1. [EdmScalarPropertyAttribute(EntityKeyProperty=false, IsNullable=true)]
        [DataMemberAttribute()]
        [DisplayName("编号")]
        //[Required(ErrorMessage="编号不能为空")]
        public global::System.String center_no
        {
            get
            {
                return _center_no;
            }
            set
            {
                Oncenter_noChanging(value);
                ReportPropertyChanging("center_no");
                _center_no = StructuralObject.SetValidValue(value, true);
                ReportPropertyChanged("center_no");
                Oncenter_noChanged();
            }
        }

这里的StructuralObject.SetValidValue(value, false)作用是什么？

2 如果数据库表里的字段是允许为空的，是不是不能将StructuralObject.SetValidValue(value, true)改为StructuralObject.SetValidValue(value, false)

望高手指点迷津，先谢过了！