So I've been following along to a fireship course on dart and I got the error: "Null check operator used on a null value". Was just wondering why? Here is the code:

String? answer;

String result = answer!;

CodePudding user response：

That's working exactly as intended.

The String? answer; declaration introduces a nullable variable with a current value of null.

The answer! expression reads that value, checks whether it's null, throws if the value is null, or evaluates to the non-null value if it isn't null. Since the value is indeed null, that operation throws the error you see.

The ! is the "null check operator". It's used on the value null. The error message is actually quite precise.

To avoid that, make sure the answer variable has a non-null value when you use the ! operator on it. (Often you won't need the ! for local variables because assigning a non-null value to the variable also promotes the variable's type to non-nullable, but there are cases where the compiler can't see that assignment, for example if it happens in a callback function.)