Geometry Union( 
   IEnumerable<Geometry> geometries
)

// union many polylines

List<Coordinate2D> coords = new List<Coordinate2D>()
{
  new Coordinate2D(1, 2), new Coordinate2D(3, 4), new Coordinate2D(4, 2),
  new Coordinate2D(5, 6), new Coordinate2D(7, 8), new Coordinate2D(8, 4),
  new Coordinate2D(9, 10), new Coordinate2D(11, 12), new Coordinate2D(12, 8),
  new Coordinate2D(10, 8), new Coordinate2D(12, 12), new Coordinate2D(14, 10)
};

// create Disjoint lines
List<Polyline> manyLines = new List<Polyline>
{
  PolylineBuilderEx.CreatePolyline(new List<Coordinate2D>(){coords[0], coords[1], coords[2]}, SpatialReferences.WGS84),
  PolylineBuilderEx.CreatePolyline(new List<Coordinate2D>(){coords[3], coords[4], coords[5]}),
  PolylineBuilderEx.CreatePolyline(new List<Coordinate2D>(){coords[6], coords[7], coords[8]})
};

Polyline polyline = GeometryEngine.Instance.Union(manyLines) as Polyline;

// union many polygons

List<Coordinate3D> coordsZ = new List<Coordinate3D>()
{
  new Coordinate3D(1, 2, 0), new Coordinate3D(3, 4, 1), new Coordinate3D(4, 2, 2),
  new Coordinate3D(5, 6, 3), new Coordinate3D(7, 8, 4), new Coordinate3D(8, 4, 5),
  new Coordinate3D(9, 10, 6), new Coordinate3D(11, 12, 7), new Coordinate3D(12, 8, 8),
  new Coordinate3D(10, 8, 9), new Coordinate3D(12, 12, 10), new Coordinate3D(14, 10, 11)
};

// create polygons
List<Polygon> manyPolygonsZ = new List<Polygon>
{
  PolygonBuilderEx.CreatePolygon(new List<Coordinate3D>(){coordsZ[0], coordsZ[1], coordsZ[2]}, SpatialReferences.WGS84),
  PolygonBuilderEx.CreatePolygon(new List<Coordinate3D>(){coordsZ[3], coordsZ[4], coordsZ[5]}),
  PolygonBuilderEx.CreatePolygon(new List<Coordinate3D>(){coordsZ[6], coordsZ[7], coordsZ[8]})
};

Polygon polygon = GeometryEngine.Instance.Union(manyPolygonsZ) as Polygon;