The Ultimate Guide to Migrating to Null-Safety: Resolving the “Expected a value of type List<ClassroomRoutineModel>, but got one of type List <dynamic>” Error

Are you tired of dealing with the frustrating “Expected a value of type List<ClassroomRoutineModel>, but got one of type List <dynamic>” error when trying to migrate to null-safety in Dart? You’re not alone! In this comprehensive guide, we’ll take you by the hand and walk you through the steps to resolve this pesky issue once and for all.

What is Null-Safety?

Before we dive into the solution, let’s quickly cover what null-safety is and why it’s essential in Dart.

Null-safety is a feature in Dart that ensures your code is free from null pointer exceptions. It does this by introducing a concept called “sound null safety,” which guarantees that a variable can never hold a null value unless you explicitly declare it as nullable.

In Dart, a nullable type is denoted by a question mark (?) after the type name, like this: `String? myString`. This tells the compiler that `myString` can hold either a string value or null.

The Problem: “Expected a value of type List<ClassroomRoutineModel>, but got one of type List <dynamic>”

So, what’s the issue here? When you try to assign a list of dynamic objects to a variable declared as `List`, the compiler throws an error. This is because the type system in Dart is designed to prevent null pointer exceptions, and it’s complaining that the types don’t match.

The error message is telling you that it expected a list of `ClassroomRoutineModel` objects, but instead, it got a list of dynamic objects. This is because the `dynamic` type is the default type in Dart when no type is specified.

The Solution: Understanding Type Casting

To resolve this issue, we need to understand type casting in Dart. Type casting is the process of converting one data type to another. In our case, we need to cast the dynamic list to a list of `ClassroomRoutineModel` objects.

There are two types of casting in Dart: implicit and explicit casting.

Implicit Casting

Implicit casting occurs when Dart infers the type of a variable based on the assigned value. For example:

var myList = [ClassroomRoutineModel(), ClassroomRoutineModel()];

In this case, Dart infers that `myList` is a list of `ClassroomRoutineModel` objects.

Explicit Casting

Explicit casting, on the other hand, involves using the `as` keyword to explicitly cast a value to a specific type. For example:

var myList = [ClassroomRoutineModel(), ClassroomRoutineModel()];
List<ClassroomRoutineModel> castedList = myList as List<ClassroomRoutineModel>;

In this case, we’re explicitly casting `myList` to a list of `ClassroomRoutineModel` objects using the `as` keyword.

Resolving the Error: Step-by-Step Guide

Now that we understand type casting, let’s walk through the steps to resolve the error:

1. Identify the problematic code:

   List<ClassroomRoutineModel> myList = [
     {'id': 1, 'name': 'Routine 1'},
     {'id': 2, 'name': 'Routine 2'},
   ];

2. Notice that the list is declared as `List`, but the elements are dynamic objects (maps).

3. Cast the dynamic objects to `ClassroomRoutineModel` objects using a factory method or a constructor:

   List<ClassroomRoutineModel> myList = [
     ClassroomRoutineModel(id: 1, name: 'Routine 1'),
     ClassroomRoutineModel(id: 2, name: 'Routine 2'),
   ];

4. If you’re using a factory method, ensure that it returns a `ClassroomRoutineModel` object:

   factory ClassroomRoutineModel.fromJson(Map<String, dynamic> json) {
     return ClassroomRoutineModel(
       id: json['id'],
       name: json['name'],
     );
   }

5. If you’re using an existing list of dynamic objects, cast it to a list of `ClassroomRoutineModel` objects using explicit casting:

   List<dynamic> dynamicList = [...];
   List<ClassroomRoutineModel> castedList = dynamicList.map((e) => e as ClassroomRoutineModel).toList();

Best Practices for Null-Safety

Now that we’ve resolved the error, let’s discuss some best practices for null-safety in Dart:

• Avoid using the `dynamic` type whenever possible. Instead, use the `Object` type or the specific type you expect.

• Use the `?` operator to denote nullable types.

• Use the `late` keyword to declare variables that will be initialized later.

• Avoid using `!` operator to force-unwrap nullable values. Instead, use the `??` operator to provide a default value.

Bad Practice	Good Practice
String myString = dynamicValue as String;	String? myString = dynamicValue as String?;
List<String> myList = [null, 'hello'];	List<String?> myList = [null, 'hello'];
String myString = nullableValue!;	String myString = nullableValue ?? 'default value';

Conclusion

Migrating to null-safety in Dart can be a challenging task, but with the right guidance, it’s a breeze. By understanding type casting and following best practices, you can ensure that your code is null-safe and error-free. Remember to avoid using the `dynamic` type, use the `?` operator to denote nullable types, and cast your values explicitly. Happy coding!

Still struggling with null-safety? Check out the official Dart documentation and null-safety guides for more resources.

Frequently Asked Question

Migrating to null-safety can be a daunting task, but don’t worry, we’ve got you covered! Here are some frequently asked questions to help you navigate the process.