Solving the NullReferenceException in C# with Dictionaries and Custom Classes

Learn how to fix the `NullReferenceException` in C- when using dictionaries in custom classes. This guide provides step-by-step solutions to problems in Xamarin Forms and UWP.
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Understanding the NullReferenceException in C- with Dictionaries

When developing applications in C-, you might encounter various exceptions, and among them, the NullReferenceException can be particularly frustrating. This guide addresses a common issue faced by developers using custom classes with dictionaries, specifically within Xamarin Forms and UWP environments. Our case centers around two custom classes named Smaller and Bigger, where the Bigger class contains a dictionary that maps strings to instances of the Smaller class.

The Problem

In the provided scenario, you have defined two classes: Smaller, which contains several instance variables (strings), and Bigger, which includes a dictionary meant to hold Smaller instances. You tried to implement an indexer in the Bigger class:

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

However, when you attempted to access elements via the indexer, the application throws a NullReferenceException, indicating that something is not initialized properly.

Why Does This Happen?

The core issue stems from the fact that the private dictionary _Info is not initialized upon the creation of a Bigger object. When you try to access it through the indexer, _Info is still null, resulting in the exception.

The Solution

Step 1: Initialize the Dictionary

To solve this problem, you need to ensure that the _Info dictionary is properly initialized before trying to access it. You can do this by initializing it directly when you declare it:

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This line of code ensures that _Info is an empty dictionary as soon as an instance of the Bigger class is created.

Step 2: Revised Code Example

Here’s how your Bigger class would look after the adjustment:

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

Step 3: Optional Improvements

Although it might be tempting to keep both private and public instances of the same variable, it can lead to confusion. Here’s what you can consider:

If the public getter and setter are sufficient, you can remove the private _Info field and directly use the public Info dictionary.

Ensure consistency in property naming and access within your class.

Conclusion

The NullReferenceException commonly arises from an uninitialized object, such as your dictionary. By initializing _Info during the declaration, you can avoid this error and improve the reliability of your code in Xamarin Forms and UWP applications.

By following these steps, you should be able to resolve the NullReferenceException issue in your custom classes effectively. Happy coding!