[c#] Why Would I Ever Need to Use C# Nested Classes

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.