为什么这个C＃代码没有编译？

Question

double? test = true ? null : 1.0;

In my book, this is the same as

if (true) {
  test = null;
} else {
  test = 1.0;
}

But the first line gives this compiler error:

Type of conditional expression cannot be determined because there is no implicit conversion between ' <null> ' and ' double '.

Answer 1

This happens because the compiler tries to evaluate the statement from right to left. This means that it sees 1.0 and it decides it is double (not double?) and then it sees null .

So there is clearly no implicit conversion between double and null (in fact there is no implicit conversion between Struct and null ).

What you can do is explicitly tell the compiler that one of the two expressions that are convertible to each other.

double? test = true ? null : (double?) 1.0;    // 1
double? test = true ? (double?)null : 1.0;     // 2
double? test = true ?  default(double?) : 1.0; // 3
double? test = true ? new double?() : 1.0;     // 4

Answer 2

double? test = true ? (double?)null : 1.0;

will work. That's because there is no conversion from the type of the first expression ( null ) to the type of the second expression ( double ).

Answer 3

The left hand side of the assignment is not used when deducing the type of an ?: expression.

In b ? A : B b ? A : B , the types of A and B must either be the same, or one must be implicitly convertible to the other.

Answer 4

Because the compiler can't figure out that for null and 1.0 to be compatible, the values need to be cast to double?. This needs to be explicitly stated.

double? test = true ? (double?) null : 1.0;