ASP.NET MVC Introduction to Filters Step by step Implementation and Top 10 Questions and Answers
 Last Update: April 01, 2025      13 mins read      Difficulty-Level: beginner

Introduction to Filters in ASP.NET MVC

ASP.NET MVC (Model-View-Controller) is a web application framework developed by Microsoft for building dynamic web applications. Filters in ASP.NET MVC play a crucial role in extending or modifying the behavior of the controller actions and the views. They provide a powerful mechanism to perform tasks before and after the execution of actions and views, thus enabling aspects such as logging, authorization, caching, and validation. In this detailed guide for beginners, we will explore the different types of filters available in ASP.NET MVC, their lifecycle, and how to implement them.

1. Understanding the Filter Lifecycle

Before diving into the types of filters, it's essential to understand how filters fit into the lifecycle of an ASP.NET MVC application. The MVC framework has a well-defined pipeline for handling HTTP requests, and filters play a critical role in this pipeline.

  1. Authorization Filters: Run first, immediately after the routing engine selects the controller and before the action method is executed. They are used for authentication and authorization checks.
  2. Action Filters: Run before and after the action method. They are used to perform tasks such as logging, validation, and modifying the inputs and outputs.
  3. Result Filters: Run before and after the action result executes but before the result is sent to the client. They are useful for modifying the content returned to the client.
  4. Exception Filters: Handle any unhandled exceptions that occur during the execution of the MVC request pipeline. They allow for custom error handling and logging.

Understanding this lifecycle helps you decide where to place your filters to achieve the desired behavior.

2. Types of Filters in ASP.NET MVC

ASP.NET MVC provides four primary types of filters: Authorization Filters, Action Filters, Result Filters, and Exception Filters.

2.1 Authorization Filters

Authorization filters are used to determine whether a user is authorized to access a particular action method or controller. They are executed before the action method and are useful for implementing security checks such as role-based access or claim-based authentication.

  • IAuthorizationFilter Interface: Implement this interface to create a custom authorization filter. You can override the OnAuthorization method to perform the necessary checks.

    public class CustomAuthorizationAttribute : FilterAttribute, IAuthorizationFilter
{
    public void OnAuthorization(AuthorizationContext filterContext)
    {
        if (!filterContext.HttpContext.User.Identity.IsAuthenticated)
        {
            filterContext.Result = new HttpUnauthorizedResult();
        }
    }
}

  • Use of AuthorizeAttribute: ASP.NET MVC includes a built-in AuthorizeAttribute that can be used to specify roles or users that have access to a particular action or controller.

    [Authorize(Roles = "Admin, Editor")]
public class ProductController : Controller
{
    public ActionResult Index()
    {
        return View();
    }
}
2.2 Action Filters

Action filters provide hooks to perform custom logic before and after the execution of an action method. They are useful for logging, validation, and modifying the input parameters or output results.

  • IActionFilter Interface: Implement this interface to create a custom action filter. Override the OnActionExecuting and OnActionExecuted methods to perform tasks before and after the action method executes.

    public class LogAttribute : FilterAttribute, IActionFilter
{
    public void OnActionExecuting(ActionExecutingContext filterContext)
    {
        // Log before action method executes
    }

    public void OnActionExecuted(ActionExecutedContext filterContext)
    {
        // Log after action method executes
    }
}

  • ActionFilterAttribute Class: Inherit from ActionFilterAttribute to create a custom action filter. This class provides a simplified way to implement the interface methods.

    public class LogAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        // Log before action method executes
    }

    public override void OnActionExecuted(ActionExecutedContext filterContext)
    {
        // Log after action method executes
    }
}
2.3 Result Filters

Result filters enable you to modify the action result just before it is rendered to the client. They are executed after the action method is executed but before the action result is executed.

  • IResultFilter Interface: Implement this interface to create a custom result filter. Override the OnResultExecuting and OnResultExecuted methods to perform tasks before and after the action result executes.

    public class LogResultAttribute : FilterAttribute, IResultFilter
{
    public void OnResultExecuting(ResultExecutingContext filterContext)
    {
        // Log before action result executes
    }

    public void OnResultExecuted(ResultExecutedContext filterContext)
    {
        // Log after action result executes
    }
}

  • ResultFilterAttribute Class: Inherit from ResultFilterAttribute to create a custom result filter. This class provides a simplified way to implement the interface methods.

    public class LogResultAttribute : ResultFilterAttribute
{
    public override void OnResultExecuting(ResultExecutingContext filterContext)
    {
        // Log before action result executes
    }

    public override void OnResultExecuted(ResultExecutedContext filterContext)
    {
        // Log after action result executes
    }
}
2.4 Exception Filters

Exception filters are used to handle any unhandled exceptions that occur during the execution of the MVC pipeline. They are executed when an exception is thrown and can be used to log errors, perform cleanup, or redirect to an error page.

  • IExceptionFilter Interface: Implement this interface to create a custom exception filter. Override the OnException method to handle the exception.

    public class CustomExceptionFilterAttribute : FilterAttribute, IExceptionFilter
{
    public void OnException(ExceptionContext filterContext)
    {
        // Log exception
        filterContext.ExceptionHandled = true;
        filterContext.Result = new RedirectResult("~/Home/Error");
    }
}

  • HandleErrorAttribute Class: Inherit from HandleErrorAttribute to create a custom exception filter. This attribute allows you to specify which controller and action should be executed when an exception occurs.

    [HandleError(View = "Error", ExceptionType = typeof(DivideByZeroException))]
public class HomeController : Controller
{
    public ActionResult Index()
    {
        return View();
    }
}

3. Applying Filters

Filters can be applied at different levels: globally, at the controller level, or at the action method level. Understanding how to apply filters is crucial for controlling their scope and behavior.

3.1 Global Filters

Global filters are applied to all controllers and action methods in the application. They are registered in the FilterConfig class in the App_Start folder.

public class FilterConfig
{
    public static void RegisterGlobalFilters(GlobalFilterCollection filters)
    {
        filters.Add(new HandleErrorAttribute());
        filters.Add(new LogAttribute());
    }
}
3.2 Controller-Level Filters

Controller-level filters affect all action methods within a specific controller. Apply them by specifying the filter attribute directly above the controller class.

[Authorize(Roles = "Admin")]
public class ProductController : Controller
{
    public ActionResult Index()
    {
        return View();
    }

    public ActionResult Details(int id)
    {
        return View();
    }
}
3.3 Action-Level Filters

Action-level filters affect only a specific action method. Apply them by specifying the filter attribute directly above the action method.

public class ProductController : Controller
{
    [Authorize(Roles = "Admin")]
    public ActionResult Index()
    {
        return View();
    }

    [Log]
    public ActionResult Details(int id)
    {
        return View();
    }
}

4. Custom Filters

While ASP.NET MVC provides built-in filters, custom filters offer the flexibility to implement specific behaviors that are unique to your application.

4.1 Creating a Custom Authorization Filter

To create a custom authorization filter, implement the IAuthorizationFilter interface or inherit from the AuthorizationAttribute class.

public class CustomAuthorizationAttribute : AuthorizationAttribute
{
    protected override bool AuthorizeCore(HttpContextBase httpContext)
    {
        // Implement custom authorization logic
        return base.AuthorizeCore(httpContext);
    }

    protected override void HandleUnauthorizedRequest(AuthorizationContext filterContext)
    {
        // Implement custom unauthorized request handling
        base.HandleUnauthorizedRequest(filterContext);
    }
}
4.2 Creating a Custom Action Filter

To create a custom action filter, implement the IActionFilter interface or inherit from the ActionFilterAttribute class.

public class LogAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        // Log before action method executes
    }

    public override void OnActionExecuted(ActionExecutedContext filterContext)
    {
        // Log after action method executes
    }
}
4.3 Creating a Custom Result Filter

To create a custom result filter, implement the IResultFilter interface or inherit from the ResultFilterAttribute class.

public class LogResultAttribute : ResultFilterAttribute
{
    public override void OnResultExecuting(ResultExecutingContext filterContext)
    {
        // Log before action result executes
    }

    public override void OnResultExecuted(ResultExecutedContext filterContext)
    {
        // Log after action result executes
    }
}
4.4 Creating a Custom Exception Filter

To create a custom exception filter, implement the IExceptionFilter interface or inherit from the HandleErrorAttribute class.

public class CustomExceptionFilterAttribute : HandleErrorAttribute
{
    public override void OnException(ExceptionContext filterContext)
    {
        // Handle exception
        filterContext.ExceptionHandled = true;
        filterContext.Result = new RedirectResult("~/Home/Error");
    }
}

5. Combining Multiple Filters

In complex applications, you may need to combine multiple filters to achieve the desired behavior. Filters can be applied at different levels and can be combined to create a powerful and flexible architecture.

[CustomAuthorization]
[Log]
[HandleError]
public class ProductController : Controller
{
    [Authorize(Roles = "Admin")]
    [LogResult]
    public ActionResult Index()
    {
        return View();
    }

    [CustomExceptionFilter]
    public ActionResult Details(int id)
    {
        return View();
    }
}

6. Best Practices for Using Filters

To ensure effective use of filters, follow these best practices:

  • Keep Filters Focused: Each filter should have a single responsibility. Avoid implementing multiple functionalities in a single filter, as it can lead to complex and difficult-to-maintain code.
  • Use Global Filters Sparingly: Global filters affect the entire application, so use them sparingly to avoid unnecessary processing.
  • Leverage Built-in Filters: Utilize the built-in filters provided by ASP.NET MVC, such as AuthorizeAttribute and HandleErrorAttribute, to reduce development effort and improve code quality.
  • Consider Filter Order: Filters are executed in a specific order, so consider the order when combining multiple filters to ensure the desired behavior.
  • Document Filters: Clearly document the purpose and behavior of your custom filters to facilitate maintenance and understanding by other developers.

7. Advanced Topics

As you become more familiar with filters, you can explore advanced topics such as dynamic filter registration, attribute routing, and custom filter providers.

  • Dynamic Filter Registration: Register filters dynamically based on certain conditions. This can be useful for applying filters based on configuration or user role.
  • Attribute Routing: Use attribute routing to specify routes directly within controller or action method attributes. This can simplify routing and improve the readability of your code.
  • Custom Filter Providers: Implement custom filter providers to manage filter registration and resolution. This can be useful for creating more advanced and flexible filter configurations.

Conclusion

Filters are a powerful feature in ASP.NET MVC that enable you to extend and modify the behavior of the framework. By understanding the different types of filters and their lifecycle, you can effectively implement custom logic to enhance the functionality and security of your web applications. Whether you are performing logging, validation, authorization, or handling exceptions, filters provide a flexible and reusable mechanism to achieve your goals. As a beginner, start by using built-in filters and gradually explore more advanced concepts as you become more comfortable with the framework. Happy coding!