Mastering Error Handling in ASP.NET Core: 30 Essential Questions and Answers

Top 30 Questions on Error Handling in ASP.NET Core

Effective error handling is crucial for building robust ASP.NET Core applications. In this guide, we cover 30 essential questions and answers to help you master error handling and improve application stability.

What is Error Handling in ASP.NET Core?

Error handling in ASP.NET Core involves managing and responding to errors that occur during the execution of an application. Proper error handling ensures that applications can fail gracefully, providing meaningful feedback to users and developers.

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Why is Error Handling Important in ASP.NET Core?

Error handling is crucial for maintaining application stability, improving user experience, and facilitating debugging and maintenance. It helps prevent application crashes and provides insights into issues that need resolution.

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What are the Types of Error Handling in ASP.NET Core?

ASP.NET Core supports several types of error handling, including:

• Synchronous Error Handling: Using try-catch blocks to handle exceptions in sequential code.
• Asynchronous Error Handling: Managing exceptions in async methods using try-catch and await.
• Global Error Handling: Using middleware to catch and handle exceptions application-wide.

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How Does the try-catch-finally Block Work in ASP.NET Core?

The try-catch-finally block is used to handle exceptions and ensure that resources are cleaned up properly.

• try: Contains code that might throw an exception.
• catch: Handles the exception if one occurs in the try block.
• finally: Executes code regardless of whether an exception was thrown, useful for cleanup tasks.

Example:

try
{
    // Code that may throw an exception
    int result = 10 / 0;
}
catch (DivideByZeroException ex)
{
    // Handle the exception
    Console.WriteLine("Error: " + ex.Message);
}
finally
{
    // Cleanup code
    Console.WriteLine("Execution completed.");
}

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What is the Difference Between throw and throw ex?

throw	throw ex
Preserves the original stack trace.	Resets the stack trace to the current method.
Recommended for rethrowing exceptions.	Not recommended as it loses original context.
Syntax: throw;	Syntax: throw ex;

Example:

try
{
    // Simulate an error
    throw new InvalidOperationException("Original error");
}
catch (InvalidOperationException ex)
{
    // Rethrow the exception
    throw;
}

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Looking for more ASP.NET Core resources? Read our Most important ASP.NET Web Forms Interview Questions to build a strong foundation!

What is Exception Middleware in ASP.NET Core?

Exception middleware is used to handle exceptions globally in an ASP.NET Core application. It catches unhandled exceptions that occur during the processing of a request.

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How Do You Implement Exception Middleware in ASP.NET Core?

Exception middleware is typically added in the Startup.cs file using the UseExceptionHandler or UseDeveloperExceptionPage methods.

Example:

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseExceptionHandler("/Home/Error");
    }

    app.UseMvc();
}

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What is Custom Middleware for Exception Handling?

Custom middleware allows you to define specific logic for handling exceptions, providing more control over the error handling process.

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How Do You Create Custom Middleware for Exception Handling?

Creating custom middleware involves defining a class that implements the IMiddleware interface or using inline middleware in the Startup.cs file.

Example:

public class CustomExceptionMiddleware
{
    private readonly RequestDelegate _next;

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

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

    private Task HandleExceptionAsync(HttpContext context, Exception exception)
    {
        context.Response.ContentType = "application/json";
        context.Response.StatusCode = (int)HttpStatusCode.InternalServerError;
        return context.Response.WriteAsync(new ErrorDetails()
        {
            StatusCode = context.Response.StatusCode,
            Message = "Internal Server Error."
        }.ToString());
    }
}

// In Startup.cs
public void Configure(IApplicationBuilder app)
{
    app.UseMiddleware<CustomExceptionMiddleware>();
    app.UseMvc();
}

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What are the Benefits of Using Custom Exception Middleware?

• Flexibility: Tailor error responses to specific application needs.
• Consistency: Ensure uniform error handling across the application.
• Control: Implement custom logging, monitoring, and error reporting.

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How Do You Handle Exceptions in Controllers?

Exceptions in controllers can be handled using try-catch blocks or by leveraging global exception handling middleware.

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What is the Role of [ApiController] and [Route] Attributes in Error Handling?

The [ApiController] attribute in ASP.NET Core automatically handles model validation errors, reducing the need for manual error handling in controllers. The [Route] attribute defines the route for the controller.

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How Do You Handle Model Validation Errors in ASP.NET Core?

Model validation errors can be handled using the ModelState property in controllers. The [ApiController] attribute automatically returns a 400 Bad Request response for invalid models.

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What is the ProblemDetails Class in ASP.NET Core?

The ProblemDetails class is used to provide machine-readable error details in the response, adhering to the RFC 7807 standard for problem details.

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How Do You Use ProblemDetails for Error Responses?

You can use the ProblemDetails class to return structured error responses from your controllers or middleware.

Example:

[ApiController]
[Route("[controller]")]
public class SampleController : ControllerBase
{
    [HttpGet]
    public IActionResult Get()
    {
        try
        {
            // Simulate an error
            throw new InvalidOperationException("An error occurred.");
        }
        catch (InvalidOperationException ex)
        {
            var problemDetails = new ProblemDetails
            {
                Status = (int)HttpStatusCode.BadRequest,
                Type = "https://example.com/probs/bad-request",
                Title = "Bad Request",
                Detail = ex.Message,
                Instance = HttpContext.Request.Path
            };

            return BadRequest(problemDetails);
        }
    }
}

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What is the Difference Between app.UseExceptionHandler and app.UseDeveloperExceptionPage?

app.UseExceptionHandler	app.UseDeveloperExceptionPage
Used in production to show custom error pages.	Used in development to show detailed error information.
Redirects to a specified path for errors.	Displays a detailed error page with stack trace.
Suitable for end-users.	Suitable for developers.

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How Do You Log Exceptions in ASP.NET Core?

Exceptions can be logged using built-in logging providers like ILogger. Logging can be configured in the appsettings.json file.

Example:

public class SampleController : ControllerBase
{
    private readonly ILogger<SampleController> _logger;

    public SampleController(ILogger<SampleController> logger)
    {
        _logger = logger;
    }

    [HttpGet]
    public IActionResult Get()
    {
        try
        {
            // Simulate an error
            throw new InvalidOperationException("An error occurred.");
        }
        catch (InvalidOperationException ex)
        {
            _logger.LogError(ex, "An error occurred while processing the request.");
            return StatusCode(500, "Internal server error.");
        }
    }
}

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What is the IExceptionFilter Interface?

The IExceptionFilter interface is used to handle exceptions that occur during the execution of controllers or actions. It allows for centralized exception handling logic.

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How Do You Implement a Custom Exception Filter?

Implementing a custom exception filter involves creating a class that implements the IExceptionFilter interface and registering it in the Startup.cs file.

Example:

public class CustomExceptionFilter : IExceptionFilter
{
    public void OnException(ExceptionContext context)
    {
        context.Result = new ObjectResult(new
        {
            context.Exception.Message,
            StatusCode = 500
        })
        {
            StatusCode = 500
        };
        context.ExceptionHandled = true;
    }
}

// In Startup.cs
public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers(options =>
    {
        options.Filters.Add<CustomExceptionFilter>();
    });
}

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What are the Best Practices for Error Handling in ASP.NET Core?

• Use Global Exception Handling: Implement exception middleware for consistent error handling.
• Log Exceptions: Use logging to track and diagnose issues.
• Provide Meaningful Error Messages: Return user-friendly error messages without exposing sensitive information.
• Validate Input: Use model validation to catch and handle invalid input early.
• Use ProblemDetails: Return structured error responses using the ProblemDetails class.

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How Do You Handle Unhandled Exceptions Globally?

Unhandled exceptions can be handled globally using exception middleware or custom exception filters. This ensures that all exceptions are caught and processed consistently.

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What is the Purpose of the UseStatusCodePages Middleware?

The UseStatusCodePages middleware is used to handle HTTP status code responses, such as 404 Not Found, by redirecting to custom error pages.

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How Do You Configure Custom Error Pages in ASP.NET Core?

Custom error pages can be configured using the UseExceptionHandler and UseStatusCodePages middleware in the Startup.cs file.

Example:

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    app.UseExceptionHandler("/Home/Error");
    app.UseStatusCodePagesWithReExecute("/Home/Error/{0}");

    app.UseMvc();
}

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What is the HttpStatusCode Enum in ASP.NET Core?

The HttpStatusCode enum represents the standard HTTP status codes used in responses. It is part of the System.Net namespace.

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How Do You Return Custom HTTP Status Codes in ASP.NET Core?

Custom HTTP status codes can be returned using the StatusCode method in controllers.

Example:

[HttpGet]
public IActionResult Get()
{
    return StatusCode(418, "I'm a teapot.");
}

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What is the UseStatusCodePagesWithReExecute Middleware?

The UseStatusCodePagesWithReExecute middleware is used to re-execute the request pipeline with a different path for specific HTTP status codes.

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How Do You Handle 404 Errors in ASP.NET Core?

404 errors can be handled using the UseStatusCodePages or UseStatusCodePagesWithReExecute middleware to redirect to a custom error page.

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What is the UseDeveloperExceptionPage Middleware?

The UseDeveloperExceptionPage middleware is used to display detailed error information, including the stack trace, during development.

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If you're interested in learning more about ASP.NET Core, check out our article on 

Most Asked C# Interview Questions

How Do You Disable Detailed Error Pages in Production?

Detailed error pages should be disabled in production by using the UseExceptionHandler middleware instead of UseDeveloperExceptionPage.

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What are Some Common Mistakes in Error Handling in ASP.NET Core?

• Exposing Sensitive Information: Avoid returning stack traces or internal error details to end-users.
• Ignoring Exceptions: Failing to handle or log exceptions can lead to unresolved issues.
• Inconsistent Error Responses: Inconsistent error handling across the application can confuse users and developers.
• Overusing Exception Handling: Using try-catch blocks excessively can lead to performance issues and make the code harder to maintain.

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