Welcome to the final lesson of this unit! We've explored many essentials of ASP.NET Core, from setting up a basic application to building APIs and managing dependencies. In this last lesson, we will focus on an often overlooked but critical aspect of web development: error handling. Errors are inevitable, occurring due to user interactions or developer mistakes. While we strive to minimize errors, a robust error-handling strategy is essential to maintain a smooth user experience and facilitate debugging. By the end of this lesson, you’ll understand how to handle errors effectively in both development and production environments using middleware in ASP.NET Core.

ASP.NET Core uses an environment variable named ASPNETCORE_ENVIRONMENT to specify the current hosting environment. This variable helps configure the application based on the environment it is running in.

Common values for ASPNETCORE_ENVIRONMENT include "Development", "Staging", and "Production", but you can specify any custom name you want. ASP.NET Core provides extension methods to check the environment:

• app.Environment.IsDevelopment()
• app.Environment.IsStaging()
• app.Environment.IsProduction()
• app.Environment.IsEnvironment("<custom name>")

These methods perform case-insensitive checks, ensuring a robust way to verify the running environment without issues related to string comparison. This setup allows you to customize configurations and behaviors based on the environment, enhancing both the development and deployment workflows.

In the development environment, we need detailed error messages to facilitate debugging. ASP.NET Core provides a middleware specifically for this purpose called DeveloperExceptionPageMiddleware, which is added by default.

For demonstration purposes, let's explicitly add it to our application:

C#
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1WebApplicationBuilder builder = WebApplication.CreateBuilder(args);
2WebApplication app = builder.Build();
3
4if (app.Environment.IsDevelopment())
5{
6    app.UseDeveloperExceptionPage(); // Detailed error page with information useful for debugging
7}
8
9app.MapGet("/", () => "Hello World!");
10
11app.Run();

In this snippet:

• app.Environment.IsDevelopment(): Checks if the application is running in the development environment.
• app.UseDeveloperExceptionPage(): Displays a detailed, developer-friendly error page in case of exceptions.

While this is useful during development, it’s crucial to disable this middleware in production to avoid exposing sensitive information, which can be exploited by attackers.

For handling errors in the production environment, the DeveloperExceptionPage is not suitable due to the risk of exposing sensitive details. Instead, we use the ExceptionHandlerMiddleware to route exceptions to a predefined endpoint that can safely display a user-friendly error message.

Here’s how to implement it:

C#
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1WebApplicationBuilder builder = WebApplication.CreateBuilder(args);
2WebApplication app = builder.Build();
3
4if (!app.Environment.IsDevelopment())
5{
6    app.UseExceptionHandler("/error"); // Route exceptions to the /error endpoint
7}
8
9app.MapGet("/error", () => "Sorry, an error occurred!"); // Error handling endpoint
10app.MapGet("/", () => "Hello World!");
11
12app.Run();

In this code:

• app.UseExceptionHandler("/error"): Routes exceptions to the /error endpoint, which we define to handle errors securely.
• app.MapGet("/error", () => "Sorry, an error occurred"): Maps the /error endpoint to return a simple error message.

This setup ensures that in production, users receive a generic error message and no sensitive information is exposed.

Congratulations on completing this lesson and the unit! You’ve learned the significance of error handling and how to implement it using middleware in ASP.NET Core. We covered error handling in the development environment using DeveloperExceptionPage and secure error handling in the production environment using ExceptionHandlerMiddleware. Next, you’ll apply these concepts in practice exercises, setting up error handling in an ASP.NET Core application. This was the last lesson of the course, and you've done an excellent job making it this far. Keep up the great work as you solidify your skills through hands-on practice!