Exception handling is a critical aspect of building robust ASP.NET Core applications. Properly managing exceptions helps ensure that your application can gracefully handle errors and provide meaningful feedback to users. This guide will cover various strategies for handling exceptions in ASP.NET Core.

1. Use Middleware for Global Exception Handling

One of the most effective ways to handle exceptions globally in an ASP.NET Core application is to use middleware. This allows you to catch exceptions that occur in the application pipeline and respond accordingly.

Sample Code for Global Exception Handling Middleware

        
            public class ExceptionHandlingMiddleware
            {
                private readonly RequestDelegate _next;

                public ExceptionHandlingMiddleware(RequestDelegate next)
                {
                    _next = next;
                }

                public async Task<Task> InvokeAsync(HttpContext context)
                {
                    try
                    {
                        await _next(context);
                    }
                    catch (Exception ex)
                    {
                        await HandleExceptionAsync(context, ex);
                    }
                }

                private Task HandleExceptionAsync(HttpContext context, Exception ex)
                {
                    context.Response.ContentType = "application/json";
                    context.Response.StatusCode = (int)HttpStatusCode.InternalServerError;

                    var result = JsonConvert.SerializeObject(new { error = ex.Message });
                    return context.Response.WriteAsync(result);
                }
            }
            

In this example, the ExceptionHandlingMiddleware class catches exceptions thrown during the request processing pipeline. It sets the response status code to 500 (Internal Server Error) and returns a JSON object containing the error message.

Registering the Middleware

        
            public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
            {
                app.UseMiddleware<ExceptionHandlingMiddleware>();
                // Other middleware
            }
            

In the Configure method of the Startup class, the custom exception handling middleware is registered. This ensures that it is invoked for every request.

2. Use Built-in Exception Handling Middleware

ASP.NET Core provides built-in middleware for handling exceptions, such as UseExceptionHandler and UseHsts. You can use these to redirect users to a specific error page or return a custom error response.

Sample Code for Using Built-in Exception Handling

        
            public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
            {
                app.UseExceptionHandler("/Home/Error"); // Redirect to error page
                app.UseHsts();
                // Other middleware
            }
            

In this example, the UseExceptionHandler middleware is configured to redirect users to the Error action in the HomeController when an unhandled exception occurs.

Creating an Error Handling Action

        
            public class HomeController : Controller
            {
                public IActionResult Error()
                {
                    return View(); // Return error view
                }
            }
            

This action can be used to display a user-friendly error message when an exception occurs.

3. Exception Filters

ASP.NET Core also supports exception filters, which allow you to handle exceptions at the action or controller level. This is useful for providing specific error handling logic for certain parts of your application.

Sample Code for Creating an Exception Filter

        
            public class CustomExceptionFilter : IExceptionFilter
            {
                public void OnException(ExceptionContext context)
                {
                    context.Result = new JsonResult(new { error = context.Exception.Message })
                    {
                        StatusCode = (int)HttpStatusCode.InternalServerError
                    };
                    context.ExceptionHandled = true; // Mark exception as handled
                }
            }
            

In this example, the CustomExceptionFilter class implements the IExceptionFilter interface to handle exceptions. It returns a JSON response with the error message and sets the status code to 500.

Registering the Exception Filter

        
            public void ConfigureServices(IServiceCollection services)
            {
                services.AddControllers(options =>
                {
                    options.Filters.Add<CustomExceptionFilter>(); // Register the filter globally
                });
            }
            

In the ConfigureServices method of the Startup class, the custom exception filter is registered globally, meaning it will apply to all controllers and actions.

4. Logging Exceptions

Logging is an essential part of exception handling. It allows you to track errors and diagnose issues in your application. ASP.NET Core integrates with various logging providers, such as Serilog, NLog, and the built-in logging framework.

Sample Code for Logging Exceptions

        
            public class ExceptionHandlingMiddleware
            {
                private readonly RequestDelegate _next;
                private readonly ILogger<ExceptionHandlingMiddleware> _logger;

                public ExceptionHandlingMiddleware(RequestDelegate next, ILogger<ExceptionHandlingMiddleware> logger)
                {
                    _next = next;
                    _logger = logger;
                }

                public async Task<Task> InvokeAsync(HttpContext context)
                {
                    try
                    {
                        await _next(context);
                    }
                    catch (Exception ex)
                    {
                        _logger.LogError(ex, "An unhandled exception occurred.");
                        await HandleExceptionAsync(context, ex);
                    }
                }
            }
            

In this modified version of the ExceptionHandlingMiddleware, an ILogger instance is injected to log the exception details before handling the response.

Conclusion

Handling exceptions in ASP.NET Core is crucial for building resilient applications. By using middleware, built-in exception handling, exception filters, and logging, you can effectively manage errors and provide a better experience for your users. Implementing these strategies will help you create a robust error handling mechanism in your ASP.NET Core applications.