How to cast in opening an ESRI.ArcGIS.Location.RouteMeasureLineLocationClass?

I want to open an ESRI.ArcGIS.Location.RouteMeasureLineClass from a GeoDB. From
http://resources.esri.com/help/9.3/a...n_overview.htm
I got Open LocalLocatorWorkspace. Then I used the code below:
public ESRI.ArcGIS.Location.RouteMeasureLineLocationClass GetRouteMeasureClass(ESRI.ArcGIS.Location.IDatabaseLocatorWorkspace ws, string fName)
{
IFeatureWorkspace fWs = (IFeatureWorkspace)ws;
return (ESRI.ArcGIS.Location.RouteMeasureLineLocationClass)(fWs.OpenFeatureClass(fName));
}
But got error: Cannot implicitly convert type 'ESRI.ArcGIS.Geodatabase.IFeatureClass' to 'ESRI.ArcGIS.Location.RouteMeasureLineLocationClass'. An explicit conversion exists (are you missing a cast?)
How can a proper casting be applied? Thanks.