Why Would I Ever Need to Use C# Nested Classes

c# inner-classes nested-class
Dan Norton picture Dan Norton · Jul 4, 2009 · Viewed 169.7k times · Source

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.

Answer

Eric Lippert picture Eric Lippert · Jul 5, 2009

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.

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