Decoding the "_InternalLinkedHashMap<String, dynamic>" Error: A Guide to Type Safety in Dart
Have you encountered the error "type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'Map<String, String>' in type cast"? This frustrating message often pops up during Dart development, especially when working with maps. This article will guide you through understanding the error and how to address it effectively.
Understanding the Problem
The error message tells us that Dart cannot convert a map of type _InternalLinkedHashMap<String, dynamic> to a map of type Map<String, String>. Let's break it down:
_InternalLinkedHashMap<String, dynamic>: This signifies a specific implementation of a map in Dart where keys areStrings, but values can be of any type (dynamic).Map<String, String>: This represents a map where both keys and values are strictlyStrings.
The error arises because Dart enforces type safety. This means that variables and data structures can only hold values of their defined type. Trying to cast a map containing values of any type (dynamic) to a map requiring only String values results in a type mismatch.
Scenario and Code Example
Let's imagine a scenario where you're fetching data from an API and storing it in a map. The API response might contain both strings and numbers.
import 'dart:convert';
void main() {
String jsonData = '{"name": "Alice", "age": 30}';
Map<String, dynamic> data = jsonDecode(jsonData);
// Attempting to cast 'data' to Map<String, String>
Map<String, String> userData = data as Map<String, String>;
// Error: type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'Map<String, String>' in type cast
}
In this example, jsonDecode creates a map (data) with keys of type String and values of type dynamic. However, when attempting to cast it to Map<String, String>, the error occurs because the age value (30) is an integer, not a string.
Solutions
There are multiple ways to address this type mismatch:
- Explicit type casting: While less ideal, you can cast individual values to
Stringbefore using them.
String name = data['name'] as String;
String age = data['age'].toString(); // Convert int to String
- Type checking: Before casting, you can verify if all values are indeed strings using
is.
if (data.values.every((value) => value is String)) {
Map<String, String> userData = data as Map<String, String>;
} else {
// Handle cases where values are not all strings
}
- Type-aware map creation: During JSON decoding, you can specify the expected type of the values.
Map<String, String> userData = jsonDecode(jsonData, (key, value) => value is String ? value : value.toString());
- Define custom data structures: If you're dealing with complex data, consider defining custom data classes to hold your data in a structured way.
class User {
final String name;
final int age;
User({required this.name, required this.age});
}
User user = User(name: data['name'], age: data['age']);
Best Practices
- Understand the data structure: Always be aware of the type of data you're working with, whether it's fetched from an API, file, or database.
- Use type safety: Leverage Dart's type system to ensure your code is robust and avoids unexpected errors.
- Choose appropriate data structures: Select the right data structure for the task.
- Document your code: Clear comments and documentation help you and others understand the intended data types.
Additional Resources
- Dart Documentation: https://dart.dev/
- Flutter Documentation: https://flutter.dev/
By understanding type safety and using the appropriate techniques, you can avoid "type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'Map<String, String>' in type cast" and write robust, maintainable Dart code.
