Handling Exceptions in ASP.NET Core: Custom Exception Handling for API Endpoints

Learn how to implement `app.UseExceptionHandler()` in ASP.NET Core to handle exceptions for specific endpoints, especially for API controllers versus regular pages.
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Handling Exceptions in ASP.NET Core: Custom Exception Handling for API Endpoints

When developing web applications, managing exceptions effectively is crucial for enhancing user experience and debugging efficiency. In the world of ASP.NET Core, a common challenge arises when you have both traditional web pages and API endpoints within the same application. Specifically, you might want different approaches to exception handling based on whether the request comes from an API controller or a regular page. This article explores how to implement app.UseExceptionHandler() for such scenarios, focusing on an elegant solution tailored for ASP.NET Core applications.

Understanding the Problem

Imagine you are adding new API methods to an existing ASP.NET Core application. The web API controller(s) will use the [ApiController] attribute, which means the output expected by calling clients will be in JSON format. However, you want to ensure that exceptions are handled in a specific way:

For exceptions arising from an API controller, the response should be a JSON object containing relevant error information.

For exceptions from standard page controllers, the response should remain an HTML error page, redirecting users to a custom error view.

This leads us to the question: How can you set up an exception handler that understands the source of the exception and renders the response accordingly?

Solution Overview

In ASP.NET Core 6, developers have the option to retrieve the endpoint directly from the IExceptionHandlerFeature. For earlier versions or when additional customization is necessary, we can capture the endpoint ourselves after the routing middleware. Here's how to implement this solution step-by-step:

Step 1: Set Up Exception Handling

First, you will need to use the app.UseExceptionHandler() middleware to define a common error handling route. This route will catch exceptions before they reach the client.

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Step 2: Configure Routing Middleware

Next, you must configure the routing middleware in your application. This process will help capture the endpoint related to incoming requests:

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Step 3: Create a Custom Middleware for Endpoint Storage

You will create a custom middleware that captures the endpoint and stores it in the context features. This information will be used later during exception handling:

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Step 4: Handle Error Responses Based on Endpoint Type

Finally, you will define the error handling endpoint. Within this definition, you will extract the stored endpoint metadata to determine if the exception originated from an API controller or a standard page:

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Conclusion

Implementing a customized exception handling mechanism in ASP.NET Core allows developers to tailor responses based on the request origin effectively. The separation of error responses for API controllers versus regular pages ensures that clients receive appropriate feedback in the format they expect, whether it's JSON or HTML.

By following the steps outlined in this article, you can enhance your application's error handling strategies, contributing to smoother user experiences and easier troubleshooting.

Final Notes

Ensure you test your error handling thoroughly to ensure it correctly captures and responds to various error scenarios.

Consider logging errors for further analysis and improvements in your API.

With this setup, you're now ready to handle exceptions gracefully in your ASP.NET Core applications, providing a better experience for your users and clients alike