public override DbExpression Visit(DbIsNullExpression expression)
        {
            Check.NotNull(expression, "expression");

            return(VisitUnary(
                       expression,
                       exp =>
                       TypeSemantics.IsRowType(exp.ResultType)
                    ? CqtBuilder.CreateIsNullExpressionAllowingRowTypeArgument(exp)
                    : CqtBuilder.IsNull(exp)));
        }