Dart Exception Handling
Handle errors gracefully in your Dart applications
🛡️ What is Exception Handling?
Exception handling helps your program deal with unexpected errors gracefully instead of crashing. Use try-catch blocks to catch and handle exceptions that might occur during program execution.
void main() {
try {
int result = 10 ~/ 0; // Division by zero
print(result);
} catch (e) {
print('Error caught: $e');
}
print('Program continues...');
}
Output:
Error caught: IntegerDivisionByZeroException
Program continues...
Exception Handling Concepts
Try-Catch
Basic exception handling structure
try {
// risky code
} catch (e) {
// handle error
}Finally Block
Code that always executes
try {
// code
} finally {
// cleanup
}Throw Exception
Create custom exceptions
if (age < 0) {
throw Exception('Invalid age');
}Specific Catches
Handle different exception types
} on FormatException {
// handle format error
}🔹 Basic Try-Catch
Handle exceptions with try-catch blocks:
void main() {
// Example 1: Division by zero
try {
int a = 10;
int b = 0;
int result = a ~/ b;
print('Result: $result');
} catch (e) {
print('Caught exception: $e');
}
// Example 2: Invalid string to number conversion
try {
String text = 'abc';
int number = int.parse(text);
print('Number: $number');
} catch (e) {
print('Cannot convert "$text" to number: $e');
}
print('Program finished successfully!');
}Output:
Caught exception: IntegerDivisionByZeroException
Cannot convert "abc" to number: FormatException: Invalid radix-10 number (at character 1)
Program finished successfully!
🔹 Specific Exception Types
Catch specific types of exceptions:
void processData(String data) {
try {
// Try to parse as number
int number = int.parse(data);
// Try to access list element
List<int> numbers = [1, 2, 3];
print('Value at index $number: ${numbers[number]}');
} on FormatException {
print('Error: "$data" is not a valid number');
} on RangeError {
print('Error: Index out of range');
} catch (e) {
print('Unexpected error: $e');
}
}
void main() {
processData('abc'); // Format exception
processData('5'); // Range exception
processData('1'); // Success
}Output:
Error: "abc" is not a valid number
Error: Index out of range
Value at index 1: 2
🔹 Finally Block
Code that runs regardless of exceptions:
void readFile(String filename) {
print('Opening file: $filename');
try {
if (filename.isEmpty) {
throw Exception('Filename cannot be empty');
}
if (filename == 'missing.txt') {
throw Exception('File not found');
}
print('Reading file content...');
print('File read successfully!');
} catch (e) {
print('Error reading file: $e');
} finally {
print('Closing file resources...');
print('Cleanup completed.');
}
}
void main() {
readFile('data.txt'); // Success case
print('---');
readFile('missing.txt'); // Error case
print('---');
readFile(''); // Another error case
}Output:
Opening file: data.txt
Reading file content...
File read successfully!
Closing file resources...
Cleanup completed.
---
Opening file: missing.txt
Error reading file: Exception: File not found
Closing file resources...
Cleanup completed.
---
Opening file:
Error reading file: Exception: Filename cannot be empty
Closing file resources...
Cleanup completed.
🔹 Throwing Custom Exceptions
Create and throw your own exceptions:
// Custom exception class
class InvalidAgeException implements Exception {
final String message;
InvalidAgeException(this.message);
@override
String toString() => 'InvalidAgeException: $message';
}
class Person {
String name;
int age;
Person(this.name, this.age) {
if (age < 0) {
throw InvalidAgeException('Age cannot be negative: $age');
}
if (age > 150) {
throw InvalidAgeException('Age seems unrealistic: $age');
}
}
@override
String toString() => '$name (age: $age)';
}
void main() {
try {
var person1 = Person('Alice', 25);
print('Created: $person1');
var person2 = Person('Bob', -5); // This will throw
print('Created: $person2');
} on InvalidAgeException catch (e) {
print('Custom exception caught: $e');
} catch (e) {
print('Other exception: $e');
}
try {
var person3 = Person('Charlie', 200); // This will also throw
} catch (e) {
print('Caught: $e');
}
}Output:
Created: Alice (age: 25)
Custom exception caught: InvalidAgeException: Age cannot be negative: -5
Caught: InvalidAgeException: Age seems unrealistic: 200