Convert between nullable int and int

问题

I would like to do something like this :

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

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 ''.

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

回答1:

You have to cast null as well:

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

By the way, that's one of the differences between the conditional operator and an if-else:

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

The relevant C# specification is 7.14 Conditional operator:

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

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.

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

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

int? l = (int?)lc;

回答2:

Simply cast your nullable enum value to nullable integer:

int? l = (int?)lc;

Due to C# specification 6.2.3 Explicit nullable conversions:

Evaluation of a nullable conversion based on an underlying conversion from S to T proceeds as follows if the nullable conversion is from S? to T?:

If the source value is null (HasValue property is false), the result is the null value of type T?.

Otherwise, the conversion is evaluated as an unwrapping from S? to S, followed by the underlying conversion from S to T, followed by a wrapping from T to T?.

回答3:

You should this:

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

来源：https://stackoverflow.com/questions/21041519/convert-between-nullable-int-and-int

标签

int

nullable

enumeration