错误:参数类型为“字符串?"无法分配给参数类型“字符串"，因为“字符串?"为可空值，而'String'不是
美好的一天，我一直在尝试下面的代码:
Good day, I have been trying the below code:
import 'dart:io';
main (){
print ("write your birth year:");
var birthyear = stdin.readLineSync();
var birthyearint = int.parse(birthyear);
var age = 2021-birthyearint;
print(age);
}
当我运行它时，出现以下错误:
when I run it I receive the following error:
5:30:错误:参数类型为字符串?"无法分配给参数类型字符串"，因为字符串?"是可以为空的，而字符串"不是.var birthyearint = int.parse(birthyear);^
5:30: Error: The argument type 'String?' can't be assigned to the parameter type 'String' because 'String?' is nullable and 'String' isn't. var birthyearint = int.parse(birthyear); ^
The error is caused by the null safety feature in Dart, see https://dart.dev/null-safety.
方法 stdin.readLineSync()的结果为 String?，即它可以是 String ，也可以是空.方法 int.parse()需要一个(非空) String .您应该检查用户是否提供了一些输入，然后可以使用 birthyear！断言该值不为空.更好的是，您应该使用 int.tryParse()来检查用户输入是否为有效整数，例如:
The result of method stdin.readLineSync() is String?, i.e. it may be a String, or it may be null. The method int.parse() requires a (non-null) String. You should check that the user gave some input, then you can assert that the value is non-null using birthyear!. Better still, you should use int.tryParse() to check that the user input is a valid integer, for example:
var birthyearint = int.tryParse(birthyear ?? "");
if (birthyearint == null) {
print("bad year");
} else {
var age = 2021-birthyearint;
print(age);
}