Overview
Understanding the lifecycle of an MVC request from routing to action execution is crucial in MVC architecture. It encompasses the journey of a request from its entry into the MVC application to the execution of a specific action in a controller. This knowledge is essential for optimizing application performance, debugging, and customizing the request processing pipeline.
Key Concepts
- Routing: The process of identifying the appropriate controller and action method to handle a request.
- Controller Initialization: Instantiation of the identified controller and preparation for action execution.
- Action Execution: The execution of the action method, including model binding, validation, and result generation.
Common Interview Questions
Basic Level
- What is the first step in the MVC request lifecycle?
- How does MVC differentiate between two methods with the same name in a controller?
Intermediate Level
- How does model binding work in the context of an MVC request?
Advanced Level
- How can you customize the pipeline in MVC to add a piece of middleware or a filter?
Detailed Answers
1. What is the first step in the MVC request lifecycle?
Answer: The first step in the MVC request lifecycle is routing. When a request is made to an MVC application, the routing engine examines the request's URL to determine which controller and action method should handle the request. This process involves parsing the URL and matching it against predefined routes in the application's RouteConfig file or attribute routes defined on controllers and actions.
Key Points:
- Routing is the entry point of the MVC request lifecycle.
- It maps URLs to controllers and actions.
- Routes can be defined using RouteConfig or attributes.
Example:
public class RouteConfig
{
public static void RegisterRoutes(RouteCollection routes)
{
routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
routes.MapRoute(
name: "Default",
url: "{controller}/{action}/{id}",
defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
);
}
}
2. How does MVC differentiate between two methods with the same name in a controller?
Answer: MVC differentiates between two methods with the same name (method overloading) in a controller by their signatures, specifically the parameters they accept. However, because HTTP does not support method overloading directly, MVC relies on attributes such as [HttpGet] and [HttpPost] to distinguish between actions intended for different HTTP methods. Additionally, route configuration and action selectors like [ActionName] can be used for further differentiation.
Key Points:
- Method overloading in MVC is based on action method signatures.
- HTTP method attributes ([HttpGet], [HttpPost], etc.) help distinguish actions.
- [ActionName] attribute can provide a unique name for overloaded methods.
Example:
public class HomeController : Controller
{
[HttpGet]
public ActionResult Register()
{
// GET version of Register
return View();
}
[HttpPost]
public ActionResult Register(UserModel model)
{
// POST version of Register
if (ModelState.IsValid)
{
// Handle registration
return RedirectToAction("Success");
}
return View(model);
}
}
3. How does model binding work in the context of an MVC request?
Answer: Model binding in MVC automatically maps data from HTTP requests to action method parameters. When an MVC action is invoked, the model binder inspects the incoming HTTP request and binds data from various sources (such as form values, route data, and query strings) to the parameters of the action method. This process involves converting data types, validating input against the model, and handling errors.
Key Points:
- Model binding links request data to action parameters.
- It supports complex types and collections.
- Validation is integrated into the model binding process.
Example:
public class UserController : Controller
{
public ActionResult Edit(int id)
{
User model = userRepository.GetUser(id);
return View(model);
}
[HttpPost]
public ActionResult Edit(User model) // Model binding maps form values to this User object
{
if (ModelState.IsValid)
{
userRepository.UpdateUser(model);
return RedirectToAction("Index");
}
return View(model);
}
}
4. How can you customize the pipeline in MVC to add a piece of middleware or a filter?
Answer: Customizing the MVC pipeline involves adding custom middleware or filters to intercept and process requests at various stages. Middleware is used in ASP.NET Core MVC for cross-cutting concerns like logging, authentication, and error handling, and is configured in the Startup.cs file. Filters, on the other hand, provide a way to run pre- and post-processing logic around MVC actions. They can be applied globally, to controllers, or to specific actions.
Key Points:
- Middleware is configured in Startup.cs and affects all HTTP requests.
- Filters can be used for action-specific processing.
- Custom filters can be created by implementing filter interfaces.
Example:
// Adding custom middleware in Startup.cs
public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
app.UseMiddleware<CustomMiddleware>(); // Custom middleware
app.UseMvc(routes =>
{
routes.MapRoute(
name: "default",
template: "{controller=Home}/{action=Index}/{id?}");
});
}
// Creating a custom action filter
public class CustomActionFilter : ActionFilterAttribute
{
public override void OnActionExecuting(ActionExecutingContext context)
{
// Logic before the action executes
}
public override void OnActionExecuted(ActionExecutedContext context)
{
// Logic after the action executes
}
}
// Applying the custom filter to a controller
[CustomActionFilter]
public class HomeController : Controller
{
public IActionResult Index()
{
return View();
}
}
