A particular problem is determining which of the columns in the DataWindow actually have a control present on the surface of the DataWindow object. The DataWindow will throw an exception if you attempt to access certain properties of a column and it doesn't have those properties because they're visible control properties. I found that attempting to access the Band property works, and columns without a visible control report back "?" for the band. The GetVisibleColumns method then returns an array containing only those columns that have a visible control.

GetVisibleColumns

Public Function GetVisibleColumns() As ArrayList
Dim columns As ArrayList = GetColumns()
Dim visiblecolumns As New ArrayList
Dim band, col As String
For Each col In columns
   band = MyBase.GetProperty(col + ".Band")
   If band <> "?" Then
    visiblecolumns.Add(col)
   End If
Next
Return visiblecolumns
End Function

Another native feature of the DataGrid control is the display of a sort arrow indicator in the column headers when the column header click is used to sort that column. I've incorporated that using a .NET implementation of the grid sort indicator sample from TopWizProgramming. The CreateHeaderSortArrows method dynamically creates a set of computed columns for every column header (at the same position and same size as the column headers) that automatically displays an up or down arrow based on the DataWindow object sort expression (See Listing 1).

Another modification was made directly to the DataWindow object. The DataWindow control has a SetRowFocusIndicator that takes a bitmap that can be used to indicate what the current row is. In most grid DataWindow implementations turning on row selection for the row usually indicates this. However, I was trying to mimic the DataGrid style as much as possible. The SetRowFocusIndicator wasn't giving me quite the results I wanted. So instead, I added a computed column to the DataWindow with an expression of:

if ( currentRow() = getRow(), '4', '' )

and then set the font for that column to Marlett.

Conclusion
As we've seen, the DataWindow provides much of the same functionality as the DataGrid control. Most of the areas where the DataGrid provides functionality not native to the DataWindow - sort on column header click and sort indicators - are easily added by subclassing the control and adding a few lines of code. On the other hand, the DataWindow offers functionality not easily implemented in the DataGrid primarily because:

1. The separate DataWindow object lets the developer create the visual presentation of the data via the DataWindow Designer painter rather than having to code it in script.
2. Most of the DataWindow object properties accept expressions that self-evaluate so properties can automatically change based on changes to the state or value of data.