ILine to IPolyline

You need to use an ISegmentCollection and add the ILIne to it, see the example below.

' ---- Assumes pLine exists as an ILine ----

Dim pSegColl as ISegmentCollection
Set pSegColl = New Polyline
pSegColl .AddSegment pPline

Dim pPolyline As IPolyline
Set pPolyline = pSegColl

' ---- You can now use pPolyline ----

ILine to IPolyline