私人int？ _City_Id; 在不知道你的目标语言回应的情况下,在 C# 2.0中？表示可空值的类型. Nullable value types (denoted by a question mark, e.g. int? i = null;) which add null to the set of allowed values for any val
Nullable value types (denoted by a question mark, e.g. int? i = null;) which add null to the set of allowed values for any value type.
正如Calum指出的那样(对他而言都是信用),意味着可以将变量赋值为null.通常像int和double这样的原语不能为null,
int? x = 10; double? d = 4.108