Question

I’d like to create an IList<Child> that maintains its Child objects in a default/implicit sort order at all times (i.e. regardless of additions/removals to the underlying list).

What I’m specifically trying to avoid is the need for all consumers of said IList<Child> to explicitly invoke IEnumerable<T>.OrderBy() every time they want to enumerate it. Apart from violating DRY, such an approach would also break encapsulation as consumers would have to know that my list is even sorted, which is really none of their business 🙂

The solution that seemed most logical/efficient was to expose IList<Child> as IEnumerable<Child> (to prevent List mutations) and add explicit Add/Remove methods to the containing Parent. This way, I can intercept changes to the List that necessitate a re-sort, and apply one via Linq:

public class Child {
  public string StringProperty;
  public int IntProperty;
}

public class Parent{
private IList<Child> _children = new List<Child>();

      public IEnumerable<Child> Children{
      get
         {

            return _children;
         }
      }
      private void ReSortChildren(){
        _children = new List<Child>(child.OrderBy(c=>c.StringProperty));
      }
      public void AddChild(Child c){
          _children.Add();
          ReSortChildren()
      }
      public void RemoveChild(Child c){
          _children.Remove(c);
          ReSortChildren()
      }
}

Still, this approach doesn’t intercept changes made to the underlying Child.StringProperty (which in this case is the property driving the sort). There must be a more elegant solution to such a basic problem, but I haven’t been able to find one.

EDIT:
I wasn’t clear in that I would preferable a LINQ compatible solution. I’d rather not resort to using .NET 2.0 constructs (i.e. SortedList)

Answer 1

One way you could go about it is to have Child publish an event OnStringPropertyChanged which passes along the previous value of StringProperty. Then create a derivation of SortedList that overrides the Add method to hookup a handler to that event. Whenever the event fires, remove the item from the list and re-add it with the new value of StringProperty. If you can’t change Child, then I would make a proxy class that either derives from or wraps Child to implement the event.

If you don’t want to do that, I would still use a SortedList, but internally manage the above sorting logic anytime the StringProperty needs to be changed. To be DRY, it’s preferable to route all updates to StringProperty through a common method that correctly manages the sorting, rather than accessing the list directly from various places within the class and duplicating the sort management logic.

I would also caution against allowing the controller to pass in a reference to Child, which allows him to manipulate StringProperty after it’s added to the list.

public class Parent{
  private SortedList<string, Child> _children = new SortedList<string, Child>();

  public ReadOnlyCollection<Child> Children{
    get { return new ReadOnlyCollection<Child>(_children.Values); }
  }

  public void AddChild(string stringProperty, int data, Salamandar sal){
    _children.Add(stringProperty, new Child(stringProperty, data, sal));
  }

  public void RemoveChild(string stringProperty){
    _children.Remove(stringProperty);
  }

  private void UpdateChildStringProperty(Child c, string newStringProperty) {
    if (c == null) throw new ArgumentNullException("c");

    RemoveChild(c);
    c.StringProperty = newStringProperty;
    AddChild(c);
  }

  public void CheckSalamandar(string s) {
    if (_children.ContainsKey(s))
      var c = _children[s];
      if (c.Salamandar.IsActive) {
        // update StringProperty through our method
        UpdateChildStringProperty(c, c.StringProperty.Reverse());
        // update other properties directly
        c.Number++;
    }
  }
}