Скачать или смотреть Resolving NullReferenceException in ASP.NET Core 3.1

Learn how to troubleshoot and resolve the `NullReferenceException` in your ASP.NET Core 3.1 application. Step-by-step guidance for setting up the Startup configuration correctly.
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This video is based on the question https://stackoverflow.com/q/62577420/ asked by the user 'Jake12342134' ( https://stackoverflow.com/u/10460880/ ) and on the answer https://stackoverflow.com/a/62577863/ provided by the user 'Legacy Code' ( https://stackoverflow.com/u/5209014/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving NullReferenceException in ASP.NET Core 3.1: A Guide for Beginners

Creating a new ASP.NET Core 3.1 application can be an exciting venture, especially when you start adding your own code and configurations. However, it's not uncommon to run into issues along the way, especially if you're a beginner. One such common problem that may arise is a NullReferenceException during the startup process of your application.

In this guide, we will take a closer look at this issue, identify its causes, and walk you through step-by-step solutions to resolve it effectively.

Understanding the Problem

When you attempt to run your newly created ASP.NET Core 3.1 application, you might encounter an exception like the following:

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This error arises in the Startup.Configure method, often indicating that the application is trying to access something that hasn't been initialized or is set to null. In your case, the problem is likely connected to the configuration used in setting up Cross-Origin Resource Sharing (CORS).

Cause of the Exception

In your Configure method, you have the following line:

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Here, you are trying to retrieve a setting from your application's configuration, specifically one that defines the allowed origins for CORS requests. If the key "CorsAllowedOrigins" does not exist in your configuration, origins will be null. This leads to the next line origins.Split(...) attempting to operate on a null object, which is what throws the NullReferenceException.

Steps to Resolve the Issue

1. Verify Your Configuration File

Make sure your appsettings.json (or whichever configuration source you're using) includes the following key:

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Ensure there are no typos and that the JSON format is correct. If this key is missing, the application will fail to retrieve the necessary information, leading to the NullReferenceException you encountered.

2. Implement Null Checks

To prevent this exception from halting your application, you can implement a null check before using the origins variable:

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3. Test Your Application

After making these changes, restart your application to see if it runs without throwing a NullReferenceException. If implemented correctly, your application should start without encountering the same error.

Conclusion

Dealing with NullReferenceException in your ASP.NET Core 3.1 application can be frustrating, especially for beginners. By ensuring that your configuration settings are correctly defined and implementing simple null checks, you can effectively prevent these types of errors from occurring in the future.

Should you encounter any other issues, remember to consult the official documentation or reach out to the community for support. Happy coding!