Hi
When calling a BooleanOperation on a solid instance against another one, if the operation succeed the first solid is changed and the the second one is erased.
solid1.BooleanOperation(BooleanOperationType.BoolUnite, solid2);
solid2 is unioned to solid1, only solid1 remains in the database.
You can see another complete example in the help;
http://help.autodesk.com/view/ACD/2015/ENU/?guid=GUID-5F8D4718-D0A5-41F9-9178-A4508133855A