How to inherit constructors

Question

Imagine a base class with many constructors and a virtual method  public class Foo             public Foo            public Foo int i                  public virtual void SomethingElse                    and now i want to create a descendant class that overrides the virtual method   public class Bar   Foo       public override void SomethingElse             And another descendant that does some more stuff   public class Bah   Bar      public void DoMoreStuff             Do i really have to copy all constructors from Foo into Bar and Bah  And then if i change a constructor signature in Foo  do i have to update it in Bar and Bah   Is there no way to inherit constructors  Is there no way to encourage code reuse

User · Answer

Too bad we re kind of forced to tell the compiler the obvious   Subclass    base      Subclass int x   base x     Subclass int x y   base x y       I only need to do 3 constructors in 12 subclasses  so it s no big deal  but I m not too fond of repeating that on every subclass  after being used to not having to write it for so long   I m sure there s a valid reason for it  but I don t think I ve ever encountered a problem that requires this kind of restriction

User · Answer

As Foo is a class can you not create virtual overloaded Initialise   methods  Then they would be available to sub-classes and still extensible   public class Foo             public Foo             public virtual void Initialise int i           public virtual void Initialise int i  int i           public virtual void Initialise int i  int i  int i                   public virtual void Initialise int i  int i       int i                    public virtual void SomethingElse                    This shouldn t have a higher performance cost unless you have lots of default property values and you hit it a lot

User · Answer

Yes  you will have to implement the constructors that make sense for each derivation and then use the base keyword to direct that constructor to the appropriate base class or the this keyword to direct a constructor to another constructor in the same class  If the compiler made assumptions about inheriting constructors  we wouldn t be able to properly determine how our objects were instantiated  In the most part  you should consider why you have so many constructors and consider reducing them to only one or two in the base class  The derived classes can then mask out some of them using constant values like null and only expose the necessary ones through their constructors  Update In C 4 you could specify default parameter values and use named parameters to make a single constructor support multiple argument configurations rather than having one constructor per configuration

User · Answer

public class BaseClass       public BaseClass params int   parameters                     public class ChildClass   BaseClass       public ChildClass params int   parameters            base parameters

User · Answer

Don t forget that you can also redirect constructors to other constructors at the same level of inheritance   public Bar int i  int j    this i

User · Answer

Yes  you will have to implement the constructors that make sense for each derivation and then use the base keyword to direct that constructor to the appropriate base class or the this keyword to direct a constructor to another constructor in the same class  If the compiler made assumptions about inheriting constructors  we wouldn t be able to properly determine how our objects were instantiated  In the most part  you should consider why you have so many constructors and consider reducing them to only one or two in the base class  The derived classes can then mask out some of them using constant values like null and only expose the necessary ones through their constructors  Update In C 4 you could specify default parameter values and use named parameters to make a single constructor support multiple argument configurations rather than having one constructor per configuration

User · Answer

Another simple solution could be to use a structure or simple data class that contains the parameters as properties  that way you can have all the default values and behaviors set up ahead of time  passing the  parameter class  in as the single constructor parameter   public class FooParams       public int Size        protected myCustomStruct  ReasonForLife       public class Foo       private FooParams  myParams      public Foo FooParams myParams                   myParams   myParams            This avoids the mess of multiple constructors  sometimes  and gives strong typing  default values  and other benefits not provided by a parameter array   It also makes it easy to carry forward since anything that inherits from Foo can still get to  or even add to  FooParams as needed   You still need to copy the constructor  but you always  most of the time  only  as a general rule  ever  at least  for now  need one constructor   public class Bar   Foo       public Bar FooParams myParams    base myParams         I really like the overloaded Initailize   and Class Factory Pattern approaches better  but sometimes you just need to have a smart constructor   Just a thought

User · Answer

Too many constructors is a sign of a broken design   Better a class with few constructors and the ability to set properties   If you really need control over the properties  consider a factory in the same namespace and make the property setters internal   Let the factory decide how to instantiate the class and set its properties   The factory can have methods that take as many parameters as necessary to configure the object properly

User · Answer

Don t forget that you can also redirect constructors to other constructors at the same level of inheritance   public Bar int i  int j    this i

User · Answer

Too many constructors is a sign of a broken design   Better a class with few constructors and the ability to set properties   If you really need control over the properties  consider a factory in the same namespace and make the property setters internal   Let the factory decide how to instantiate the class and set its properties   The factory can have methods that take as many parameters as necessary to configure the object properly

User · Answer

Yes  you will have to implement the constructors that make sense for each derivation and then use the base keyword to direct that constructor to the appropriate base class or the this keyword to direct a constructor to another constructor in the same class  If the compiler made assumptions about inheriting constructors  we wouldn t be able to properly determine how our objects were instantiated  In the most part  you should consider why you have so many constructors and consider reducing them to only one or two in the base class  The derived classes can then mask out some of them using constant values like null and only expose the necessary ones through their constructors  Update In C 4 you could specify default parameter values and use named parameters to make a single constructor support multiple argument configurations rather than having one constructor per configuration

User · Answer

Yes  you have to copy all 387 constructors   You can do some reuse by redirecting them     public Bar int i   base i       public Bar int i  int j    base i  j       but that s the best you can do

User · Answer

Don t forget that you can also redirect constructors to other constructors at the same level of inheritance   public Bar int i  int j    this i

User · Answer

Do i really have to copy all constructors from Foo into Bar and Bah  And then if i change a constructor signature in Foo  do i have to update it in Bar and Bah    Yes  if you use constructors to create instances      Is there no way to inherit constructors    Nope      Is there no way to encourage code reuse    Well  I won t get into whether inheriting constructors would be a good or bad thing and whether it would encourage code reuse  since we don t have them and we re not going to get them   -   But here in 2014  with the current C   you can get something very much like inherited constructors by using a generic create method instead  It can be a useful tool to have in your belt  but you wouldn t reach for it lightly  I reached for it recently when faced with needing to pass something into the constructor of a base type used in a couple of hundred derived classes  until recently  the base didn t need any arguments  and so the default constructor was fine nbsp  mdash  the derived classes didn t declare constructors at all  and got the automatically-supplied one    It looks like this      In Foo  public T create lt T gt  int i  where  where T   Foo  new         T obj   new T           Do whatever you would do with  i  in  Foo i   here  for instance         if you save it as a data member    obj dataMember   i       return obj      That says that you can call the generic create function using a type parameter which is any subtype of Foo that has a zero-arguments constructor   Then  instead of doing Bar b new Bar 42   you d do this   var b   Foo create lt Bar gt  42      or Bar b   Foo create lt Bar gt  42      or var b   Bar create lt Bar gt  42      But you still need the  lt Bar gt  bit    or Bar b   Bar create lt Bar gt  42     There I ve shown the create method being on Foo directly  but of course it could be in a factory class of some sort  if the information it s setting up can be set by that factory class   Just for clarity  The name create isn t important  it could be makeThingy or whatever else you like   Full Example  using System IO  using System   class Program       static void Main                 Bar b1   Foo create lt Bar gt  42           b1 ShowDataMember  b1             Bar b2   Bar create lt Bar gt  43      Just to show  Foo create  vs   Bar create  doesn t matter         b2 ShowDataMember  b2               class Foo               public int DataMember   get  private set             public static T create lt T gt  int i  where T  Foo  new                         T obj   new T                obj DataMember   i              return obj                       class Bar   Foo               public void ShowDataMember string prefix                        Console WriteLine prefix     DataMember       this DataMember

User · Answer

As Foo is a class can you not create virtual overloaded Initialise   methods  Then they would be available to sub-classes and still extensible   public class Foo             public Foo             public virtual void Initialise int i           public virtual void Initialise int i  int i           public virtual void Initialise int i  int i  int i                   public virtual void Initialise int i  int i       int i                    public virtual void SomethingElse                    This shouldn t have a higher performance cost unless you have lots of default property values and you hit it a lot

User · Answer

You may be able to adapt a version of the C   virtual constructor idiom   As far as I know  C  doesn t support covariant return types   I believe that s on many peoples  wish lists

User · Answer

The problem is not that Bar and Bah have to copy 387 constructors  the problem is that Foo has 387 constructors   Foo clearly does too many things - refactor quick   Also  unless you have a really good reason to have values set in the constructor  which  if you provide a parameterless constructor  you probably don t   I d recommend using property getting setting

User · Answer

Yes  you will have to implement the constructors that make sense for each derivation and then use the base keyword to direct that constructor to the appropriate base class or the this keyword to direct a constructor to another constructor in the same class  If the compiler made assumptions about inheriting constructors  we wouldn t be able to properly determine how our objects were instantiated  In the most part  you should consider why you have so many constructors and consider reducing them to only one or two in the base class  The derived classes can then mask out some of them using constant values like null and only expose the necessary ones through their constructors  Update In C 4 you could specify default parameter values and use named parameters to make a single constructor support multiple argument configurations rather than having one constructor per configuration

User · Answer

No  you don t need to copy all 387 constructors to Bar and Bah  Bar and Bah can have as many or as few constructors as you want independent of how many you define on Foo  For example  you could choose to have just one Bar constructor which constructs Foo with Foo s 212th constructor   Yes  any constructors you change in Foo that Bar or Bah depend on will require you to modify Bar and Bah accordingly   No  there is no way in  NET to inherit constructors  But you can achieve code reuse by calling a base class s constructor inside the subclass s constructor or by calling a virtual method you define  like Initialize

User · Answer

Too many constructors is a sign of a broken design   Better a class with few constructors and the ability to set properties   If you really need control over the properties  consider a factory in the same namespace and make the property setters internal   Let the factory decide how to instantiate the class and set its properties   The factory can have methods that take as many parameters as necessary to configure the object properly

User · Answer

Do i really have to copy all constructors from Foo into Bar and Bah  And then if i change a constructor signature in Foo  do i have to update it in Bar and Bah    Yes  if you use constructors to create instances      Is there no way to inherit constructors    Nope      Is there no way to encourage code reuse    Well  I won t get into whether inheriting constructors would be a good or bad thing and whether it would encourage code reuse  since we don t have them and we re not going to get them   -   But here in 2014  with the current C   you can get something very much like inherited constructors by using a generic create method instead  It can be a useful tool to have in your belt  but you wouldn t reach for it lightly  I reached for it recently when faced with needing to pass something into the constructor of a base type used in a couple of hundred derived classes  until recently  the base didn t need any arguments  and so the default constructor was fine nbsp  mdash  the derived classes didn t declare constructors at all  and got the automatically-supplied one    It looks like this      In Foo  public T create lt T gt  int i  where  where T   Foo  new         T obj   new T           Do whatever you would do with  i  in  Foo i   here  for instance         if you save it as a data member    obj dataMember   i       return obj      That says that you can call the generic create function using a type parameter which is any subtype of Foo that has a zero-arguments constructor   Then  instead of doing Bar b new Bar 42   you d do this   var b   Foo create lt Bar gt  42      or Bar b   Foo create lt Bar gt  42      or var b   Bar create lt Bar gt  42      But you still need the  lt Bar gt  bit    or Bar b   Bar create lt Bar gt  42     There I ve shown the create method being on Foo directly  but of course it could be in a factory class of some sort  if the information it s setting up can be set by that factory class   Just for clarity  The name create isn t important  it could be makeThingy or whatever else you like   Full Example  using System IO  using System   class Program       static void Main                 Bar b1   Foo create lt Bar gt  42           b1 ShowDataMember  b1             Bar b2   Bar create lt Bar gt  43      Just to show  Foo create  vs   Bar create  doesn t matter         b2 ShowDataMember  b2               class Foo               public int DataMember   get  private set             public static T create lt T gt  int i  where T  Foo  new                         T obj   new T                obj DataMember   i              return obj                       class Bar   Foo               public void ShowDataMember string prefix                        Console WriteLine prefix     DataMember       this DataMember

User · Answer

387 constructors    That s your main problem   How about this instead   public Foo params int   list

User · Answer

387 constructors    That s your main problem   How about this instead   public Foo params int   list

User · Answer

The problem is not that Bar and Bah have to copy 387 constructors  the problem is that Foo has 387 constructors   Foo clearly does too many things - refactor quick   Also  unless you have a really good reason to have values set in the constructor  which  if you provide a parameterless constructor  you probably don t   I d recommend using property getting setting

User · Answer

No  you don t need to copy all 387 constructors to Bar and Bah  Bar and Bah can have as many or as few constructors as you want independent of how many you define on Foo  For example  you could choose to have just one Bar constructor which constructs Foo with Foo s 212th constructor   Yes  any constructors you change in Foo that Bar or Bah depend on will require you to modify Bar and Bah accordingly   No  there is no way in  NET to inherit constructors  But you can achieve code reuse by calling a base class s constructor inside the subclass s constructor or by calling a virtual method you define  like Initialize

User · Answer

Too many constructors is a sign of a broken design   Better a class with few constructors and the ability to set properties   If you really need control over the properties  consider a factory in the same namespace and make the property setters internal   Let the factory decide how to instantiate the class and set its properties   The factory can have methods that take as many parameters as necessary to configure the object properly

User · Answer

387 constructors    That s your main problem   How about this instead   public Foo params int   list

User · Answer

As Foo is a class can you not create virtual overloaded Initialise   methods  Then they would be available to sub-classes and still extensible   public class Foo             public Foo             public virtual void Initialise int i           public virtual void Initialise int i  int i           public virtual void Initialise int i  int i  int i                   public virtual void Initialise int i  int i       int i                    public virtual void SomethingElse                    This shouldn t have a higher performance cost unless you have lots of default property values and you hit it a lot

User · Answer

No  you don t need to copy all 387 constructors to Bar and Bah  Bar and Bah can have as many or as few constructors as you want independent of how many you define on Foo  For example  you could choose to have just one Bar constructor which constructs Foo with Foo s 212th constructor   Yes  any constructors you change in Foo that Bar or Bah depend on will require you to modify Bar and Bah accordingly   No  there is no way in  NET to inherit constructors  But you can achieve code reuse by calling a base class s constructor inside the subclass s constructor or by calling a virtual method you define  like Initialize

User · Answer

Personally I think this is a mistake on Microsofts part  they should have allowed the programmer to override visibility of Constructors  Methods and Properties in base classes  and then make it so that Constructors are always inherited   This way we just simply override  with lower visibility - ie  Private  the constructors we DONT want instead of having to add all the constructors we DO want  Delphi does it this way  and I miss it   Take for example if you want to override the System IO StreamWriter class  you need to add all 7 constructors to your new class  and if you like commenting you need to comment each one with the header XML  To make it worse  the metadata view dosnt put the XML comments as proper XML comments  so we have to go line by line and copy and paste them  What was Microsoft thinking here   I have actually written a small utility where you can paste in the metadata code and it will convert it to XML comments using overidden visibility

User · Answer

Yes  you have to copy all 387 constructors   You can do some reuse by redirecting them     public Bar int i   base i       public Bar int i  int j    base i  j       but that s the best you can do

User · Answer

As Foo is a class can you not create virtual overloaded Initialise   methods  Then they would be available to sub-classes and still extensible   public class Foo             public Foo             public virtual void Initialise int i           public virtual void Initialise int i  int i           public virtual void Initialise int i  int i  int i                   public virtual void Initialise int i  int i       int i                    public virtual void SomethingElse                    This shouldn t have a higher performance cost unless you have lots of default property values and you hit it a lot

User · Answer

387 constructors    That s your main problem   How about this instead   public Foo params int   list

User · Answer

You may be able to adapt a version of the C   virtual constructor idiom   As far as I know  C  doesn t support covariant return types   I believe that s on many peoples  wish lists

User · Answer

Another simple solution could be to use a structure or simple data class that contains the parameters as properties  that way you can have all the default values and behaviors set up ahead of time  passing the  parameter class  in as the single constructor parameter   public class FooParams       public int Size        protected myCustomStruct  ReasonForLife       public class Foo       private FooParams  myParams      public Foo FooParams myParams                   myParams   myParams            This avoids the mess of multiple constructors  sometimes  and gives strong typing  default values  and other benefits not provided by a parameter array   It also makes it easy to carry forward since anything that inherits from Foo can still get to  or even add to  FooParams as needed   You still need to copy the constructor  but you always  most of the time  only  as a general rule  ever  at least  for now  need one constructor   public class Bar   Foo       public Bar FooParams myParams    base myParams         I really like the overloaded Initailize   and Class Factory Pattern approaches better  but sometimes you just need to have a smart constructor   Just a thought

User · Answer

The problem is not that Bar and Bah have to copy 387 constructors  the problem is that Foo has 387 constructors   Foo clearly does too many things - refactor quick   Also  unless you have a really good reason to have values set in the constructor  which  if you provide a parameterless constructor  you probably don t   I d recommend using property getting setting

User · Answer

public class BaseClass       public BaseClass params int   parameters                     public class ChildClass   BaseClass       public ChildClass params int   parameters            base parameters

User · Answer

Personally I think this is a mistake on Microsofts part  they should have allowed the programmer to override visibility of Constructors  Methods and Properties in base classes  and then make it so that Constructors are always inherited   This way we just simply override  with lower visibility - ie  Private  the constructors we DONT want instead of having to add all the constructors we DO want  Delphi does it this way  and I miss it   Take for example if you want to override the System IO StreamWriter class  you need to add all 7 constructors to your new class  and if you like commenting you need to comment each one with the header XML  To make it worse  the metadata view dosnt put the XML comments as proper XML comments  so we have to go line by line and copy and paste them  What was Microsoft thinking here   I have actually written a small utility where you can paste in the metadata code and it will convert it to XML comments using overidden visibility

User · Answer

You may be able to adapt a version of the C   virtual constructor idiom   As far as I know  C  doesn t support covariant return types   I believe that s on many peoples  wish lists

User · Answer

You may be able to adapt a version of the C   virtual constructor idiom   As far as I know  C  doesn t support covariant return types   I believe that s on many peoples  wish lists

User · Answer

Yes  you have to copy all 387 constructors   You can do some reuse by redirecting them     public Bar int i   base i       public Bar int i  int j    base i  j       but that s the best you can do

User · Answer

The problem is not that Bar and Bah have to copy 387 constructors  the problem is that Foo has 387 constructors   Foo clearly does too many things - refactor quick   Also  unless you have a really good reason to have values set in the constructor  which  if you provide a parameterless constructor  you probably don t   I d recommend using property getting setting

User · Answer

Yes  you have to copy all 387 constructors   You can do some reuse by redirecting them     public Bar int i   base i       public Bar int i  int j    base i  j       but that s the best you can do

User · Answer

Don t forget that you can also redirect constructors to other constructors at the same level of inheritance   public Bar int i  int j    this i

User · Answer

No  you don t need to copy all 387 constructors to Bar and Bah  Bar and Bah can have as many or as few constructors as you want independent of how many you define on Foo  For example  you could choose to have just one Bar constructor which constructs Foo with Foo s 212th constructor   Yes  any constructors you change in Foo that Bar or Bah depend on will require you to modify Bar and Bah accordingly   No  there is no way in  NET to inherit constructors  But you can achieve code reuse by calling a base class s constructor inside the subclass s constructor or by calling a virtual method you define  like Initialize

User · Answer

Too bad we re kind of forced to tell the compiler the obvious   Subclass    base      Subclass int x   base x     Subclass int x y   base x y       I only need to do 3 constructors in 12 subclasses  so it s no big deal  but I m not too fond of repeating that on every subclass  after being used to not having to write it for so long   I m sure there s a valid reason for it  but I don t think I ve ever encountered a problem that requires this kind of restriction

[c#] How to inherit constructors?

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