Question

I try to use the following code

double d;
double? a = double.TryParse("3.14", out d) ? d : null;

But it won’t compile as “there is no implicit conversion between double and null’. Splitting up the code above as follows works:

        double d;
        double? a;
        if ( double.TryParse("3.14", out d))
            a = d;
        else
            a = null;

How come there is a difference when using the ?-operator ?

Answer 1

The reason is, that you can’t assign null to a double and your ternary expression returns a double, not a double?. Because null doesn’t have an implicit type, the return type of your ternary expression is determined by the part, that has an implicit type, that is the part that returns d. As d is a double, your whole ternary expression evaluates to returning a double.

Fix it by casting either one of the returns to double?, e.g.

double d;
double? a = double.TryParse("3.14", out d) ? (double?)d : null;