winforms - Check for null via BinaryOperator -

groupoperator groupapplyon = new groupoperator();          groupapplyon.operatortype = groupoperatortype.and;         groupapplyon.operands.add(isb);         groupapplyon.operands.add(isa);         groupapplyon.operands.add(isc); 

where isx binaryoperator ..

binaryoperator isb = new binaryoperator("validity", null,             binaryoperatortype.equal); 

this code gives me error - how check public unaryoperator isnull() if "validity" null?

use nulloperator instead.