Скачать или смотреть Resolving the String Not Subtype of List String Error in Flutter

Learn how to fix the common error "type 'String' is not a subtype of type 'List String '" in Flutter when handling lists in Dart.
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This video is based on the question https://stackoverflow.com/q/63411501/ asked by the user 'Meggy' ( https://stackoverflow.com/u/1293894/ ) and on the answer https://stackoverflow.com/a/63411561/ provided by the user 'MickaelHrndz' ( https://stackoverflow.com/u/3925977/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the String Not Subtype of List<String> Error

As a Flutter developer, you may encounter various types of errors while coding. One such common issue is the error message: "type 'String' is not a subtype of type 'List String '." This error typically occurs when you are trying to assign a String value to a variable that expects a List<String>. Let's break down a scenario where this might happen and how to resolve it effectively.

The Scenario: Handling Tags in Flutter

Assume you have a widget that uses a variable, widget.tagname, to hold a string of comma-separated tags. For example, it could be something like:

[[See Video to Reveal this Text or Code Snippet]]

You aim to convert this String into a List<String> using the .split(',') method:

[[See Video to Reveal this Text or Code Snippet]]

However, if widget.tagname is null or if no tags are provided, it throws the frustrating error. This stems from the fact that if the tagname variable is null, the fallback value is a string (''), which cannot be assigned to a List<String>.

The Problem Explained

When the code encounters a situation where tagname is null or an empty string, Dart tries to assign '' (a String) to split, which is expected to be of type List<String>. Since a String cannot be treated as a List<String>, you get the error.

The Solution: Providing the Correct Fallback Value

To prevent this error, you need to ensure that your fallback value is indeed a List<String>. Instead of an empty string, you should return an empty list. This can be done by modifying your code as follows:

Revised Code Example

[[See Video to Reveal this Text or Code Snippet]]

In this updated code:

If widget.tagname is null, split will now default to an empty list []—an appropriate subtype of List<String>.

Summary

In summary, the key takeaway is to always ensure that your fallback values in Dart are of the expected type. When dealing with potential null values that could result in a type mismatch, returning an empty list instead of a string allows your application to handle cases gracefully without throwing runtime errors.

Final Thoughts

By updating your code as suggested, you should no longer encounter the "type 'String' is not a subtype of type 'List String '" error. Remember this solution as a best practice whenever you're working with variables that can potentially be null. Happy coding in Flutter!