private void FlashFeature(IActiveView _activeView, IFeature _feature)
{
    IActiveView activeView = _activeView;
    IFeatureIdentifyObj featIdentify = new FeatureIdentifyObject();
    featIdentify.Feature = _feature;
    IIdentifyObj identify = featIdentify as IIdentifyObj;
    identify.Flash(activeView.ScreenDisplay);
}

Dim pFCursor As ESRI.ArcGIS.Geodatabase.IFeatureCursor
    Dim pFeature As ESRI.ArcGIS.Geodatabase.IFeature
    Dim pGeometry As ESRI.ArcGIS.Geometry.IGeometry
    Dim pGeometryBag As ESRI.ArcGIS.Geometry.IGeometryCollection = New ESRI.ArcGIS.Geometry.GeometryBag
    Dim pPolygon As ESRI.ArcGIS.Geometry.IPolygon
    Dim pPolyline As ESRI.ArcGIS.Geometry.IPolyline
    Dim pQFilter As New ESRI.ArcGIS.Geodatabase.QueryFilter
    Dim pRgbColor As New ESRI.ArcGIS.Display.RgbColor
    Dim pTopoOp As ESRI.ArcGIS.Geometry.ITopologicalOperator4

    Using releaser As New ESRI.ArcGIS.ADF.ComReleaser
       releaser.ManageLifetime(pFCursor)

       pQFilter.WhereClause = [String].Format("{0} in ({1})", pFClass.OIDFieldName, sender.Rows(e.RowIndex).Cells("colAdjacentOIDs").value)
       pFCursor = pFClass.Search(pQFilter, True)
       pFeature = pFCursor.NextFeature

       Do Until pFeature Is Nothing
           pGeometryBag.AddGeometry(pFeature.ShapeCopy)
           pFeature = pFCursor.NextFeature
       Loop

       Select Case pFClass.ShapeType
           Case ESRI.ArcGIS.Geometry.esriGeometryType.esriGeometryPolygon
                pTopoOp = New ESRI.ArcGIS.Geometry.Polygon
                pTopoOp.ConstructUnion(pGeometryBag)
                pPolygon = pTopoOp
                pGeometry = pPolygon
           Case ESRI.ArcGIS.Geometry.esriGeometryType.esriGeometryPolyline
                pTopoOp = New ESRI.ArcGIS.Geometry.Polyline
                pTopoOp.ConstructUnion(pGeometryBag)
                pPolyline = pTopoOp
                pGeometry = pPolyline
       End Select

       pRgbColor.Red = 255

       FlashGeometry(pGeometry, pRgbColor, My.ArcMap.Document.ActiveView.ScreenDisplay, 500)
   End Using