Troubleshooting the Error: ‘The class ‘PreferredSizeWidget’ cannot be used as a mixin’ in Flutter


During the development process, encountering errors is a common occurrence. Upgrading to a new version of a framework or programming language can sometimes introduce unexpected issues. In this blog post, we will delve into a specific error that arises when attempting to use the ‘PreferredSizeWidget’ class as a mixin in Flutter. Furthermore, we will explore a solution to overcome this error.

The Error:

Following the upgrade to Flutter 3.10 and Dart 3, several developers have reported an error message similar to the following:

“The class ‘PreferredSizeWidget’ cannot be used as a mixin because it is neither a mixin class nor a mixin.”

This error typically arises when trying to mix in the ‘PreferredSizeWidget’ class into another class using the ‘with’ keyword.

Resolving the Error:

To rectify this error, a simple modification needs to be made to the code. Instead of using the ‘with’ keyword, we will employ the ‘implements’ keyword. Let’s examine the adjusted code snippet:

class MyAppBar extends StatelessWidget implements PreferredSizeWidget {

By implementing the ‘PreferredSizeWidget’ interface, we can attain the desired functionality without encountering the error. This adjustment adheres to the stricter rules governing mixins in Dart 3.


In prior versions of Dart, any class could be used as a mixin provided it had no declared constructors and its superclass was exclusively ‘Object.’ However, Dart 3 introduced more stringent rules regarding mixins. Starting from Dart 3, classes declared in libraries with language version 3.0 or later cannot be used as mixins unless explicitly marked with the ‘mixin’ keyword.

The ‘PreferredSizeWidget’ class lacks the ‘mixin’ keyword in its definition and is not explicitly declared as a mixin. Consequently, using it as a mixin in the original code triggers the aforementioned error message.


If you encounter the error message “‘The class ‘PreferredSizeWidget’ cannot be used as a mixin” in Flutter, the solution involves modifying the code to implement the ‘PreferredSizeWidget’ interface instead of using it as a mixin. By making this simple adjustment, you can continue developing your Flutter application without encountering this error. It is essential to stay up-to-date with the latest changes and guidelines in programming languages and frameworks to ensure seamless development experiences.

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