Error: A value of type 'num?' can't be assigned to a variable of type 'num' because 'num? - flutter

Question

I found this error while running the dart project
Error: A value of type 'num?' can't be assigned to a variable of type 'num' because 'num?'
import 'dart:io';
void main() {
print("Enter your birth-year");
var n = num.tryParse(stdin.readLineSync() ?? "");
if(n=="") {
print("Bad Year");
}
var age = DateTime.now().year-n;
print(" ==> You are $age year old!");
}

Answer 1

As you can read in the documentation of num.tryParse, the method has an return type of num?. This implies that your variable n is also of the type num? (Nullable num).
So the error points you to the line, where you want to subtract an nullable number from DateTime.now().year, which is not allowed.
You can workaround this limitation by using the ?? (Null coalescing) operator and checking for null and if your n is null subtract 0.
var age = DateTime.now().year - (n ?? 0) ;
Another solution is, just using an if checking for null followed by an else block, like:
if(n == null) {
print("Bad Year");
}
else {
var age = DateTime.now(). year - n ;
print(" ==> You are $age year old!");
}
Taking up your comment:
i added ! after n and works fine... that is the same solution?
Adding the null-assertion operator (!) to the n in DateTime.now().year - n! is not the same solution, but it is a possible solution. And as you already mentioned it compiles and work.
But be careful:
You are telling the compiler that you are sure, that n will never be null.
With your current code you will get an runtime error, when the user enters a letter or nothing. Then the parsing of that string fails and your n is null. And now you want to subtract null from the DateTime.now().year, which is again not allowed.
You can read more about Null-Safety on https://dart.dev/codelabs/null-safety .
Side note:
The following check is not correct:
if(n=="") {
print("Bad Year");
}
If an incorrect number was entered, your n is null and this is not equal to an empty string. To get your if clause work, simply use a null check. I also recommend to add a return after the output, otherwise your person gets quite old.
if(n == null) {
print("Bad Year");
return;
}