Does C have extension properties

Question

Does C  have extension properties   For example  can I add an extension property to DateTimeFormatInfo called ShortDateLongTimeFormat which would return ShortDatePattern         LongTimePattern

User · Answer

No they do not exist in C  3 0 and will not be added in 4 0   It s on the list of feature wants for C  so it may be added at a future date   At this point the best you can do is GetXXX style extension methods

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For the moment it is still not supported out of the box by Roslyn compiler      Until now  the extension properties were not seen as valuable enough to be included in the previous versions of C  standard  C  7 and C  8 0 have seen this as proposal champion but it wasn t released yet  most of all because even if there is already an implementation  they want to make it right from the start   But it will      There is an extension members item in the C  7 work list so it may be supported in the near future  The current status of extension property can be found on Github under the related item   However  there is an even more promising topic which is the  extend everything  with a focus on especially properties and static classes or even fields   Moreover you can use a workaround  As specified in this article  you can use the TypeDescriptor capability to attach an attribute to an object instance at runtime  However  it is not using the syntax of the standard properties  It s a little bit different from just syntactic sugar adding a possibility to define an extended property like string Data this MyClass instance  as an alias for extension method string GetData this MyClass instance  as it stores data into the class   I hope that C 7 will provide a full featured extension everything  properties and fields   however on that point  only time will tell    And feel free to contribute as the software of tomorrow will come from the community   Update  August 2016  As dotnet team published what s new in C  7 0 and from a comment of Mads Torgensen      Extension properties  we had a  brilliant   intern implement them over   the summer as an experiment  along with other kinds of extension   members  We remain interested in this  but it   s a big change and we   need to feel confident that it   s worth it    It seems that extension properties and other members  are still good candidates to be included in a future release of Roslyn  but maybe not the 7 0 one   Update  May 2017  The extension members has been closed as duplicate of extension everything issue which is closed too  The main discussion was in fact about Type extensibility in a broad sense  The feature is now tracked here as a proposal and has been removed from 7 0 milestone   Update  August  2017 - C  8 0 proposed feature  While it still remains only a proposed feature  we have now a clearer view of what would be its syntax  Keep in mind that this will be the new syntax for extension methods as well   public interface IEmployee        public decimal Salary   get  set       public class Employee       public decimal Salary   get  set       public extension MyPersonExtension extends Person   IEmployee       private static readonly ConditionalWeakTable lt Person  Employee gt   employees            new ConditionalWeakTable lt Person  Employee gt           public decimal Salary               get                            this  is the instance of Person             return  employees GetOrCreate this  Salary                     set                        Employee employee   null              if    employees TryGetValue this  out employee                                employee    employees GetOrCreate this                             employee Salary   value                     IEmployee person   new Person    var salary   person Salary    Similar to partial classes  but compiled as a separate class type in a different assembly  Note you will also be able to add static members and operators this way  As mentioned in Mads Torgensen podcast  the extension won t have any state  so it cannot add private instance members to the class  which means you won t be able to add private instance data linked to the instance  The reason invoked for that is it would imply to manage internally dictionaries and it could be difficult  memory management  etc      For this  you can still use the TypeDescriptor ConditionalWeakTable technique described earlier and with the property extension  hides it under a nice property   Syntax is still subject to change as implies this issue  For example  extends could be replaced by for which some may feel more natural and less java related   Update December 2018 - Roles  Extensions and static interface members  Extension everything didn t make it to C  8 0  because of some of drawbacks explained as the end of this GitHub ticket  So  there was an exploration to improve the design  Here  Mads Torgensen explains what are roles and extensions and how they differs      Roles allow interfaces to be implemented on specific values of a given   type  Extensions allow interfaces to be implemented on all values of a   given type  within a specific region of code    It can be seen at a split of previous proposal in two use cases  The new syntax for extension would be like this   public extension ULongEnumerable of ulong       public IEnumerator lt byte gt  GetEnumerator                 for  int i   sizeof ulong   i  gt  0  i--                        yield return unchecked  byte  this  gt  gt   i-1  8                        then you would be able to do this   foreach  byte b in 0x 3A 9E F1 C5 DA F7 30 16ul        WriteLine    e Current X         And for a static interface   public interface IMonoid lt T gt  where T   IMonoid lt T gt        static T operator   T t1  T t2       static T Zero   get        Add an extension property on int and treat the int as IMonoid lt int gt    public extension IntMonoid of int   IMonoid lt int gt        public static int Zero   gt  0

User · Answer

As  Psyonity mentioned  you can use the conditionalWeakTable to add properties to existing objects  Combined with the dynamic ExpandoObject  you could implement dynamic extension properties in a few lines   using System Dynamic  using System Runtime CompilerServices   namespace ExtensionProperties            lt summary gt          Dynamically associates properies to a random object instance          lt  summary gt           lt example gt          var jan   new Person  Jan                    jan Age   24     regular property of the person object          jan DynamicProperties   NumberOfDrinkingBuddies   27     not originally scoped to the person object                  if  jan Age  amp lt  jan DynamicProperties   NumberOfDrinkingBuddies          Console WriteLine  Jan drinks too much             lt  example gt           lt remarks gt          If you get  Microsoft CSharp RuntimeBinder CSharpArgumentInfo Create  you should reference Microsoft CSharp          lt  remarks gt      public static class ObjectExtensions                   lt summary gt Stores extended data for objects lt  summary gt          private static ConditionalWeakTable lt object  object gt  extendedData   new ConditionalWeakTable lt object  object gt                   lt summary gt              Gets a dynamic collection of properties associated with an object instance              with a lifetime scoped to the lifetime of the object              lt  summary gt               lt param name  obj  gt The object the properties are associated with lt  param gt               lt returns gt A dynamic collection of properties associated with an object instance  lt  returns gt          public static dynamic DynamicProperties this object obj    gt  extendedData GetValue obj      gt  new ExpandoObject               A usage example is in the xml comments   var jan   new Person  Jan     jan Age   24     regular property of the person object  jan DynamicProperties   NumberOfDrinkingBuddies   27     not originally scoped to the person object   if  jan Age  lt  jan DynamicProperties   NumberOfDrinkingBuddies        Console WriteLine  Jan drinks too much       jan   null     NumberOfDrinkingBuddies will also be erased during garbage collection

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Update  thanks to  chaost for pointing this update out       Mads Torgersen   Extension everything didn   t make it into C  8 0  It got    caught up     if you will  in a very exciting debate about the further future of the language  and now we want to make sure we don   t add it in a way that inhibits those future possibilities  Sometimes language design is a very long game     Source  comments section in https   blogs msdn microsoft com dotnet 2018 11 12 building-c-8-0     I stopped counting how many times over the years I opened this question with hopes to have seen this implemented   Well  finally we can all rejoice  Microsoft is going to introduce this in their upcoming C  8 release   So instead of doing this     public static class IntExtensions         public static bool Even this int value                          return value   2    0              We ll be finally able to do it like so     public extension IntExtension extends int       public bool Even   gt  this   2    0      Source  https   blog ndepend com c-8-0-features-glimpse-future

User · Answer

No  they don t exist   I know that the C  team was considering them at one point  or at least Eric Lippert was  - along with extension constructors and operators  those may take a while to get your head around  but are cool     However  I haven t seen any evidence that they ll be part of C  4     EDIT  They didn t appear in C  5  and as of July 2014 it doesn t look like it s going to be in C  6 either   Eric Lippert  the Principal Developer on the C  compiler team at Microsoft thru November 2012  blogged about this in October of 2009    Why No Extension Properties      Fabulous Adventures In Coding

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Because I recently needed this  I looked at the source of the answer in    c  extend class by adding properties  and created a more dynamic version   public static class ObjectExtenders       static readonly ConditionalWeakTable lt object  List lt stringObject gt  gt  Flags   new ConditionalWeakTable lt object  List lt stringObject gt  gt          public static string GetFlags this object objectItem  string key                return Flags GetOrCreateValue objectItem  Single x   gt  x Key    key  Value             public static void SetFlags this object objectItem  string key  string value                if  Flags GetOrCreateValue objectItem  Any x   gt  x Key    key                         Flags GetOrCreateValue objectItem  Single x   gt  x Key    key  Value   value                    else                       Flags GetOrCreateValue objectItem  Add new stringObject                                 Key   key                  Value   value                                      class stringObject               public string Key          public string Value            It can probably be improved a lot  naming  dynamic instead of string   I currently use this in CF 3 5 together with a hacky ConditionalWeakTable  https   gist github com Jan-WillemdeBruyn db79dd6fdef7b9845e217958db98c4d4

[c#] Does C# have extension properties?

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