arcobjects - How do I zoom to the overall extent of two (or more) selected features from two layers?

I am creating an application where the user picks a value from a combo box, the application then selects that feature and then selects the corresponding features from a second layer, (both selections are in a different colour). What I want to do is as soon as the selection is made, the map will zoom to the envelope of the selected features. I know how to zoom to 2 individual layers (using pEnvlope.union...), but i need to do it on features.

Has anyone got any idea of the best way to do this? (I'm using ArcGIS 9.3.1 and VBA).

Thanks

Answer

You could just use the ArcMap Command, Zoom To Selected Features.

  Dim pUID As New UID
  Dim pCmdItem As ICommandItem
  ' Use the GUID of the Save command
  ' Zoom to all selected features
  pUID.Value = "{AB073B49-DE5E-11D1-AA80-00C04FA37860}"

  ' or you can use the ProgID
  ' pUID.Value = "esriArcMapUI.ZoomToSelectedCommand"
  pUID.SubType = 3
  Set pCmdItem = Application.Document.CommandBars.Find(pUID)
  pCmdItem.Execute