arcgis 10.0 - Why does ITopologicalOperator2 .ConstructUnion produce a somehow invalid polygon?

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.