// example 2: proposed change

public class ClassMap<T>
{
  private ClassMapping classMapping = new ClassMapping();

  public PropertyPart Map(...)
  {
    var propertyMapping = new PropertyMapping();

    classMapping.AddProperty(propertyMapping);

    return new PropertyPart(propertyMapping);
  }

  public ClassMapping GetMapping()
  {
    return classMapping;
  }
}