Why Would I Ever Need to Use C Nested Classes

Question

I m trying to understand about nested classes in C    I understand that a nested class is a class that is defined within another class  what I don t get is why I would ever need to do this

User · Answer

Nested classes are very useful for implementing internal details that should not be exposed  If you use Reflector to check classes like Dictionary lt Tkey TValue gt  or Hashtable you ll find some examples

User · Answer

There are times when it s useful to implement an interface that will be returned from within the class  but the implementation of that interface should be completely hidden from the outside world   As an example - prior to the addition of yield to C   one way to implement enumerators was to put the implementation of the enumerator as a private class within a collection   This would provide easy access to the members of the collection  but the outside world would not need see the details of how this is implemented

User · Answer

A pattern that I particularly like is to combine nested classes with the factory pattern   public abstract class BankAccount     private BankAccount         prevent third-party subclassing    private sealed class SavingsAccount   BankAccount           private sealed class ChequingAccount   BankAccount           public static BankAccount MakeSavingAccount             public static BankAccount MakeChequingAccount               By nesting the classes like this  I make it impossible for third parties to create their own subclasses  I have complete control over all the code that runs in any bankaccount object  And all my subclasses can share implementation details via the base class

User · Answer

There is good uses of public nested members too    Nested classes have access to the private members of the outer class  So a scenario where this is the right way would be when creating a Comparer  ie  implementing the IComparer interface   In this example  the FirstNameComparer has access to the private  firstName member  which it wouldn t if the class was a separate class    public class Person       private string  firstName      private string  lastName      private DateTime  birthday                      public class FirstNameComparer   IComparer lt Person gt                public int Compare Person x  Person y                        return x  firstName CompareTo y  firstName

User · Answer

A nested class can have private  protected and protected internal access modifiers along with public and internal    For example  you are implementing the GetEnumerator   method that returns an IEnumerator lt T gt  object  The consumers wouldn t care about the actual type of the object  All they know about it is that it implements that interface  The class you want to return doesn t have any direct use  You can declare that class as a private nested class and return an instance of it  this is actually how the C  compiler implements iterators    class MyUselessList   IEnumerable lt int gt                   private List lt int gt  internalList      private class UselessListEnumerator   IEnumerator lt int gt            private MyUselessList obj          public UselessListEnumerator MyUselessList o               obj   o                    private int currentIndex   -1          public int Current              get   return obj internalList currentIndex                       public bool MoveNext                 return   currentIndex  lt  obj internalList Count                      public IEnumerator lt int gt  GetEnumerator             return new UselessListEnumerator this

User · Answer

Maybe this is a good example of when to use nested classes      ORIGINAL class ImageCacheSettings     class ImageCacheEntry     class ImageCache       ImageCacheSettings mSettings      List lt ImageCacheEntry gt  mEntries      And      REFACTORED class ImageCache       Settings mSettings      List lt Entry gt  mEntries       class Settings        class Entry        PS  I ve not taken into account which access modifiers should be applied  private  protected  public  internal

User · Answer

The purpose is typically just to restrict the scope of the nested class  Nested classes compared to normal classes have the additional possibility of the private modifier  as well as protected of course    Basically  if you only need to use this class from within the  parent  class  in terms of scope   then it is usually appropiate to define it as a nested class  If this class might need to be used from without the assembly library  then it is usually more convenient to the user to define it as a separate  sibling  class  whether or not there is any conceptual relationship between the two classes  Even though it is technically possible to create a public class nested within a public parent class  this is in my opinion rarely an appropiate thing to implement

User · Answer

what I don t get is why I would ever need to do this   I think you never need to do this  Given a nested class like this      class A       B is used to help implement A   class B            etc             etc            you can always move the inner nested class to global scope  like this      class A        etc         B is used to help implement A class B        etc        However  when B is only used to help implement A  then making B an inner nested class has two advantages    It doesn t pollute the global scope  e g  client code which can see A doesn t know that the B class even exists  The methods of B implicitly have access to private members of A  whereas if B weren t nested inside A  B wouldn t be able to access members of A unless those members were internal or public  but then making those members internal or public would expose them to other classes too  not just B   so instead  keep those methods of A private and let B access them by declaring B as a nested class  If you know C    this is like saying that in C  all nested classes are automatically a  friend  of the class in which they re contained  and  that declaring a class as nested is the only way to declare friendship in C   since C  doesn t have a friend keyword     When I say that B can access private members of A  that s assuming that B has a reference to A  which it often does  since nested classes are often declared like this      class A       used to help implement A   class B         A m a      internal B A a    m a   a           methods of B can access private members of the m a instance             etc            and constructed from a method of A using code like this        create an instance of B  whose implementation can access members of self B b   new B this     You can see an example in Mehrdad s reply

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