How to find self-intersecting polylines using ArcObjects?

Question

If I have a polyline, how do I figure out whether it is self-intersecting? I tried to use ITopologicalOperator.IsSimple Property, but it always returns true.

Are there any ways of doing these kind of topological checks over individual geometries?

Answer 1

I think you can do with with the ITopologicalOperator2 interface:

dim pTopoLogicalOp as ITopologicalOperator2
Set pPolyline = pFeature.ShapeCopy
Set pTopoLogicalOp = pPolyline
Let pTopoLogicalOp.IsKnownSimple = False
If pTopoLogicalOp.IsSimple = False Then
   ' Polyline is self intersecting
End If

Answer 2

The Polyline should be converted to a Polygon, and then the ITopologicalOperator3:get_IsSimpleEx() method can be used

Here is the sample code

private bool IsLineSelfIntersecting(IPolyline line)
    {
        bool isSelfIntersecting = false;

        IPointCollection polygonVertices = new PolygonClass();
        IPointCollection lineVertices = line as IPointCollection;

        polygonVertices.AddPointCollection(lineVertices); //convert to Polygon

        ITopologicalOperator3 topo = polygonVertices as ITopologicalOperator3;
        esriNonSimpleReasonEnum reason =esriNonSimpleReasonEnum.esriNonSimpleSelfIntersections;
        topo.IsKnownSimple_2 = false;
        if (!topo.get_IsSimpleEx(out reason))
        {
            if (reason == esriNonSimpleReasonEnum.esriNonSimpleSelfIntersections)
            {
                isSelfIntersecting = true;
            }
        }

        return isSelfIntersecting;
    }