Why doesn't this C# code compile?

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

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 2

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

Answer 3

Because null is not, actually, implicitly convertable to a double?. Funny as that sounds.

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

Answer 4

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

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