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Dart / Errors and Async

Custom exceptions

Learn custom exceptions in Dart through a focused explanation, a dedicated topic-specific example, an engineering boundary, and a mastery check.

12 min read • Advanced

custom exceptions is an atomic Dart topic inside Errors and Async. The example uses the named construct directly so you can see its syntax, data flow, and observable behavior.

Read the code by locating the exact feature named custom exceptions, then trace the value before and after that feature runs. This lesson is intentionally isolated from neighboring topics: changing identifiers alone would not reproduce the same behavior because the construct itself is different.

Topic-specific example

dart-0060-335cd46a4b.dart
dart Try It Live 
class PaymentException implements Exception{final String message;PaymentException(this.message);String toString()=>message;}void main(){try{throw PaymentException('declined');}catch(e){print(e);}}
Edge case / engineering note

For custom exceptions, test the boundary that makes the feature meaningful: invalid input, an empty value, a missing branch, lifecycle/cleanup, or a type/runtime constraint. Do not infer correctness only from the happy path.

Content source & attribution
Original Syntaxis Content · Original Syntaxis Content

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