Can I add extension methods to an existing static class

Question

I m a fan of extension methods in C   but haven t had any success adding an extension method to a static class  such as Console   For example  if I want to add an extension to Console  called  WriteBlueLine   so that I can go   Console WriteBlueLine  This text is blue      I tried this by adding a local  public static method  with Console as a  this  parameter    but no dice       public static class Helpers       public static void WriteBlueLine this Console c  string text                Console ForegroundColor   ConsoleColor Blue          Console WriteLine text           Console ResetColor              This didn t add a  WriteBlueLine  method to Console    am I doing it wrong  Or asking for the impossible

User · Answer

I tried to do this with System Environment back when I was learning extension methods and was not successful  The reason is  as others mention  because extension methods require an instance of the class

User · Answer

Can you add static extensions to classes in C   No but you can do this   public static class Extensions       public static T Create lt T gt  this T  this          where T   class  new                 return Utility lt T gt  Create             public static class Utility lt T gt      where T   class  new         static Utility                 Create   Expression Lambda lt Func lt T gt  gt  Expression New typeof T  GetConstructor Type EmptyTypes    Compile              public static Func lt T gt  Create   get  private set        Here s how it works   While you can t technically write static extension methods  instead this code exploits a loophole in extension methods   That loophole being that you can call extension methods on null objects without getting the null exception  unless you access anything via  this    So here s how you would use this       var ds1    null as DataSet  Create       as oppose to DataSet Create          or     DataSet ds2   null      ds2   ds2 Create            using some of the techniques above you could have this       null as Console  WriteBlueLine          as oppose to Console WriteBlueLine        Now WHY did I pick calling the default constructor as an example  and AND why don t I just return new T   in the first code snippet without doing all of that Expression garbage  Well todays your lucky day because you get a 2fer   As any advanced  NET developer knows  new T   is slow because it generates a call to System Activator which uses reflection to get the default constructor before calling it   Damn you Microsoft  However my code calls the default constructor of the object directly   Static extensions would be better than this but desperate times call for desperate measures

User · Answer

You CAN do this if you are willing to  frig  it a little by making a variable of the static class and assigning it to null  However  this method would not be available to static calls on the class  so not sure how much use it would be   Console myConsole   null  myConsole WriteBlueLine  my blue line     public static class Helpers       public static void WriteBlueLine this Console c  string text                Console ForegroundColor   ConsoleColor Blue          Console WriteLine text           Console ResetColor

User · Answer

Nope   Extension method definitions require an instance of the type you re extending   It s unfortunate  I m not sure why it s required

User · Answer

yes  in a limited sense    public class DataSet   System Data DataSet       public static void SpecialMethod           This works but Console doesn t because it s static   public static class Console              public static void WriteLine String x        System Console WriteLine x          public static void WriteBlueLine String x                System Console ForegroundColor   ConsoleColor Blue          System Console Write  x                        This works because as long as it s not on the same namespace  The problem is that you have to write a proxy static method for every method that System Console have  It s not necessarily a bad thing as you can add something like this       public static void WriteLine String x        System Console WriteLine x Replace  Fck                or   public static void WriteLine String x                System Console ForegroundColor   ConsoleColor Blue          System Console WriteLine x            The way it works is that you hook something into the standard WriteLine  It could be a line count or bad word filter or whatever  Whenever you just specify Console in your namespace say WebProject1 and import the namespace System  WebProject1 Console will be chosen over System Console as default for those classes in namespace WebProject1  So this code will turn all the Console WriteLine calls into blue insofar as you never specified System Console WriteLine

User · Answer

I tried to do this with System Environment back when I was learning extension methods and was not successful  The reason is  as others mention  because extension methods require an instance of the class

User · Answer

You can t add static methods to a type   You can only add  pseudo- instance methods to an instance of a type   The point of the this modifier is to tell the C  compiler to pass the instance on the left-side of the   as the first parameter of the static extension method   In the case of adding static methods to a type  there is no instance to pass for the first parameter

User · Answer

I tried to do this with System Environment back when I was learning extension methods and was not successful  The reason is  as others mention  because extension methods require an instance of the class

User · Answer

Maybe you could add a static class with your custom namespace and the same class name   using CLRConsole   System Console   namespace ExtensionMethodsDemo       public static class Console               public static void WriteLine string value                        CLRConsole WriteLine value                      public static void WriteBlueLine string value                        System ConsoleColor currentColor   CLRConsole ForegroundColor               CLRConsole ForegroundColor   System ConsoleColor Blue              CLRConsole WriteLine value                CLRConsole ForegroundColor   currentColor                     public static System ConsoleKeyInfo ReadKey bool intercept                        return CLRConsole ReadKey intercept                       class Program               static void Main string   args                        try                               Console WriteBlueLine  This text is blue                                 catch  System Exception ex                                Console WriteLine ex Message                   Console WriteLine ex StackTrace                              Console WriteLine  Press any key to continue                   Console ReadKey true

User · Answer

You can t add static methods to a type   You can only add  pseudo- instance methods to an instance of a type   The point of the this modifier is to tell the C  compiler to pass the instance on the left-side of the   as the first parameter of the static extension method   In the case of adding static methods to a type  there is no instance to pass for the first parameter

User · Answer

As of C 7 this isn t supported  There are however discussions about integrating something like that in C 8 and proposals worth supporting

User · Answer

Nope   Extension method definitions require an instance of the type you re extending   It s unfortunate  I m not sure why it s required

User · Answer

yes  in a limited sense    public class DataSet   System Data DataSet       public static void SpecialMethod           This works but Console doesn t because it s static   public static class Console              public static void WriteLine String x        System Console WriteLine x          public static void WriteBlueLine String x                System Console ForegroundColor   ConsoleColor Blue          System Console Write  x                        This works because as long as it s not on the same namespace  The problem is that you have to write a proxy static method for every method that System Console have  It s not necessarily a bad thing as you can add something like this       public static void WriteLine String x        System Console WriteLine x Replace  Fck                or   public static void WriteLine String x                System Console ForegroundColor   ConsoleColor Blue          System Console WriteLine x            The way it works is that you hook something into the standard WriteLine  It could be a line count or bad word filter or whatever  Whenever you just specify Console in your namespace say WebProject1 and import the namespace System  WebProject1 Console will be chosen over System Console as default for those classes in namespace WebProject1  So this code will turn all the Console WriteLine calls into blue insofar as you never specified System Console WriteLine

User · Answer

I tried to do this with System Environment back when I was learning extension methods and was not successful  The reason is  as others mention  because extension methods require an instance of the class

User · Answer

As for extension methods  extension methods themselves are static  but they are invoked as if they are instance methods   Since a static class is not instantiable  you would never have an instance of the class to invoke an extension method from   For this reason the compiler does not allow extension methods to be defined for static classes   Mr  Obnoxious wrote   As any advanced  NET developer knows  new T   is slow because it generates a call to System Activator which uses reflection to get the default constructor before calling it    New   is compiled to the IL  newobj  instruction if the type is known at compile time   Newobj takes a constructor for direct invocation   Calls to System Activator CreateInstance   compile to the IL  call  instruction to invoke System Activator CreateInstance     New   when used against generic types will result in a call to System Activator CreateInstance     The post by Mr  Obnoxious was unclear on this point    and well  obnoxious     This code   System Collections ArrayList  al   new System Collections ArrayList    System Collections ArrayList  al2    System Collections ArrayList System Activator CreateInstance typeof System Collections ArrayList      produces this IL      locals init   0  class  mscorlib System Collections ArrayList  al              1  class  mscorlib System Collections ArrayList  al2    IL 0001   newobj     instance void  mscorlib System Collections ArrayList   ctor     IL 0006   stloc 0   IL 0007   ldtoken     mscorlib System Collections ArrayList   IL 000c   call       class  mscorlib System Type  mscorlib System Type  GetTypeFromHandle valuetype  mscorlib System RuntimeTypeHandle    IL 0011   call       object  mscorlib System Activator  CreateInstance class  mscorlib System Type    IL 0016   castclass   mscorlib System Collections ArrayList   IL 001b   stloc 1

User · Answer

As of C 7 this isn t supported  There are however discussions about integrating something like that in C 8 and proposals worth supporting

User · Answer

The following was rejected as an edit to tvanfosson s answer  I was asked to contribute it as my own answer  I used his suggestion and finished the implementation of a ConfigurationManager wrapper  In principle I simply filled out the     in tvanfosson s answer      No  Extension methods require an instance of an object  You can   however  write a static wrapper around the ConfigurationManager   interface  If you implement the wrapper  you don t need an extension   method since you can just add the method directly    public static class ConfigurationManagerWrapper       public static NameValueCollection AppSettings               get   return ConfigurationManager AppSettings               public static ConnectionStringSettingsCollection ConnectionStrings               get   return ConfigurationManager ConnectionStrings               public static object GetSection string sectionName                return ConfigurationManager GetSection sectionName              public static Configuration OpenExeConfiguration string exePath                return ConfigurationManager OpenExeConfiguration exePath              public static Configuration OpenMachineConfiguration                 return ConfigurationManager OpenMachineConfiguration               public static Configuration OpenMappedExeConfiguration ExeConfigurationFileMap fileMap  ConfigurationUserLevel userLevel                return ConfigurationManager OpenMappedExeConfiguration fileMap  userLevel              public static Configuration OpenMappedMachineConfiguration ConfigurationFileMap fileMap                return ConfigurationManager OpenMappedMachineConfiguration fileMap              public static void RefreshSection string sectionName                ConfigurationManager RefreshSection sectionName

User · Answer

It s not possible  And yes  I think MS made a mistake here  Their decision does not make sense and forces programmers to write  as described above  a pointless wrapper class  Here is a good example  Trying to extend static MS Unit testing class Assert  I want 1 more Assert method AreEqual x1 x2   The only way to do this is to point to different classes or write a wrapper around 100s of different Assert methods  Why   If the decision was being made to allow extensions of instances  I see no logical reason to not allow static extensions  The arguments about sectioning libraries does not stand up once instances can be extended

User · Answer

You can use a cast on null to make it work   public static class YoutTypeExtensionExample       public static void Example                   YourType null  ExtensionMethod              The extension   public static class YourTypeExtension       public static void ExtensionMethod this YourType x          YourType   public class YourType

User · Answer

Can you add static extensions to classes in C   No but you can do this   public static class Extensions       public static T Create lt T gt  this T  this          where T   class  new                 return Utility lt T gt  Create             public static class Utility lt T gt      where T   class  new         static Utility                 Create   Expression Lambda lt Func lt T gt  gt  Expression New typeof T  GetConstructor Type EmptyTypes    Compile              public static Func lt T gt  Create   get  private set        Here s how it works   While you can t technically write static extension methods  instead this code exploits a loophole in extension methods   That loophole being that you can call extension methods on null objects without getting the null exception  unless you access anything via  this    So here s how you would use this       var ds1    null as DataSet  Create       as oppose to DataSet Create          or     DataSet ds2   null      ds2   ds2 Create            using some of the techniques above you could have this       null as Console  WriteBlueLine          as oppose to Console WriteBlueLine        Now WHY did I pick calling the default constructor as an example  and AND why don t I just return new T   in the first code snippet without doing all of that Expression garbage  Well todays your lucky day because you get a 2fer   As any advanced  NET developer knows  new T   is slow because it generates a call to System Activator which uses reflection to get the default constructor before calling it   Damn you Microsoft  However my code calls the default constructor of the object directly   Static extensions would be better than this but desperate times call for desperate measures

User · Answer

It is not possible to write an extension method  however it is possible to mimic the behaviour you are asking for   using FooConsole   System Console   public static class Console       public static void WriteBlueLine string text                FooConsole ForegroundColor   ConsoleColor Blue          FooConsole WriteLine text           FooConsole ResetColor              This will allow you to call Console WriteBlueLine fooText  in other classes  If the other classes want access to the other static functions of Console  they will have to be explicitly referenced through their namespace   You can always add all of the methods in to the replacement class if you want to have all of them in one place   So you would have something like   using FooConsole   System Console   public static class Console       public static void WriteBlueLine string text                FooConsole ForegroundColor   ConsoleColor Blue          FooConsole WriteLine text           FooConsole ResetColor              public static void WriteLine string text                FooConsole WriteLine text            etc      This would provide the kind of behaviour you are looking for    Note Console will have to be added through the namespace that you put it in

User · Answer

Nope   Extension method definitions require an instance of the type you re extending   It s unfortunate  I m not sure why it s required

User · Answer

No   Extension methods require an instance variable  value  for an object   You can however  write a static wrapper around the ConfigurationManager interface   If you implement the wrapper  you don t need an extension method since you can just add the method directly    public static class ConfigurationManagerWrapper          public static ConfigurationSection GetSection  string name                    return ConfigurationManager GetSection  name                                public static ConfigurationSection GetWidgetSection                     return GetSection   widgets

User · Answer

No   Extension methods require an instance variable  value  for an object   You can however  write a static wrapper around the ConfigurationManager interface   If you implement the wrapper  you don t need an extension method since you can just add the method directly    public static class ConfigurationManagerWrapper          public static ConfigurationSection GetSection  string name                    return ConfigurationManager GetSection  name                                public static ConfigurationSection GetWidgetSection                     return GetSection   widgets

User · Answer

I stumbled upon this thread while trying to find an answer to the same question the OP had  I didn t find the answer I wanted  but I ended up doing this  public static class MyConsole       public static void WriteLine this ConsoleColor Color  string Text                Console ForegroundColor   Color          Console WriteLine Text               And I use it like this  ConsoleColor Cyan WriteLine  quot voil   quot

User · Answer

As for extension methods  extension methods themselves are static  but they are invoked as if they are instance methods   Since a static class is not instantiable  you would never have an instance of the class to invoke an extension method from   For this reason the compiler does not allow extension methods to be defined for static classes   Mr  Obnoxious wrote   As any advanced  NET developer knows  new T   is slow because it generates a call to System Activator which uses reflection to get the default constructor before calling it    New   is compiled to the IL  newobj  instruction if the type is known at compile time   Newobj takes a constructor for direct invocation   Calls to System Activator CreateInstance   compile to the IL  call  instruction to invoke System Activator CreateInstance     New   when used against generic types will result in a call to System Activator CreateInstance     The post by Mr  Obnoxious was unclear on this point    and well  obnoxious     This code   System Collections ArrayList  al   new System Collections ArrayList    System Collections ArrayList  al2    System Collections ArrayList System Activator CreateInstance typeof System Collections ArrayList      produces this IL      locals init   0  class  mscorlib System Collections ArrayList  al              1  class  mscorlib System Collections ArrayList  al2    IL 0001   newobj     instance void  mscorlib System Collections ArrayList   ctor     IL 0006   stloc 0   IL 0007   ldtoken     mscorlib System Collections ArrayList   IL 000c   call       class  mscorlib System Type  mscorlib System Type  GetTypeFromHandle valuetype  mscorlib System RuntimeTypeHandle    IL 0011   call       object  mscorlib System Activator  CreateInstance class  mscorlib System Type    IL 0016   castclass   mscorlib System Collections ArrayList   IL 001b   stloc 1

User · Answer

Maybe you could add a static class with your custom namespace and the same class name   using CLRConsole   System Console   namespace ExtensionMethodsDemo       public static class Console               public static void WriteLine string value                        CLRConsole WriteLine value                      public static void WriteBlueLine string value                        System ConsoleColor currentColor   CLRConsole ForegroundColor               CLRConsole ForegroundColor   System ConsoleColor Blue              CLRConsole WriteLine value                CLRConsole ForegroundColor   currentColor                     public static System ConsoleKeyInfo ReadKey bool intercept                        return CLRConsole ReadKey intercept                       class Program               static void Main string   args                        try                               Console WriteBlueLine  This text is blue                                 catch  System Exception ex                                Console WriteLine ex Message                   Console WriteLine ex StackTrace                              Console WriteLine  Press any key to continue                   Console ReadKey true

User · Answer

No   Extension methods require an instance variable  value  for an object   You can however  write a static wrapper around the ConfigurationManager interface   If you implement the wrapper  you don t need an extension method since you can just add the method directly    public static class ConfigurationManagerWrapper          public static ConfigurationSection GetSection  string name                    return ConfigurationManager GetSection  name                                public static ConfigurationSection GetWidgetSection                     return GetSection   widgets

User · Answer

You can use a cast on null to make it work   public static class YoutTypeExtensionExample       public static void Example                   YourType null  ExtensionMethod              The extension   public static class YourTypeExtension       public static void ExtensionMethod this YourType x          YourType   public class YourType

User · Answer

The following was rejected as an edit to tvanfosson s answer  I was asked to contribute it as my own answer  I used his suggestion and finished the implementation of a ConfigurationManager wrapper  In principle I simply filled out the     in tvanfosson s answer      No  Extension methods require an instance of an object  You can   however  write a static wrapper around the ConfigurationManager   interface  If you implement the wrapper  you don t need an extension   method since you can just add the method directly    public static class ConfigurationManagerWrapper       public static NameValueCollection AppSettings               get   return ConfigurationManager AppSettings               public static ConnectionStringSettingsCollection ConnectionStrings               get   return ConfigurationManager ConnectionStrings               public static object GetSection string sectionName                return ConfigurationManager GetSection sectionName              public static Configuration OpenExeConfiguration string exePath                return ConfigurationManager OpenExeConfiguration exePath              public static Configuration OpenMachineConfiguration                 return ConfigurationManager OpenMachineConfiguration               public static Configuration OpenMappedExeConfiguration ExeConfigurationFileMap fileMap  ConfigurationUserLevel userLevel                return ConfigurationManager OpenMappedExeConfiguration fileMap  userLevel              public static Configuration OpenMappedMachineConfiguration ConfigurationFileMap fileMap                return ConfigurationManager OpenMappedMachineConfiguration fileMap              public static void RefreshSection string sectionName                ConfigurationManager RefreshSection sectionName

User · Answer

You can t add static methods to a type   You can only add  pseudo- instance methods to an instance of a type   The point of the this modifier is to tell the C  compiler to pass the instance on the left-side of the   as the first parameter of the static extension method   In the case of adding static methods to a type  there is no instance to pass for the first parameter

User · Answer

It s not possible  And yes  I think MS made a mistake here  Their decision does not make sense and forces programmers to write  as described above  a pointless wrapper class  Here is a good example  Trying to extend static MS Unit testing class Assert  I want 1 more Assert method AreEqual x1 x2   The only way to do this is to point to different classes or write a wrapper around 100s of different Assert methods  Why   If the decision was being made to allow extensions of instances  I see no logical reason to not allow static extensions  The arguments about sectioning libraries does not stand up once instances can be extended

User · Answer

It s not possible  And yes  I think MS made a mistake here  Their decision does not make sense and forces programmers to write  as described above  a pointless wrapper class  Here is a good example  Trying to extend static MS Unit testing class Assert  I want 1 more Assert method AreEqual x1 x2   The only way to do this is to point to different classes or write a wrapper around 100s of different Assert methods  Why   If the decision was being made to allow extensions of instances  I see no logical reason to not allow static extensions  The arguments about sectioning libraries does not stand up once instances can be extended

User · Answer

No   Extension methods require an instance variable  value  for an object   You can however  write a static wrapper around the ConfigurationManager interface   If you implement the wrapper  you don t need an extension method since you can just add the method directly    public static class ConfigurationManagerWrapper          public static ConfigurationSection GetSection  string name                    return ConfigurationManager GetSection  name                                public static ConfigurationSection GetWidgetSection                     return GetSection   widgets

User · Answer

Nope   Extension method definitions require an instance of the type you re extending   It s unfortunate  I m not sure why it s required

User · Answer

It s not possible  And yes  I think MS made a mistake here  Their decision does not make sense and forces programmers to write  as described above  a pointless wrapper class  Here is a good example  Trying to extend static MS Unit testing class Assert  I want 1 more Assert method AreEqual x1 x2   The only way to do this is to point to different classes or write a wrapper around 100s of different Assert methods  Why   If the decision was being made to allow extensions of instances  I see no logical reason to not allow static extensions  The arguments about sectioning libraries does not stand up once instances can be extended

User · Answer

It is not possible to write an extension method  however it is possible to mimic the behaviour you are asking for   using FooConsole   System Console   public static class Console       public static void WriteBlueLine string text                FooConsole ForegroundColor   ConsoleColor Blue          FooConsole WriteLine text           FooConsole ResetColor              This will allow you to call Console WriteBlueLine fooText  in other classes  If the other classes want access to the other static functions of Console  they will have to be explicitly referenced through their namespace   You can always add all of the methods in to the replacement class if you want to have all of them in one place   So you would have something like   using FooConsole   System Console   public static class Console       public static void WriteBlueLine string text                FooConsole ForegroundColor   ConsoleColor Blue          FooConsole WriteLine text           FooConsole ResetColor              public static void WriteLine string text                FooConsole WriteLine text            etc      This would provide the kind of behaviour you are looking for    Note Console will have to be added through the namespace that you put it in

User · Answer

You CAN do this if you are willing to  frig  it a little by making a variable of the static class and assigning it to null  However  this method would not be available to static calls on the class  so not sure how much use it would be   Console myConsole   null  myConsole WriteBlueLine  my blue line     public static class Helpers       public static void WriteBlueLine this Console c  string text                Console ForegroundColor   ConsoleColor Blue          Console WriteLine text           Console ResetColor

User · Answer

I stumbled upon this thread while trying to find an answer to the same question the OP had  I didn t find the answer I wanted  but I ended up doing this  public static class MyConsole       public static void WriteLine this ConsoleColor Color  string Text                Console ForegroundColor   Color          Console WriteLine Text               And I use it like this  ConsoleColor Cyan WriteLine  quot voil   quot

User · Answer

You can t add static methods to a type   You can only add  pseudo- instance methods to an instance of a type   The point of the this modifier is to tell the C  compiler to pass the instance on the left-side of the   as the first parameter of the static extension method   In the case of adding static methods to a type  there is no instance to pass for the first parameter

[c#] Can I add extension methods to an existing static class?

Examples related to c#

Examples related to static

Examples related to extension-methods