private IPolygon GeometryBag_Example(IFeatureClass featureClass)
...{

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

    IGeoDataset geoDataset = featureClass as IGeoDataset;
    ISpatialFilter queryFilter = new SpatialFilterClass();

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

    //Define the spatial reference of the bag before adding geometries to it.
    geometryBag.SpatialReference = geoDataset.SpatialReference;

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

    IGeometryCollection geometryCollection = geometryBag as IGeometryCollection;
    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.
        object missing = Type.Missing;
        geometryCollection.AddGeometry(currentFeature.Shape, ref missing, ref missing);

        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 PolygonClass();
    unionedPolygon.ConstructUnion(geometryBag as IEnumGeometry);

    return unionedPolygon as IPolygon;
}