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4.9 Nested Types

A nested type is a type defined within the scope of another type, which is called the enclosing type. A nested type has access to all members of its enclosing type. For example, it has access to private fields defined in the enclosing type and to protected fields defined in all ascendants of the enclosing type.

// enclosing type
public class OuterType {
  private string name;

  // nested type
  public class InnerType {
    public InnerType(OuterType outer){
      // the name field is private, but it works just fine
      Console.WriteLine(outer.name);
    }
  }
}

In general, nested types should be used sparingly. There are several reasons for this. Some developers are not fully familiar with the concept. These developers might, for example, have problems with the syntax of declaring variables of nested types. Nested types are also very tightly coupled with their enclosing types, and as such are not suited to be general-purpose types.

Nested types are best suited for modeling implementation details of their enclosing types. The end user should rarely have to declare variables of a nested type and almost never explicitly instantiate nested types. For example, the enumerator of a collection can be a nested type of that collection. Enumerators are usually instantiated by their enclosing type and because many languages support the foreach statement, enumerator variables rarely have to be declared by the end user.

In general, nested types should be used sparingly, and exposure as public types should be avoided.

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