Question

I’m currently working on a wrapper for a third party component (not the best idea, I know) that we just purchased, a grid to be precise. Well, exposing most of the grid properties hasn’t been that hard so far… my real issue is with the rows/cells collection.

The seniors/leads don’t want the developers going crazy with everything in those, so having some sort CustomRowCollection of CustomRow objects wrapping the bare minimum of the actual grid’s RowCollection of Row objects is their idea… same goes for cells, a CustomRow object should have a CustomCellCollection of CustomCells wrapping the actual gridrow’s CellCollection of Cell objects.

Do you have any advice on how I should approach this? I can’t quite picture the design.
Thanks

Answer 1

The CustomRowCollection is a wrapper around the RowCollection. It’s not actually a collection at all; it’s a class that implements ICollection<CustomRow> and translates each method into the appropriate method on the underlying RowCollection. For instance:

public void Add(CustomRow r)
{
   _RowCollection.Add(r.Row);
}

The problem with this is evident if you look at, say, the indexer:

public CustomRow this[int i]
{
   get { return new CustomRow(_RowCollection[i]); }
   set { _RowCollection[i] = value.Row; }
}

You’re creating a new CustomRow object every time you get a CustomRow out of the CustomRowCollection. This can cause a lot of problems. For instance, this will fail, and it really shouldn’t:

Debug.Assert(myCustomRowCollection[0] == myCustomRowCollection[0]);

There are a couple of ways around this. You can shadow the RowCollection with a private Dictionary<Row, CustomRow> field. Your indexer would then look like this:

public CustomRow this[int i]
{
   get
   {
      Row r = RowCollection[i];
      if (!_Dictionary.ContainsKey(r))
      {
         _Dictionary[r] = CustomRow(r);
      }
      return _Dictionary[r];
   }
   set
   {
      _Dictionary[value.Row] = value;
      RowCollection[i] = value.Row;
   }
}

This prevents you from ever creating two CustomRow objects with the same underlying Row. But it’s a lot of work, because you have to deal with keeping those collections in sync with each other. (It’s especially bad if the grid ever deletes a Row from its RowCollection without notifying you, because you’re still maintaining a reference to that Row in your dictionary and nothing short of re-synching your dictionary with the RowCollection is ever going to remove it.)

A simpler way is to override Equals in the CustomRow class, so that two CustomRow objects are equal if their Row properties are equal. You also have to override GetHashCode so that it returns Row.GetHashCode(), which will let you use CustomRow objects as dictionary keys.

You have to be very careful, if you do this, not to ever implement any properties in CustomRow that aren’t wrappers around properties in Row. If you ever do this, you’re introducing a situation in which this might not fail, and it always should:

Debug.Assert(r1 == r2 && r1.Property != r2.Property);