私人int？ _City_Id;任何人都可以告诉我“？”在这里意味着什么

Question

private int? _City_Id;

Answer 1

Without knowing your target language to respond with, in C# 2.0 the ? denotes nullable value types.

Nullable value types (denoted by a question mark, eg int? i = null;) which add null to the set of allowed values for any value type.

Which, as Calum points out ( all credit to him ), means that the variable can be assigned null . Normally primitives like int and double can't be null,

int? x = 10;
double? d = 4.108