Why does ITopologicalOperator2 .ConstructUnion produce a somehow invalid polygon?

Question

I try to add a single buffer around the shape of the selected features. So I step through each selected feature and add it's shape to a GeometryBag. With ITopologicalOperator2.ConstructUnion I create a union geometry. But when I use this geometry to add an element, nothing appears after Refresh.

The code part for adding an element does work, when I directly assign element.Geometry = feature.Shape. So ConstructUnion seems to fail here. Any ideas why?

I already found out, that I have to simplify the shapes before adding it to GeometryBag and to set the SpatialReference on the GeometryBag before.

ESRI.ArcGIS.Carto.IGraphicsContainer graphicsContainer = (ESRI.ArcGIS.Carto.IGraphicsContainer)activeView.FocusMap;

ESRI.ArcGIS.Geodatabase.IEnumFeature enumFeature = (ESRI.ArcGIS.Geodatabase.IEnumFeature)map.FeatureSelection;
enumFeature.Reset();
ESRI.ArcGIS.Geodatabase.IFeature feature = enumFeature.Next();

IGeometryBag geoBag = new GeometryBagClass();
geoBag.SpatialReference = activeView.FocusMap.SpatialReference;
IGeometryCollection geometriesToUnion = geoBag as IGeometryCollection;
while (!(feature == null))
{
  ITopologicalOperator2 shape = feature.ShapeCopy as ITopologicalOperator2;
  shape.IsKnownSimple_2 = false; // see nef001's answer
  shape.Simplify();
  geometriesToUnion.AddGeometry(shape as IGeometry);
  feature = enumFeature.Next();
}

IPolygon poly = new PolygonClass();
resultPolygon.SpatialReference = activeView.FocusMap.SpatialReference; // see Petr Krebs's comment
ITopologicalOperator2 resultPolygon = poly as ITopologicalOperator2;
resultPolygon.ConstructUnion(geometriesToUnion as IEnumGeometry);
IFillShapeElement fillShapeElement = new ESRI.ArcGIS.Carto.PolygonElementClass();
fillShapeElement.Symbol = fillSymbol;
ESRI.ArcGIS.Carto.IElement element = (IElement)fillShapeElement;
topologicalOperator = (ESRI.ArcGIS.Geometry.ITopologicalOperator)resultPolygon;
//element.Geometry = topologicalOperator.Buffer(distance);
element.Geometry = resultPolygon as IGeometry; // To make sure, it's not the Buffering that goes wrong
graphicsContainer.AddElement(element, 0);

Update: To address Petr Krebs suggestions

• I changed the assignment of geometriesToUnion.AddGeometry to feature.ShapeCopy.
• I changed the assignment of element.Geometry to resultPolygon instead of the buffer output.

Both didn't help.

• For testing I use a single layer where I select three polygon features. So the are all polygons and have the same SpatialReference.
• resultPolygon.IsEmpty after ConstructUnion is false.

Update 2

• Regarding Nef001's answer I added shape.IsKnownSimple_2 = false; but it didn't change the result.

Update 3

• Regarding Petr's answer I set the spatial reference before Union() but it didn't help.

Answer 1

ESRI Documentation says that this method does not support GeometryBags.

http://help.arcgis.com/en/sdk/10.0/arcobjects_net/componenthelp/index.html#//002m000003vn000000

Answer 2

Few things to try:

1. Make sure the selection enumerator is either non-recycling (see IEnumFeatureSetup, or otherwise copy the feature geometries prior to adding them to the geometry bag (via IFeature.ShapeCopy).

2. Make sure all the input geometries, as well as the newly created Polygon have the same spatial reference.

3. Make sure all the input geometries are polygons, which you seem to be assuming.

4. Check that it is not actually the subsequent buffer operation that prevents your code snippet from working as expected.

Answer 3

I think calling Simplify() on ITopologicalOperator does nothing unless you first set IsKnownSimple to false.

Try adding to your code before the call to Simplify:

shape.IsKnownSimple_2 = false;

See the following Remarks in the documentation for ITopologicalOperator.Simplify() method:

Remarks This method first looks at the ITopologicalOperator::IsKnownSimple flag before starting processing. If the flag is 'true' then operation is interrupted and the geometry is considered simple. If the flag is 'false' then the geometry consistency is checked and the geometry is updated as needed.

Answer 4

I do not think code in question successfully builds at the moment..I made a few edits..Cast the resultpolygon to ITopologicalOperator2 and added a Type.Missing object parameter to the AddGeometry function..It works for me.

IGeometryCollection geometriesToUnion = geoBag as IGeometryCollection;
object missing = Type.Missing;
while (!(feature == null))
{
    ITopologicalOperator2 shape = feature.ShapeCopy as ITopologicalOperator2;
    shape.IsKnownSimple_2 = false; // see nef001's answer
    shape.Simplify();

    geometriesToUnion.AddGeometry(shape as IGeometry, ref missing, ref missing);
    feature = enumFeature.Next();
}

IPolygon poly = new PolygonClass();
poly.SpatialReference = activeView.FocusMap.SpatialReference;
ITopologicalOperator2 topologicalOperator;
topologicalOperator = poly as ITopologicalOperator2;

topologicalOperator.ConstructUnion(geometriesToUnion as IEnumGeometry);
IFillShapeElement fillShapeElement = new ESRI.ArcGIS.Carto.PolygonElementClass();
ISimpleFillSymbol simpleFillSymbol = new SimpleFillSymbolClass();
fillShapeElement.Symbol = simpleFillSymbol;
IElement element = (IElement)fillShapeElement;

topologicalOperator = (ITopologicalOperator2)poly;
element.Geometry = topologicalOperator.Buffer(10);
graphicsContainer.AddElement(element, 0);