   struct  IConvertible      if   typeof T  IsEnum              throw new ArgumentException  T must be an enumerated type                       This will still permit passing of value types implementing IConvertible  The chances are rare though

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Since Enum Type implements IConvertible interface  a better implementation should be something like this   public T GetEnumFromString lt T gt  string value  where T   struct  IConvertible      if   typeof T  IsEnum              throw new ArgumentException  T must be an enumerated type                       This will still permit passing of value types implementing IConvertible  The chances are rare though

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I ve encapsulated Vivek s solution into a utility class that you can reuse  Please note that you still should define type constraints  where T   struct  IConvertible  on your type   using System   internal static class EnumEnforcer            lt summary gt          Makes sure that generic input parameter is of an enumerated type           lt  summary gt           lt typeparam name  T  gt Type that should be checked  lt  typeparam gt           lt param name  typeParameterName  gt Name of the type parameter  lt  param gt           lt param name  methodName  gt Name of the method which accepted the parameter  lt  param gt      public static void EnforceIsEnum lt T gt  string typeParameterName  string methodName          where T   struct  IConvertible               if   typeof T  IsEnum                        string message   string Format                   Generic parameter  0  in  1  method forces an enumerated type  Make sure your type parameter  0  is an enum                    typeParameterName                  methodName                throw new ArgumentException message                             lt summary gt          Makes sure that generic input parameter is of an enumerated type           lt  summary gt           lt typeparam name  T  gt Type that should be checked  lt  typeparam gt           lt param name  typeParameterName  gt Name of the type parameter  lt  param gt           lt param name  methodName  gt Name of the method which accepted the parameter  lt  param gt           lt param name  inputParameterName  gt Name of the input parameter of this page  lt  param gt      public static void EnforceIsEnum lt T gt  string typeParameterName  string methodName  string inputParameterName          where T   struct  IConvertible               if   typeof T  IsEnum                        string message   string Format                   Generic parameter  0  in  1  method forces an enumerated type  Make sure your input parameter  2  is of correct type                    typeParameterName                  methodName                  inputParameterName                throw new ArgumentException message                             lt summary gt          Makes sure that generic input parameter is of an enumerated type           lt  summary gt           lt typeparam name  T  gt Type that should be checked  lt  typeparam gt           lt param name  exceptionMessage  gt Message to show in case T is not an enum  lt  param gt      public static void EnforceIsEnum lt T gt  string exceptionMessage          where T   struct  IConvertible               if   typeof T  IsEnum                        throw new ArgumentException exceptionMessage

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You can define a static constructor for the class that will check that the type T is an enum and throw an exception if it is not. This is the method mentioned by Jeffery Richter in his book CLR via C#.

internal sealed class GenericTypeThatRequiresAnEnum<T> {
    static GenericTypeThatRequiresAnEnum() {
        if (!typeof(T).IsEnum) {
        throw new ArgumentException("T must be an enumerated type");
        }
    }
}

Then in the parse method, you can just use Enum.Parse(typeof(T), input, true) to convert from string to the enum. The last true parameter is for ignoring case of the input.

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I loved Christopher Currens s solution using IL but for those who don t want to deal with tricky business of including MSIL into their build process I wrote similar function in C    Please note though that you can t use generic restriction like where T   Enum because Enum is special type  Therefore I have to check if given generic type is really enum   My function is   public static T GetEnumFromString lt T gt  string strValue  T defaultValue           Check if it realy enum at runtime      if   typeof T  IsEnum          throw new ArgumentException  Method GetEnumFromString can be used with enums only         if   string IsNullOrEmpty strValue                 IEnumerator enumerator   Enum GetValues typeof T   GetEnumerator            while  enumerator MoveNext                          T temp    T enumerator Current              if  temp ToString   ToLower   Equals strValue Trim   ToLower                     return temp                       return defaultValue

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The existing answers are true as of C   lt  7 2  However  there is a C  language feature request  tied to a corefx feature request  to allow the following   public class MyGeneric lt TEnum gt  where TEnum   System Enum       At time of writing  the feature is  In discussion  at the Language Development Meetings   EDIT  As per nawfal s info  this is being introduced in C  7 3   EDIT 2  This is now in C  7 3 forward  release notes   Sample   public static Dictionary lt int  string gt  EnumNamedValues lt T gt        where T   System Enum       var result   new Dictionary lt int  string gt         var values   Enum GetValues typeof T         foreach  int item in values          result Add item  Enum GetName typeof T   item        return result

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As stated in other answers before  while this cannot be expressed in source-code it can actually be done on IL Level   Christopher Currens answer shows how the IL do to that   With Fodys Add-In ExtraConstraints Fody there s a very simple way  complete with build-tooling  to achieve this  Just add their nuget packages  Fody  ExtraConstraints Fody  to your project and add the constraints as follows  Excerpt from the Readme of ExtraConstraints    public void MethodWithEnumConstraint lt  EnumConstraint  T gt           public void MethodWithTypeEnumConstraint lt  EnumConstraint typeof ConsoleColor    T gt            and Fody will add the necessary IL for the constraint to be present  Also note the additional feature of constraining delegates   public void MethodWithDelegateConstraint lt  DelegateConstraint  T gt            public void MethodWithTypeDelegateConstraint lt  DelegateConstraint typeof Func lt int gt     T gt             Regarding Enums  you might also want to take note of the highly interesting Enums NET

[c#] Create Generic method constraining T to an Enum

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