Question

The following code does not compile:

//int a = ...
int? b = (int?) (a != 0 ? a : null);

In order to compile, it needs to be changed to

int? b = (a != 0 ? a : (int?) null);

Since both b = null and b = a are legal, this doesn’t make sense to me.

Why do we have to cast the null into an int? and why can’t we simply provide an explicit type cast for the whole expression (which I know is possible in other cases)?

Answer 1

From chapter 7.13 of the C# Language Specification:

The second and third operands of the ?: operator control the type of the conditional expression. Let X and Y be the types of the second and third operands. Then,

• If X and Y are the same type, then this is the type of the conditional expression.
• Otherwise, 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.
• Otherwise, 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.

In your case, there is no implicit conversion from int to null nor the other way around. Your cast solves the problem, int is convertible to int?