Problems with Multipatches

Question

I have some polygonal features that have minimum and maximum z values.

I'm trying to calculate the difference between a fixed extruded polygons and the sum of extruded related polygons.

Here's how I'm doing it:

Notes: the cursor is non recycling.

IExtrude extrude = new GeometryEnvironmentClass();

        while ((tempFeature = cursor.NextFeature()) != null)
        {
            var lower = Classifier.GetNamedLevelValue(tempFeature.get_Value(lowerIdx).ToString());
            var upper = Classifier.GetNamedLevelValue(tempFeature.get_Value(upperIdx).ToString());
            var geometry = extrude.ExtrudeFromTo(lower, upper, tempFeature.ShapeCopy);
            geometries.Add(geometry);
        }

        IGeometryFactory fact = new GeometryEnvironmentClass();
        IGeometry unioned = null;
        fact.CreateEmptyGeometryByType(esriGeometryType.esriGeometryMultiPatch, out unioned);

        for (int i = 0; i <= geometries.Count - 1; i++)
        {
            var topo = (ITopologicalOperator)unioned;
            var tempGeom = topo.Union(geometries[i]);
            unioned = (IGeometry)((IClone)tempGeom).Clone();
        }

        var extrudedParent = extrude.ExtrudeFromTo(0, 999, feature.ShapeCopy);
        var topoParent = (ITopologicalOperator)extrudedParent;
        var remaining = topoParent.Difference((IGeometry)unioned);

I get a COM Exception in topoParent.Difference. I'm not sure what is the problem. Can anyone help me? Any other ways to do it?

This is the COMException: - "Exceção de HRESULT: 0x80040229"} System.Exception {System.Runtime.InteropServices.COMException}

I even whipped up a little unit test to check if it was my code or if the operation is illegal. Still crashes in the same spot.

    [TestMethod]
    public void VolumeTest()
    {
        object missing = Type.Missing;

        var p1 = PointHelper.ConstructPoint(0, 0);
        var p2 = PointHelper.ConstructPoint(0, 1);
        var p3 = PointHelper.ConstructPoint(1, 1);
        var p4 = PointHelper.ConstructPoint(1, 0);

        var p5 = PointHelper.ConstructPoint(0, 2);
        var p6 = PointHelper.ConstructPoint(2, 2);
        var p7 = PointHelper.ConstructPoint(2, 0);

        IPointCollection pointCol = new PolygonClass();
        pointCol.AddPoint(p1, ref missing, ref missing);
        pointCol.AddPoint(p2, ref missing, ref missing);
        pointCol.AddPoint(p3, ref missing, ref missing);
        pointCol.AddPoint(p4, ref missing, ref missing);
        pointCol.AddPoint(p1, ref missing, ref missing);

        IPointCollection pointCol2 = new PolygonClass();
        pointCol2.AddPoint(p1, ref missing, ref missing);
        pointCol2.AddPoint(p5, ref missing, ref missing);
        pointCol2.AddPoint(p6, ref missing, ref missing);
        pointCol2.AddPoint(p7, ref missing, ref missing);
        pointCol2.AddPoint(p1, ref missing, ref missing);

        IExtrude ext = new GeometryEnvironmentClass();
        IGeometry extruded = ext.ExtrudeFromTo(0, 1, (IPolygon)pointCol);
        IGeometry extruded2 = ext.ExtrudeFromTo(0, 2, (IPolygon)pointCol2);

        ITopologicalOperator topo = (ITopologicalOperator)extruded2;
        IGeometry result = topo.Difference(extruded);
    }

Thanks