Why are C 4 optional parameters defined on interface not enforced on implementing class

Question

I noticed that with the optional parameters in C  4 if you specify an  optional parameter on an interface you don t have to make that parameter optional on any implementing class   public interface MyInterface       void TestMethod bool flag   false      public class MyClass   MyInterface       public void TestMethod bool flag                Console WriteLine flag             and therefore   var obj   new MyClass            obj TestMethod       compiler error  var obj2   new MyClass   as MyInterface  obj2 TestMethod       prints false   Does anyone know why optional parameters are designed to work this way   On one hand I suppose the ability to override any default values specified on the interfaces is useful though to be honest I m not sure if you should even be able to specify default values on the interface as that should be an implementation decision   On the other hand  this disconnect means you can t always use the concrete class and the interface interchangeably  This of course  wouldn t be a problem if the default value is specified on the implementation  but then if you re exposing your concrete class as the interface  using some IOC framework to inject the concrete class for instance  then really there s no point having the default value as the caller will have to always provide it anyway

User · Answer

Just want to add my take here  as the other answers do provide reasonable explanations  but not ones that fully satisfy me  Optional parameters are syntactic sugar for compile-time injection of the default value at the call site  This doesn t have anything to do with interfaces implementations  and it can be seen as purely a side-effect of methods with optional parameters  So  when you call the method  public void TestMethod bool value   false               like SomeClass TestMethod    it is actually SomeClass TestMethod false   If you call this method on an interface  from static type-checking  the method signature has the optional parameter  If you call this method on a deriving class s instance that doesn t have the optional parameter  from static type-checking  the method signature does not have the optional parameter  and must be called with full arguments  Due to how optional parameters are implemented  this is the natural design result

User · Answer

UPDATE  This question was the subject of my blog on May 12th 2011  Thanks for the great question   Suppose you have an interface as you describe  and a hundred classes that implement it  Then you decide to make one of the parameters of one of the interface s methods optional  Are you suggesting that the right thing to do is for the compiler to force the developer to find every implementation of that interface method  and make the parameter optional as well   Suppose we did that  Now suppose the developer did not have the source code for the implementation        in metadata  public class B         public void TestMethod bool b              in source code interface MyInterface         void TestMethod bool b   false      class D   B  MyInterface       Legal because D s base class has a public method     that implements the interface method   How is the author of D supposed to make this work  Are they required in your world to call up the author of B on the phone and ask them to please ship them a new version of B that makes the method have an optional parameter     That s not going to fly  What if two people call up the author of B  and one of them wants the default to be true and one of them wants it to be false  What if the author of B simply refuses to play along   Perhaps in that case they would be required to say   class D   B  MyInterface        public new void TestMethod bool b   false                base TestMethod b             The proposed feature seems to add a lot of inconvenience for the programmer with no corresponding increase in representative power  What s the compelling benefit of this feature which justifies the increased cost to the user     UPDATE  In the comments below  supercat suggests a language feature that would genuinely add power to the language and enable some scenarios similar to the one described in this question  FYI  that feature -- default implementations of methods in interfaces -- will be added to C  8

User · Answer

An optional parameter is just tagged with an attribute  This attribute tells the compiler to insert the default value for that parameter at the call-site   The call obj2 TestMethod    is replaced by obj2 TestMethod false   when the C  code gets compiled to IL  and not at JIT-time   So in a way it s always the caller providing the default value with optional parameters  This also has consequences on binary versioning  If you change the default value but don t recompile the calling code it will continue to use the old default value      On the other hand  this disconnect means you can t always use the concrete class and the interface interchangeably    You already can t do that if the interface method was implemented explicitly

User · Answer

Optional parameters are kind of like a macro substitution from what I understand  They are not really optional from the method s point of view  An artifact of that is the behavior you see where you get different results if you cast to an interface

User · Answer

Because default parameters are resolved at compile time  not runtime  So the default values does not belong to the object being called  but to the reference type that it is being called through

[c#] Why are C# 4 optional parameters defined on interface not enforced on implementing class?

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