ConstructUnion method some time returning an empty geometry

ShenglinXu

I found ConstructUnion method some time returning an empty geometry even all the geometries added are not empty.
Could you help to find out why? Is it a known issue?


// Sample code
IGeometry pGeomBag = null;
IGeometryCollection geometryCollection = null;

pGeomBag = new GeometryBagClass();
                                    
pGeomBag.SpatialReference = objFeature.ShapeCopy.SpatialReference;
geometryCollection = (IGeometryCollection)pGeomBag;

//buffer each selected roads and add to geometryCollection

geometryCollection.AddGeometry(objBufferResult, ref missing, ref missing);

//Do construct union

IPolygon polygon = new PolygonClass();
ITopologicalOperator2 objTopo = polygon as ITopologicalOperator2;
if (objTopo != null)
{
objTopo.ConstructUnion((IEnumGeometry)pGeomBag);
        objTopo.IsKnownSimple_2 = false;
        objTopo.Simplify();

IGeometry objExclusion = new PolygonClass();               
         objExclusion = (IGeometry)objTopo;
}

AndreaFlesca

Same problem: ConstructUnion doesn't work.
I'm developing with ArcGIs Engine SDK 10.1  for C++/Qt and I'm on Linux.
Here is my code:

    IGeometryBagPtr cGeometryBag(CLSID_GeometryBag);
    IGeometryCollectionPtr cGeometryCollection = cGeometryBag; // QI
    IGeometryPtr * cGeomVector = new IGeometryPtr[len];
    for (i = 0; i < len; i++)
    {
        IPointCollectionPtr cPointCollection(CLSID_Polygon);
        // I fill the cPointCollection object with my polygones (triangles)
        IPolygonPtr cPolygon = cPointCollection; // QI
        cGeomVector = cPolygon;
    }
    hr = cGeometryCollection -> AddGeometries((long)len, (IGeometry**)cGeomVector);
    if (FAILED(hr))
    {
        cout << "Impossible to add geometries" << std::endl;
        return;
    }
    long cGeomCount;
    hr = cGeometryCollection -> get_GeometryCount(&cGeomCount);
    if (FAILED(hr))
    {
        cout << "Impossible to calculate the number of geometries (1)" << std::endl;
        return;
    }
    cout << "Number of geometries (1): " << cGeomCount << std::endl;
    // in this dump I see that the number of geometries (in the GeometryCollection) is OK

    IEnumGeometryPtr cEnumGeometry = cGeometryBag; // QI
    hr = cEnumGeometry -> get_Count(&cGeomCount);
    if (FAILED(hr))
    {
        cout << "Impossible to calculate the number of geometries (2)" << std::endl;
        return;
    }
    cout << "Number of geometries (2): " << cGeomCount << std::endl;
    // in this dump I see that the number of geometries (in the EnumGeometry) is OK

    IPolygonPtr cResultPolygon(CLSID_Polygon);
    ITopologicalOperator2Ptr cTopoOp = cResultPolygon;
    hr = cTopoOp -> ConstructUnion(cEnumGeometry);
    if (FAILED(hr))
    {
        cout << "Impossible to do ConstructUnion" << std::endl;
        return;
    }
    IPointCollectionPtr cFinalPointCollection = cResultPolygon; // QI

    hr = cFinalPointCollection -> get_PointCount(&cGeomCount);
    if (FAILED(hr))
    {
        cout << "Impossible to calculate the number of points (3)" << std::endl;
        return;
    }
    cout << "Number of points (3): " << cGeomCount << std::endl;
    // In this dump the number (of points of the new geometry) is not OK, is always zero!

    

Where do I go wrong?
Are there examples in C++ to refer to?
The result remains the same (negative) if I change the line:

IPointCollectionPtr cFinalPointCollection = cResultPolygon; // QI

with:

IPointCollectionPtr cFinalPointCollection = cTopoOp; // QI

Any help would be appreciated.
Cheers from Italy.