ASP.NET MVC Interview Questions

Question: What is Dependency Injection in ASP.NET Core?

Answer:

Dependency Injection (DI) in ASP.NET Core is a design pattern used to manage the dependencies of classes. It allows the developer to inject the dependencies that a class needs, rather than having the class create the dependencies itself. This helps in making the code more modular, testable, and maintainable.

In simple terms, Dependency Injection is a way to provide an object with all the services it needs (dependencies) rather than having the object create those services.

ASP.NET Core has built-in support for Dependency Injection, making it a core part of the framework. DI is used to manage the lifecycle of services, provide them where needed, and help in keeping the application loosely coupled.

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Key Concepts of Dependency Injection:

1. Dependency: A dependency is a service or object that another object (such as a controller, class, or component) depends on to function. For example, a service might depend on a repository to fetch data.

2. Injection: Injection is the process of providing the required dependency to a class. This can be done through various methods like constructor injection, property injection, or method injection.

3. Inversion of Control (IoC): Dependency Injection is a form of Inversion of Control (IoC), where the control over the creation of objects is shifted from the class itself to an external framework (such as ASP.NET Core’s DI container).

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How Dependency Injection Works in ASP.NET Core:

ASP.NET Core has a built-in Dependency Injection container that helps in configuring and resolving dependencies. The DI container is configured in the Startup.cs file (or Program.cs in .NET 6+), where services are registered and later injected into controllers, middleware, or other components.

Steps Involved in DI in ASP.NET Core:

1. Service Registration:

   • In ASP.NET Core, services (dependencies) are registered in the DI container. This is typically done in the ConfigureServices method of Startup.cs.

   public void ConfigureServices(IServiceCollection services)
   {
       // Register a service with DI container
       services.AddScoped<IMyService, MyService>();
   }

   • The AddScoped, AddSingleton, and AddTransient methods are used to register services with different lifetimes:
     • AddScoped: The service is created once per request.
     • AddSingleton: The service is created once and shared throughout the entire application’s lifetime.
     • AddTransient: The service is created each time it is requested.

2. Service Injection:

   • Once a service is registered, it can be injected into the constructor of classes (like controllers or services) that require it.

   Example:

   public class HomeController : Controller
   {
       private readonly IMyService _myService;
   
       // Constructor injection
       public HomeController(IMyService myService)
       {
           _myService = myService;
       }
   
       public IActionResult Index()
       {
           // Use _myService in the action
           var result = _myService.GetData();
           return View(result);
       }
   }

   • In the example above, IMyService is injected into the HomeController through the constructor. ASP.NET Core’s DI container automatically resolves the dependency and provides an instance of MyService.

3. Service Consumption:

   • Once the dependency is injected, the class can use the service throughout its lifetime. The service instance is automatically disposed of according to the specified lifetime (scoped, singleton, or transient).

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Types of Dependency Injection in ASP.NET Core:

ASP.NET Core supports three main types of dependency injection lifetimes:

1. Transient:

   • Services registered as transient are created each time they are requested. This is useful for lightweight, stateless services that don’t need to be shared.

   Example:

   services.AddTransient<IMyService, MyService>();

   • Use Case: Suitable for stateless services like small utility classes.

2. Scoped:

   • Services registered as scoped are created once per request. A new instance is created each time a new HTTP request is made.

   Example:

   services.AddScoped<IMyService, MyService>();

   • Use Case: This is often used for services that manage data or are related to the lifetime of an HTTP request, such as database contexts (e.g., Entity Framework DbContext).

3. Singleton:

   • Services registered as singleton are created once and shared throughout the entire application. They are created the first time they are requested and then reused throughout the application lifetime.

   Example:

   services.AddSingleton<IMyService, MyService>();

   • Use Case: Suitable for services that are expensive to create and should be shared across all components, like logging or caching services.

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Benefits of Dependency Injection:

1. Loose Coupling:

   • DI reduces the tight coupling between components by abstracting the creation of objects. Classes don’t need to create their own dependencies, and they can work with abstractions (interfaces) rather than concrete implementations.

2. Improved Testability:

   • Since services are injected, it becomes easier to replace real implementations with mock or fake objects for unit testing.

3. Centralized Configuration:

   • By registering services in the DI container, you centralize the configuration of dependencies in one place, making the application easier to manage and maintain.

4. Better Separation of Concerns:

   • DI allows for better separation of concerns, as classes no longer need to worry about instantiating their dependencies. Instead, they just consume them.

5. Easier Maintenance:

   • With DI, updating or changing the behavior of a service is easier. Since classes depend on interfaces, you can swap out the implementation without affecting the rest of the application.

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Example of Dependency Injection in ASP.NET Core:

Let’s say you have a service interface IEmailService that is implemented by EmailService:

public interface IEmailService
{
    void SendEmail(string to, string subject, string body);
}

public class EmailService : IEmailService
{
    public void SendEmail(string to, string subject, string body)
    {
        // Send email logic here
    }
}

You would register this service in Startup.cs:

public void ConfigureServices(IServiceCollection services)
{
    services.AddScoped<IEmailService, EmailService>();
}

Now, you can inject the IEmailService into any controller, for example, HomeController:

public class HomeController : Controller
{
    private readonly IEmailService _emailService;

    public HomeController(IEmailService emailService)
    {
        _emailService = emailService;
    }

    public IActionResult SendEmail()
    {
        _emailService.SendEmail("[email protected]", "Hello", "This is a test email.");
        return Ok("Email sent!");
    }
}

In this example:

• The EmailService class is registered with the DI container in ConfigureServices.
• It is then injected into HomeController through constructor injection.
• The SendEmail method can be invoked without HomeController needing to instantiate the EmailService manually.

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Conclusion:

Dependency Injection is a powerful pattern that is natively supported in ASP.NET Core to manage the creation and lifecycle of objects. It improves testability, maintainability, and modularity of applications by promoting loose coupling between components. The built-in DI container provides an efficient and flexible way to register and resolve dependencies throughout the application’s lifetime, making it easier to manage complex systems and maintain scalable code.

Question: How do you handle exceptions in ASP.NET MVC applications?

Answer:

Exception handling in ASP.NET MVC is an important aspect of building robust, fault-tolerant applications. Proper exception handling ensures that errors are caught, logged, and appropriate feedback is provided to users. It also allows for graceful failure and better system stability.

ASP.NET MVC provides several techniques to handle exceptions, including global exception handling, controller-level handling, and custom error pages. Below are the most commonly used methods to handle exceptions in ASP.NET MVC applications:

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1. Global Exception Handling (Application-Level)

The global exception handler catches unhandled exceptions across the entire application. This can be done using the Application_Error method in the Global.asax file or by using a custom Error Handling Middleware in ASP.NET Core.

a. Using Application_Error in Global.asax (ASP.NET MVC):

The Application_Error event in the Global.asax.cs file is triggered whenever an unhandled exception occurs. You can log the error, display a custom error message, or redirect the user to a generic error page.

Example:

// Global.asax.cs
protected void Application_Error()
{
    // Get the last error that occurred
    Exception exception = Server.GetLastError();

    // Log the exception (e.g., using a logging framework like NLog, log4net, etc.)
    LogException(exception);

    // Clear the error to prevent the default error page from showing
    Server.ClearError();

    // Redirect to a custom error page
    Response.Redirect("~/Error/GeneralError");
}

In this example:

• The Application_Error event captures the exception.
• You can log the error for debugging and analysis.
• You can redirect the user to a custom error page (e.g., ~/Error/GeneralError).

b. Custom Error Handling Middleware (ASP.NET Core):

In ASP.NET Core, middleware is used to handle exceptions globally. This can be done in the Startup.cs file by adding exception handling middleware.

Example:

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage(); // Developer exception page for debugging
    }
    else
    {
        app.UseExceptionHandler("/Home/Error"); // Generic error handling page in production
        app.UseHsts();
    }
}

This middleware allows for better handling based on the environment (development or production). The UseExceptionHandler("/Home/Error") will redirect the user to an Error action in the HomeController when an unhandled exception occurs in production.

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2. Exception Handling in Controllers (Action-Level)

While global exception handling handles application-wide exceptions, it’s often useful to handle exceptions within specific actions (controller methods). This can be done using try-catch blocks inside controller actions.

Example:

public class ProductController : Controller
{
    public ActionResult Details(int id)
    {
        try
        {
            // Simulate retrieving a product by id
            var product = ProductService.GetProductById(id);
            return View(product);
        }
        catch (ProductNotFoundException ex)
        {
            // Handle specific exception
            return View("ProductNotFound");
        }
        catch (Exception ex)
        {
            // Handle general exceptions
            LogException(ex);
            return View("Error");
        }
    }
}

In this example:

• A try-catch block is used to catch specific exceptions (ProductNotFoundException) as well as general exceptions.
• If a product is not found, it shows a “ProductNotFound” view.
• If any other error occurs, it logs the error and shows a generic error page (Error view).

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3. Custom Error Pages (Error Views)

For user-friendly error handling, ASP.NET MVC allows you to define custom error pages that provide helpful feedback to users when an error occurs.

a. Using HandleErrorAttribute Filter:

In ASP.NET MVC, you can globally apply error handling using the HandleErrorAttribute filter, which handles exceptions by redirecting to a custom error view.

1. Global Error Handling in FilterConfig.cs:

   In the FilterConfig.cs file, you can register the HandleErrorAttribute filter globally.

   public static void RegisterGlobalFilters(GlobalFilterCollection filters)
   {
       filters.Add(new HandleErrorAttribute());
   }

2. Custom Error View: Create an error view in the Views/Shared folder, such as _Error.cshtml, which will display when an exception is thrown.

   // _Error.cshtml
   <h2>An error occurred while processing your request.</h2>
   <p>@ViewData["ErrorMessage"]</p>

3. Controller Action for Error Handling: You can create a custom Error action in your controller to handle specific errors:

   public class ErrorController : Controller
   {
       public ActionResult GeneralError()
       {
           // Return a general error view or handle specific errors
           return View();
       }
   }

b. Custom Error Pages in ASP.NET Core:

In ASP.NET Core, you can configure custom error pages by using the UseExceptionHandler middleware or creating custom error pages based on the status codes.

Example (in Startup.cs):

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

public class HomeController : Controller
{
    public IActionResult Error()
    {
        return View();
    }
}

In this example, the Error action in the HomeController will handle exceptions globally.

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4. Logging Exceptions

Logging is a crucial part of exception handling. It helps to track errors and analyze what went wrong.

ASP.NET Core has built-in support for logging using various providers like Console, Debug, EventLog, Azure Application Insights, and Serilog.

Example (ASP.NET Core):

public class HomeController : Controller
{
    private readonly ILogger<HomeController> _logger;

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

    public IActionResult Index()
    {
        try
        {
            // Some code that may throw an exception
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "An error occurred in Index action.");
            return View("Error");
        }

        return View();
    }
}

In this example:

• The ILogger service is injected into the controller.
• If an exception occurs, it is logged using the LogError method.

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5. Using Try-Catch with Specific Exceptions

It’s a good practice to catch specific exceptions rather than a general Exception class, as this provides better control over the error-handling flow. For example, if a database connection fails, catch a SqlException, or if a user enters invalid input, catch a FormatException.

Example:

try
{
    // Some database operation
}
catch (SqlException ex)
{
    // Handle SQL exceptions
    LogError(ex);
}
catch (Exception ex)
{
    // Handle generic exceptions
    LogError(ex);
}

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6. Custom Exception Filters (ASP.NET MVC)

In addition to using HandleErrorAttribute, you can create custom exception filters to catch and process exceptions in a more granular way.

Example (Custom Exception Filter):

public class CustomExceptionFilter : IExceptionFilter
{
    public void OnException(ExceptionContext filterContext)
    {
        // Log the exception
        LogException(filterContext.Exception);

        // Redirect to a custom error page
        filterContext.Result = new RedirectResult("/Error/GeneralError");
        filterContext.ExceptionHandled = true;
    }
}

You can register this filter globally or at the controller/action level.

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Conclusion:

Handling exceptions properly in ASP.NET MVC applications ensures a better user experience and maintainability. Some of the best practices include:

• Using global exception handling via Application_Error or UseExceptionHandler.
• Handling exceptions locally within controllers using try-catch.
• Using custom error pages and views for user-friendly error messages.
• Logging exceptions for future debugging and analysis.
• Handling specific exceptions instead of general ones to provide better control over error scenarios.

By adopting these strategies, you can create robust applications that handle errors gracefully and provide valuable feedback to both users and developers.

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