可空int和INT之间的转换 [英] Convert between nullable int and int

问题描述

我愿做这样的事情：

int? l = lc.HasValue ? (int)lc.Value : null; 

，其中LC是可空枚举类型，说EMyEnumeration？所以我想测试是否LC有一个值，所以如果再给予其int值升，否则l为null。但我这样做的时候，C＃抱怨道：条件表达式的错误类型不能被确定为之间不存在隐式转换'廉政'和''。

where lc is a nullable enumeration type, say EMyEnumeration?. So I want to test if lc has a value, so if then give its int value to l, otherwise l is null. But when I do this, C# complains that 'Error type of conditional expression cannot be determined as there is no implicit conversion between 'int' and ''.

我怎样才能使它正确吗？在此先感谢！

How can I make it correct? Thanks in advance!!

推荐答案

您已经投空，以及

int? l = lc.HasValue ? (int)lc.Value : (int?)null; 

顺便说一句，这是的的条件运算符和的if-else ：

if (lc.HasValue)
    l = (int)lc.Value;
else
    l = null;  // works

有关C＃规范的 7.14条件运算符

第二个和第三个操作数，x和y的？：运营商控制条件表达式的类型。

The second and third operands, x and y, of the ?: operator control the type of the conditional expression.

• 如果x具有X型和Y具有Y型则
  
  
  
  • 如果隐式转换（第6.1节）从X到Y存在，但不能从Y到X，那么Y就是条件表达式的类型。
  
  
  • 如果（第6.1节）从Y到存在的隐式转换X，但不能从X到Y，则X就是条件表达式的类型。
  
  
  • 否则，毫无表情的类型可确定和编制─发生错误。
  
  
  • If x has type X and y has type Y then
    • If an implicit conversion (§6.1) exists from X to Y, but not from Y to X, then Y is the type of the conditional expression.
    • If an implicit conversion (§6.1) exists from Y to X, but not from X to Y, then X is the type of the conditional expression.
    • Otherwise, no expression type can be determined, and a compile-time error occurs.

    由于 INT 是无法转换为空含蓄，反之亦然，你得到一个编译错误。但你也可以施放枚举到 INT？而不是 INT 其中是的敞篷车。那么编译器可以推导出类型，它就会编译：

    Since int is not convertible to null implicitly and vice-versa you get a compiler error. But you could also cast the enum to int? instead of int which is convertible. Then the compiler can derive the type and it'll compile:

    int? l = lc.HasValue ? (int?)lc.Value : null; // works also
    

    或像谢尔盖在他的answer 直接：

    or, as Sergey has mentioned in his answer, directly:

    int? l = (int?)lc;
    

    这篇关于可空int和INT之间的转换的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！