The argument type 'double?' can't be assigned to the parameter type 'num', even if I check if the value is not null

Question

I have this function:

double discount() {
    return cost_min_discounted != null ? cost_min - cost_min_discounted : 0;
}

cost_min_discounted is defined as double?
I get the error:

The argument type 'double?' can't be assigned to the parameter type 'num'

What's the best way to write code like that? I check for the null value, so the code seems correct to me.

score 1 · Accepted Answer · answered May 28 '22 at 14:34

1

While you've already checking null at beginning, use ! at the end.

double discount() {
  return cost_min_discounted != null ? cost_min - cost_min_discounted! : 0;
}

answered May 28 '22 at 14:34

Md. Yeasin Sheikh

1

Since it is a class field it can theoretically change between your check and the usage. In a multithreaded application this can be a really hard bug to track down. Assigning to a local variable first guarantees it will never be null when used. – Dan Harms May 28 '22 at 14:50
What about `return cost_min - (cost_min_discounted ?? 0)` – Giacomo M May 28 '22 at 15:13
@DanHarms as it already checked the null value, I can't see any possibility to get it null and throw the error, It will return the right expression on null cases. – Md. Yeasin Sheikh May 28 '22 at 15:44
@GiacomoM yes it will return `cost_min` if `cost_min_discounted` becomes null. – Md. Yeasin Sheikh May 28 '22 at 15:46
Another thread can change the value. There's a reason you have to add the bang to the use because it's not guaranteed non-null there. – Dan Harms May 28 '22 at 16:26

1 Answers1