Chapter 13 — Exceptions and Error Handling

Level: beginner / intermediate Estimated time: 5 h Prerequisites: Chapter 12 — Null safety What you will be able to do by the end: write a Dart program that never crashes abruptly on invalid data, by signaling problems with throw and catching them in the right place with try / on / catch / finally.

13.0 — Chapter Objectives

By the end of this chapter, you will be able to:

• explain, step by step, what happens when a Dart program crashes;
• distinguish an Error (a programmer's fault) from an Exception (a foreseeable accident);
• read a Dart VM error trace without panicking;
• protect a piece of code with try;
• catch a problem with catch;
• retrieve the thrown object with catch (e) and the call stack with catch (e, s);
• filter a specific exception type with on;
• chain several on clauses in the right order;
• combine on and catch with on ... catch (e);
• guarantee that cleanup code runs with finally;
• describe precisely the execution order of try, catch, and finally;
• signal a problem yourself with throw;
• propagate an already-caught exception with rethrow;
• create your own domain-specific exception classes for your game;
• understand and trigger a FormatException;
• explain why int.parse() fails and how its message is built;
• replace an exception with a check using int.tryParse() and double.tryParse();
• validate a complete user input, from raw text to a usable value;
• use ArgumentError and assert to protect your own functions;
• combine exceptions and null safety;
• decide, in a real project, when to catch an exception and when to let the program crash.