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.

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dart analyze . --fatal-infos

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

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# 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.

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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):

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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):

dartCopy code
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):

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

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

Output (Passes Static Analysis):

dartCopy code
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):

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

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

Output (Passes Static Analysis):

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

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