c# - Type of conditional expression cannot be determined because there is no implicit conversion between 'int' and <null> -

why not compile?

int? number = true ? 5 : null; 

type of conditional expression cannot determined because there no implicit conversion between 'int' , <null>

the spec (§7.14) says conditional expression b ? x : y, there 3 possibilities, either x , y both have type and good conditions met, 1 of x , y has type and good conditions met, or compile-time error occurs. here, "certain conditions" means conversions possible, details of below.

now, let's turn germane part of spec:

if 1 of x , y has type, , both x , y implicitly convertible type, type of conditional expression.

the issue here in

int? number = true ? 5 : null; 

only 1 of conditional results has type. here x int literal, , y null not have type and null not implicitly convertible int¹. therefore, "certain conditions" aren't met, , compile-time error occurs.

there are 2 ways around this:

int? number = true ? (int?)5 : null; 

here still in case 1 of x , y has type. note null still not have type yet compiler won't have problem because (int?)5 , null both implicitly convertible int? (§6.1.4 , §6.1.5).

the other way obviously:

int? number = true ? 5 : (int?)null; 

but have read different clause in spec understand why okay:

if x has type x , y has type y then

• if implicit conversion (§6.1) exists x y, not y x, y type of conditional expression.

• if implicit conversion (§6.1) exists y x, not x y, x type of conditional expression.

• otherwise, no expression type can determined, , compile-time error occurs.

here x of type int , y of type int?. there no implicit conversion int? int, there implicit conversion int int? type of expression int?.

¹: note further type of left-hand side ignored in determining type of conditional expression, common source of confusion here.