protected virtual Expression VisitIsNull(IsNullExpression isnull)
        {
            Expression expr = this.Visit(isnull.Expression);

            if (expr != isnull.Expression)
            {
                return(new IsNullExpression(expr));
            }
            return(isnull);
        }
 protected override Expression VisitIsNull(IsNullExpression isnull)
 {
     this.Visit(isnull.Expression);
     sb.Append(" IS NULL");
     return(isnull);
 }