Spatial Index is lost but never found unless I restart Arcmap

Hornbydd

All,

Odd problem here and I have not found a solution to it. I'm using ArcGIS 10.1, VB .net and the source data is a shapefile.At some point in my tool I add a field using the IFeatureClass.AddField method and populate it with values but when I add this field it drops the spatial index on the dataset. In ArcMap if I open the Attribute table I see the * in the Shape field which suggests its still there but I know its not. Later on in my tool I test if a dataset has a spatial index, if not I add one. I test if the dataset has a spatial index with the following code:

Dim pFeatureClass As IFeatureClass
 pFeatureClass = m_pFeatureLayer_River.FeatureClass
 Dim pIndexes As IIndexes 
Dim pEnumIndex As IEnumIndex 
pIndexes = pFeatureClass.Indexes 
pEnumIndex = pIndexes.FindIndexesByFieldName(pFeatureClass.ShapeFieldName) 
Dim pIndex As IIndex 
pIndex = pEnumIndex.Next 
If pIndex Is Nothing Then 
   ' Add Spatial Index 
End If

This always fails suggesting the spatial index exists but I know it does not as another tool runs very slowly without it.Only when I restart ArcMap, load the shapefile, open attribute table do I see that the * in the shape field has disappeared.So my question is knowing that I use the IFeatureClass.AddField method to add a field how do I "refresh" ArcMap to pick up on the fact that the spatial index has been lost and allow my check for spatial index code to work?

Duncan