Why must the types match in a ?: ternary conditional operator

Question

If i try to do this:

// As an example. char? c value is actually given by the user.
char? c = null;

WriteLine((c == null) ? "null" : c);

Where i am saying that if c equals null (c == null) then WriteLine() should output null to the console. Else it should output the value of c.

However i get this compiler error:

Type of conditional expression cannot be determined because there is no implicit conversion between 'string' and char?.

My workaround was to do this:

char? c = null;            
WriteLine((c == null) ? "null" : "{0}",c);

Or with C#6 and using String Interpolation

char? c = null;            
WriteLine((c == null) ? "null" : $"{c}");

Reading through similar compiler errors here in stackoverflow e.g.

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

or

Type of conditional expression cannot be determined because there is no implicit conversion between 'string' and 'System.DBNull'

Tells that the reason is the types must match. My question is why must the types match? Also since the error says there is no implicit conversion between string and char?, does that mean that there is an explicit conversion of string to char? (which obviously by logic can't be tha case here) or does it mean that there is from char? to string ?

Answer 1

The compiler has to know what the type of the expression is. A character, and a pointer to character, are not the same thing.

It's obvious to you what you mean, but the compiler has to assign a type to every expression, so it will know which version of the WriteLine subroutine will be called. It cannot postpone that decision to run time - it has to decide it at compile time, so it has to know what type the ternary expression is.