How to create a union of several polygons

Creating a union of several polygons

The following code example constructs a polygon representing the topological union of several polygons. The source polygons come from a feature class. References to the polygons are inserted into a geometry bag. The geometry bag is then used as the input parameter to the ConstructUnion method.
The spatial reference of the geometry bag is defined before adding geometries to it.
static IPolygon geometryBag_Example(IFeatureClass featureClass)throws Exception{

    //Check input objects.
    if (featureClass == null)
        return null;

    IGeoDataset geoDataset = (IGeoDataset)featureClass;
    ISpatialFilter queryFilter = new SpatialFilter();

    //Set the properties of the spatial filter here.
    IGeometry geometryBag = new GeometryBag();

    //Define the spatial reference of the bag before adding geometries to it.

    //Use a nonrecycling cursor so each returned geometry is a separate object. 
    IFeatureCursor featureCursor =, false);

    IGeometryCollection geometryCollection = (IGeometryCollection)geometryBag;
    IFeature currentFeature = featureCursor.nextFeature();

    while (currentFeature != null){
        //Add a reference to this feature's geometry into the bag.
        //You don't specify the before or after geometry (missing),
        //so the currentFeature.Shape IGeometry is added to the end of the geometryCollection.
        geometryCollection.addGeometry(currentFeature.getShape(), null, null);

        currentFeature = featureCursor.nextFeature();

    // Create the polygon that will be the union of the features returned from the search cursor.
    // The spatial reference of this feature does not need to be set ahead of time. The 
    // ConstructUnion method defines the constructed polygon's spatial reference to be the same as 
    // the input geometry bag.
    ITopologicalOperator unionedPolygon = new Polygon();

    return (IPolygon)unionedPolygon;

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