Dart Fix Runtime Errors

Resolving Dart Static Analysis Errors

Contents

• Core Concepts & Guidelines
  • Type System & Soundness
  • Null Safety
  • Error Handling
• Workflows
  • Workflow: Static Analysis Resolution
• Examples

Core Concepts & Guidelines

Type System & Soundness

Enforce Dart's sound type system to prevent runtime invalid states.

• Method Overrides: Maintain sound return types (covariant) and parameter types (contravariant). Never tighten a parameter type in a subclass unless explicitly marked with the covariant keyword.
• Generics & Collections: Add explicit type annotations to generic classes (e.g., List<T>, Map<K, V>). Never assign a List<dynamic> to a typed list (e.g., List<Cat>).
• Downcasting: Avoid implicit downcasts from dynamic. Use explicit casts (e.g., as List<Cat>) when necessary, but ensure the underlying runtime type matches to prevent TypeError exceptions.
• Strict Casts: Enable strict-casts: true in analysis_options.yaml under analyzer: language: to force explicit casting and catch implicit downcast errors at compile time.

Null Safety

Eliminate static errors related to null safety by correctly managing variable initialization and nullability.

• Modifiers: Apply ? for nullable types, ! for null assertions, and required for named parameters that cannot be null.
• Late Initialization: Use the late keyword for non-nullable variables guaranteed to be initialized before use. Apply this specifically to top-level or instance variables where Dart's control flow analysis cannot definitively prove initialization.
• Wildcards: Use the _ wildcard variable (Dart 3.7+) for non-binding local variables or parameters to avoid unused variable warnings.

Error Handling

Distinguish between recoverable exceptions and unrecoverable errors.

• Catching: Catch Exception subtypes for recoverable failures.
• Errors: Never explicitly catch Error or its subtypes (e.g., TypeError, ArgumentError). Errors indicate programming bugs that must be fixed, not caught. Enforce this by enabling the avoid_catching_errors linter rule.
• Rethrowing: Use rethrow inside a catch block to propagate an exception while preserving its original stack trace.

Workflows

Workflow: Static Analysis Resolution

Use this sequential workflow to identify, fix, and verify static analysis errors in a Dart project. Copy the checklist to track your progress.

Task Progress:

• 1. Run static analyzer.
• 2. Apply automated fixes.
• 3. Resolve remaining errors manually.
• 4. Verify fixes (Feedback Loop).

1. Run static analyzer Execute the Dart analyzer to identify all static errors in the target directory or file.

dart analyze . --fatal-infos

2. Apply automated fixes Use the dart fix tool to automatically resolve standard linting and analysis issues.

# Preview changes
dart fix --dry-run
# Apply changes
dart fix --apply

3. Resolve remaining errors manually Review the remaining analyzer output and apply conditional logic based on the error type:

• If the error is a Null Safety issue (e.g., "Property cannot be accessed on a nullable receiver"):
  • Verify if the variable can logically be null.
  • If yes, use optional chaining (?.) or provide a fallback (??).
  • If no, and initialization is guaranteed elsewhere, mark the declaration with late.
• If the error is a Type Mismatch (e.g., "The argument type 'List' can't be assigned..."):
  • Trace the variable's initialization.
  • Add explicit generic type annotations to the instantiation (e.g., <int>[] instead of []).
• If the error is an Invalid Override (e.g., "The parameter type doesn't match the overridden method"):
  • Widen the parameter type to match the superclass, OR
  • Add the covariant keyword to the parameter if tightening the type is intentionally required by the domain logic.

4. Verify fixes (Feedback Loop) Run the validator. Review errors. Fix.

dart analyze .
dart test

• If dart analyze reports errors: Return to Step 3.
• If dart test fails with a TypeError: You have introduced an invalid explicit cast (as T) or accessed an uninitialized late variable. Locate the runtime failure and correct the type hierarchy or initialization order.

Examples

Example: Fixing Dynamic List Assignments

Input (Fails Static Analysis):

void printInts(List<int> a) => print(a);

void main() {
  final list = []; // Inferred as List<dynamic>
  list.add(1);
  list.add(2);
  printInts(list); // Error: List<dynamic> can't be assigned to List<int>
}

Output (Passes Static Analysis):

void printInts(List<int> a) => print(a);

void main() {
  final list = <int>[]; // Explicitly typed
  list.add(1);
  list.add(2);
  printInts(list);
}

Example: Fixing Method Overrides (Contravariance)

Input (Fails Static Analysis):

class Animal {
  void chase(Animal a) {}
}

class Cat extends Animal {
  @override
  void chase(Mouse a) {} // Error: Tightening parameter type
}

Output (Passes Static Analysis):

class Animal {
  void chase(Animal a) {}
}

class Cat extends Animal {
  @override
  void chase(covariant Mouse a) {} // Explicitly marked covariant
}

Example: Fixing Null Safety with late

Input (Fails Static Analysis):

class Thermometer {
  String temperature; // Error: Non-nullable instance field must be initialized

  void read() {
    temperature = '20C';
  }
}

Output (Passes Static Analysis):

class Thermometer {
  late String temperature; // Defers initialization check to runtime

  void read() {
    temperature = '20C';
  }
}