How to Throw and Handle Custom Exceptions in Dart Using try and on

Learn how to effectively `throw` a custom `FormatException` in Dart and handle it using `try` and `on` with practical examples.
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Understanding Custom Exceptions in Dart: FormatException

In programming, handling errors gracefully is crucial for creating robust applications. In the Dart language, developers can define and manage their own exceptions, allowing them to better control the flow of their applications when unexpected issues arise. One common scenario is throwing a custom exception, such as a FormatException. In this guide, we will explore how to throw and handle a FormatException using the try and on keywords in Dart.

The Challenge: Throwing a Custom Exception

You might have encountered a situation where you tried to throw a custom FormatException using the following code:

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

However, the output of your code doesn't seem to handle the exception as expected, and you've noticed that using catch(e) worked fine instead. This raises a critical question: Why isn't the on FormatException block catching the exception?

Dissecting the Problem

To understand what went wrong, we need to look closely at how Dart handles throwing and catching exceptions.

Throwing vs. Catching Exceptions

Throwing an Exception: When you use the throw keyword to create an exception, you are expected to throw an instance of the exception class. In your code, you used throw FormatException, which is incorrect because it merely refers to the type itself and does not instantiate it.

Catching an Exception: The on statement is used to catch specific instances of exceptions. Your current use of on FormatException is looking for an existing instance to handle.

The Solution: Correctly Throwing a FormatException

To properly throw a FormatException, you should use the constructor of FormatException which instantiates the exception. Here’s how you can do that:

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

Key Takeaways

Instantiate Exceptions: Always create an instance of the exception to throw, for example, throw FormatException('Your custom message here').

Using Catch with On: You can handle specific errors more comprehensively by combining on with catch, giving you access to the details of the exception.

Error Messages: This approach can help provide users with meaningful feedback when they make errors, enhancing user experience.

Conclusion

Throwing and catching exceptions correctly in Dart is essential for building applications that handle errors gracefully. By properly instantiating exceptions and understanding the relationship between throwing and catching, you can create a more resilient codebase. Understanding these concepts will not only help you handle FormatExceptions effectively but also prepare you to manage other types of exceptions in Dart. Remember, it's all about catching the right instance at the right time!

Keep experimenting with your Dart code, and don't hesitate to throw and handle exceptions effectively!