Decide if it makes sense to close off the curve by moving the endpoint to the start based on start-end gap size and length of curve as approximated by chord defined by 6 points.
Rhino.IsCurveClosable (strObject [, dblTolerance])
| strObject | Required. String. The object's identifier. | 
| dblTolerance | Optional. Number. The maximum allowable distance between start point and end point of the curve. If omitted, the document's current absolute tolerance is used. | 
| Boolean | True if successful, otherwise False. | 
| Null | On error. | 
Const rhObjectCurve = 4
Dim strObject
strObject = Rhino.GetObject("Select curve", rhObjectCurve)
If Not Rhino.IsCurveClosed(strObject) And Rhino.IsCurveClosable(strObject) Then
Rhino.CloseCurve strObject
End If