How to Handle NullReferenceException in Visual Basic.Net When Closing Stream Objects

Learn how to efficiently manage file streams in Visual Basic.Net and avoid the common `NullReferenceException` error during cleanup.
---

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding the Scope Issue with Stream Objects in Visual Basic.Net

In programming, particularly in Visual Basic.Net, managing file input and output efficiently can sometimes lead to unexpected errors. One such error you'll encounter is the System.NullReferenceException, which can be particularly frustrating when dealing with stream objects. In this post, we will investigate a common scenario—a coding example that leads to this specific error—and provide a clear solution to fix it.

The Issue: System.NullReferenceException

In the given example, the programmer attempts to work with a StreamWriter object to write to a text file. However, when an exception occurs while creating the StreamWriter, the object reference is never initialized. As a result, when the code attempts to call Close() on the myFileStreamWriter variable in the Finally block, it throws a NullReferenceException because myFileStreamWriter is still Nothing (or null).

Example Code Snippet

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

The problem becomes evident as the Close() method is called both in the Try block and in the Finally block. If there's an exception, the reference to the file stream is not created, leading to a situation where one attempts to close something that does not exist.

Solution: Properly Managing Stream Objects

To resolve the issue above, we need to restructure our code to ensure that we correctly manage the lifecycle of the StreamWriter object. Here, we outline a couple of recommended approaches.

1. Use the Using Statement

One efficient way to handle resource management is by using the Using statement. This ensures that the stream object is disposed of properly without needing to manually call Close(). Here’s how you can modify the code:

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

2. Eliminate Manual StreamWriter Creation

Another approach replaces the manual creation of a StreamWriter. You can simplify the code even further by using the static method File.AppendAllText, which handles writing to a file in a more straightforward manner:

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

Best Practices in Exception Handling

Exceptions can occur for a myriad of reasons, and it's essential to be mindful of how we handle them. Here are some best practices to keep in mind when coding in Visual Basic.Net:

Avoid Catching General Exceptions: Catch specific exceptions that your code can reasonably handle. This helps in debugging and understanding where issues may arise.

Log Exceptions: Always consider logging exceptions for diagnostic purposes, enabling you to track issues over time.

Graceful Handling of Unhandled Exceptions: Consider implementing an UnhandledException event to manage any unexpected failures in your application gracefully.

Conclusion

In this post, we've analyzed how NullReferenceException could arise in your Visual Basic.Net code while handling file streams. By utilizing the Using statement or static methods like File.AppendAllText, you can avoid these common pitfalls and write cleaner, more maintainable code. Happy coding!