Question

Can someone please explain me the logic reason why I must cast null to int?

When the left argument type can have their both types ?

Insted of doing

  int? k = (DateTime.Now.Ticks%5 > 3 ? 1 : null);

I must do

  int? k = (DateTime.Now.Ticks%5 > 3 ? 1 : (int?) null);

although int? k = null is perfectly valid.

An opposite example :

I didn’t have to do it in:

string k = (DateTime.Now.Ticks%5 > 3 ? "lala" : null);

Answer 1

int? k = (DateTime.Now.Ticks%5 > 3 ? 1 : (int?) null);

In this case what we have is 1 is int and null is actually null
Now the confusion is the ternary operator is confused as what is the return type int or well null and since its int it won’t accept null

So you would need to cast it to a nullable int

Now in the other case you have a string and null is perfectly acceptable for a string

Further explanation can be found at Type inference – Eric

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 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 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.