Filters in Asp.Net Core MVC

In Asp.Net Core, we use filters to run some custom logic before or after specific stages in the request processing pipeline. For example, for tasks like authentication, logging, and exception handling, there is some repetitive code that needs to be applied to multiple controllers or actions. Instead of writing repetitive code in multiple controllers or actions, filters provide a clean and reusable way to handle cross-cutting concerns. Now, let's look at the different types of filters and how they work in real-world scenarios.

1) Authorization Filters

public class ProSubscriptionFilter : IAsyncAuthorizationFilter
{
    private readonly ApplicationDbContext _dbContext;

    public ProSubscriptionFilter(ApplicationDbContext dbContext)
    {
        _dbContext = dbContext;
    }

    public async Task OnAuthorizationAsync(AuthorizationFilterContext context)
    {
        var user = context.HttpContext.User;
        if (user == null || user.Identity == null || !user.Identity.IsAuthenticated)
        {
            context.Result = new RedirectToActionResult("AccessDenied", "Home", null);
            return;
        }

        var userId = user.FindFirst(ClaimTypes.NameIdentifier)?.Value;
        if (string.IsNullOrEmpty(userId))
        {
            context.Result = new RedirectToActionResult("AccessDenied", "Home", null);
            return;
        }

        // Get the user's subscription plan from the database
        var userPlan = await _dbContext.UserSubscriptions.Where(u => u.UserId == userId).Select(u => u.SubscriptionPlan).FirstOrDefaultAsync();

        if (userPlan == null || userPlan != "Pro")
        {
            context.Result = new RedirectToActionResult("Subscribe", "Account", null); // Redirect to subscription page
        }
    }
}

Apply to the controllers:


    [ServiceFilter(typeof(ProSubscriptionFilter))]
    public class ProController : Controller
    {

    }

    [ServiceFilter(typeof(ProSubscriptionFilter))]
    public class ExclusiveController : Controller
    {

    }

    [ServiceFilter(typeof(ProSubscriptionFilter))]
    public class AdvancedController : Controller
    {

    }

2) Resource Filters

public class CacheResourceFilter : IResourceFilter
{
    private readonly IMemoryCache _cache;
    private readonly int _cacheDurationInMinutes = 5;

    public CacheResourceFilter(IMemoryCache cache)
    {
        _cache = cache;
    }

    public void OnResourceExecuting(ResourceExecutingContext context)
    {
        var cacheKey = context.HttpContext.Request.Path.ToString();

        if (_cache.TryGetValue(cacheKey, out var cachedResponse))
        {
            context.Result = (IActionResult)cachedResponse;
            return;
        }
    }

    public void OnResourceExecuted(ResourceExecutedContext context)
    {
        var cacheKey = context.HttpContext.Request.Path.ToString();
        _cache.Set(cacheKey, context.Result, TimeSpan.FromMinutes(_cacheDurationInMinutes));
    }
}

Apply to BlogController.cs

[ServiceFilter(typeof(CacheResourceFilter))]
public IActionResult Index()
{
    return View();
}

Register in Program.cs

builder.Services.AddMemoryCache();  // Enable in-memory caching
builder.Services.AddScoped<CacheResourceFilter>();  // Register the filter

3) Action Filters

public class ProfilePictureFilter : IActionFilter
{
    private readonly IGeneralService _generalService;
    public ProfilePictureFilter(IGeneralService generalService)
    {
        _generalService = generalService;
    }

    public void OnActionExecuting(ActionExecutingContext context)
    {
        Controller? controller = context.Controller as Controller;
        if (controller != null && context.HttpContext.User.Identity != null
            && context.HttpContext.User.Identity.IsAuthenticated)
        {
            ClaimsPrincipal currentUser = context.HttpContext.User;
            if (currentUser != null)
            {
                string? currentUserID = currentUser.FindFirst(ClaimTypes.NameIdentifier)?.Value;
                if (!string.IsNullOrEmpty(currentUserID))
                {
                    controller.ViewBag.ProfilePictureUrl =
                        _generalService.GetUserProfilePictureUrl(currentUserID);
                }
            }
        }
    }

    public void OnActionExecuted(ActionExecutedContext context)
    {
        // No action needed after execution
    }
}

Register in Program.cs

builder.Services.AddControllersWithViews(options =>
{
    options.Filters.Add<ProfilePictureFilter>();
});

Apply in _LoginPartial.cshtml

<img src="@ViewBag.ProfilePictureUrl" />

4) Exception Filters

public class CustomExceptionFilter : IExceptionFilter
{
    public void OnException(ExceptionContext context)
    {
        string userFriendlyMessage;
        int statusCode;

        // Determine the type of exception and set appropriate response
        switch (context.Exception)
        {
            case ArgumentNullException:
                userFriendlyMessage = "Required data was missing. Please check your input.";
                statusCode = 400; // Bad Request
                break;

            case UnauthorizedAccessException:
                userFriendlyMessage = "You are not authorized to perform this action.";
                statusCode = 403; // Forbidden
                break;

            case InvalidOperationException:
                userFriendlyMessage = "The operation could not be completed. Please try again.";
                statusCode = 400; // Bad Request
                break;

            case NullReferenceException:
                userFriendlyMessage = "An unexpected error occurred while processing your request.";
                statusCode = 500; // Internal Server Error
                break;

            case Exception:
                userFriendlyMessage = "Something went wrong. Please try again later.";
                statusCode = 500; // Internal Server Error
                break;

            default:
                userFriendlyMessage = "An unknown error occurred.";
                statusCode = 500; // Internal Server Error
                break;
        }

        // Set the error message in TempData
        context.Result = new ViewResult
        {
            ViewName = "CustomError", // Change to your custom error view name
            ViewData = new ViewDataDictionary(new EmptyModelMetadataProvider(), context.ModelState)
            {
                ["Message"] = userFriendlyMessage,
                ["StatusCode"] = statusCode
            }
        };

        // Mark the exception as handled to prevent further propagation
        context.ExceptionHandled = true;
    }
}

Create CustomError.cshtml view page

@{
    ViewBag.Title = "Error";
}

<h2>An Error Occurred</h2>
<p>Status Code: @ViewData["StatusCode"]</p>
<p>@ViewData["Message"]</p>

Register in Program.cs

builder.Services.AddControllersWithViews(options =>
{
    options.Filters.Add<CustomExceptionFilter>();
});

5) Result Filters

public class PerformanceMonitorFilter : IResultFilter
{
    private readonly ILogger<PerformanceMonitorFilter> _logger;
    private Stopwatch _stopwatch;

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

    public void OnResultExecuting(ResultExecutingContext context)
    {
        _stopwatch = Stopwatch.StartNew();
        var actionName = context.ActionDescriptor.DisplayName;
        _logger.LogInformation($"Starting execution of action: {actionName}");
    }

    public void OnResultExecuted(ResultExecutedContext context)
    {
        _stopwatch.Stop();
        var actionName = context.ActionDescriptor.DisplayName;
        var elapsedTime = _stopwatch.ElapsedMilliseconds;
        var statusCode = context.HttpContext.Response.StatusCode;

        _logger.LogInformation($"Action '{actionName}' executed in {elapsedTime} ms with status code {statusCode}");
    }
}

Register in Program.cs

builder.Services.AddControllersWithViews(options =>
{
    options.Filters.Add<PerformanceMonitorFilter>();
});

Run the project and investigate the performance in console output.

That's a wrap! Hope this helped! Thank you for tuning in!

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