Within the ESRI SDK resources there is an example of combining polygons by use of a geometry bag. The following code appears:
'Define the spatial reference of the bag before adding geometries to it. geometryBag.SpatialReference = geoDataset.SpatialReference
That makes sense, based on some of the other documentation surrounding geometry bags, but when I tried it myself, I got a compile error saying that the method didn't exist. After looking at the geometry object model diagram, it appears as if the spatial reference property of a geometry bag is read-only... so what gives?