GeometryCollection
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
A GeometryCollection is a collection of zero or more geometry or geography instances. A GeometryCollection can be empty.
GeometryCollection Instances
Accepted Instances
For a GeometryCollection instance to be accepted, it must either be an empty GeometryCollection instance or all the instances comprising the GeometryCollection instance must be accepted instances. The following example shows accepted instances.
DECLARE @g1 geometry = 'GEOMETRYCOLLECTION EMPTY';
DECLARE @g2 geometry = 'GEOMETRYCOLLECTION(LINESTRING EMPTY,POLYGON((-1 -1, -1 -5, -5 -5, -5 -1, -1 -1)))';
DECLARE @g3 geometry = 'GEOMETRYCOLLECTION(LINESTRING(1 1, 3 5),POLYGON((-1 -1, -1 -5, -5 -5, -5 -1, -1 -1)))';
The following example throws a System.FormatException because the LinesString instance in the GeometryCollection instance is not accepted.
DECLARE @g geometry = 'GEOMETRYCOLLECTION(LINESTRING(1 1), POLYGON((-1 -1, -1 -5, -5 -5, -5 -1, -1 -1)))';
Valid Instances
A GeometryCollection instance is valid when all instances that comprise the GeometryCollection instance are valid. The following shows three valid GeometryCollection instances and one instance that is not valid.
DECLARE @g1 geometry = 'GEOMETRYCOLLECTION EMPTY';
DECLARE @g2 geometry = 'GEOMETRYCOLLECTION(LINESTRING EMPTY,POLYGON((-1 -1, -1 -5, -5 -5, -5 -1, -1 -1)))';
DECLARE @g3 geometry = 'GEOMETRYCOLLECTION(LINESTRING(1 1, 3 5),POLYGON((-1 -1, -1 -5, -5 -5, -5 -1, -1 -1)))';
DECLARE @g4 geometry = 'GEOMETRYCOLLECTION(LINESTRING(1 1, 3 5),POLYGON((-1 -1, 1 -5, -5 5, -5 -1, -1 -1)))';
SELECT @g1.STIsValid(), @g2.STIsValid(), @g3.STIsValid(), @g4.STIsValid();
@g4 is not valid because the Polygon instance in the GeometryCollection instance is not valid.
For more information on accepted and valid instances, see Point, MultiPoint, LineString, MultiLineString, Polygon, and MultiPolygon.
Examples
The following example instantiates a geometry``GeometryCollection with Z values in SRID 1 containing a Point instance and a Polygon instance.
DECLARE @g geometry;
SET @g = geometry::STGeomCollFromText('GEOMETRYCOLLECTION(POINT(3 3 1), POLYGON((0 0 2, 1 10 3, 1 0 4, 0 0 2)))', 1);