Dart项目运行报错：类型'num?'无法赋值给类型'num'的变量问题求助

Let's break down and fix the two key issues in your code:

1. The Type Mismatch Error #

num.tryParse() returns a nullable num? type—it gives null if the input can't be parsed as a valid number. You can't directly subtract this nullable value from DateTime.now().year (a non-nullable num), which is why you're seeing the type mismatch error.

2. The Incorrect Input Validation #

Your code checks if n == "", but n is a num?—it will never be an empty string. When parsing fails, tryParse returns null, not "". This means your error message will never trigger, even for invalid inputs.

Corrected Code #

Here's the fixed version with clear improvements:

import 'dart:io';

void main() {
  print("Enter your birth-year");
  // Capture raw input, default to empty string if input is null
  var input = stdin.readLineSync() ?? "";
  var birthYear = num.tryParse(input);

  // Check if parsing failed (birthYear is null)
  if (birthYear == null) {
    print("Bad Year");
    return; // Exit early to avoid running invalid age calculation
  }

  // Now birthYear is guaranteed non-null, so subtraction is safe
  var currentYear = DateTime.now().year;
  var age = currentYear - birthYear;
  print(" ==> You are $age year old!");
}

Key Improvements: #

• We first capture the raw input string separately, making the parsing logic clearer.
• We validate by checking if birthYear is null (the actual result of a failed parse) instead of comparing to an empty string.
• Added return after the error message to stop execution early—this prevents the code from trying to calculate age with an invalid value.
• Renamed variables to be more descriptive (birthYear instead of n) for better readability.

内容的提问来源于stack exchange，提问作者Ahmed Farag