What are reasons why one would want to use nested classes? [duplicate]

Answer 1

One very common pattern where this technique is used is in scenarios where a class returns an interface or base class type from one of its properties or methods, but the concrete type is a private nested class. Consider the following example.

public class MyCollection : IEnumerable
{
  public IEnumerator GetEnumerator()
  {
    return new MyEnumerator();
  }

  private class MyEnumerator
  {
  }
}