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Filters in.NET Core: An Overview with Examples

Posted on by Pablo Francesco

Because they enable developers to execute custom code either before or after specific steps in the request processing pipeline, filters are a crucial component of ASP.NET Core. They offer a mechanism to add logic to the way requests are processed for functions like error correction, authentication, logging, and more. This article will define filters, describe the many kinds of filters that are available in.NET Core, and provide examples of how to develop and use them.

What do .NET Core Filters do?
In ASP.NET Core, filters are properties that can be applied globally, to controllers, or to actions to change how requests are processed. In a consistent and reusable way, they can be used to add cross-cutting issues like logging, caching, exception handling, or authentication.

Types of Filters in .NET Core

ASP.NET Core provides several built-in filter types, which are executed at different points in the request processing pipeline:

  1. Authorization Filters
    • Run before anything else and are used to ensure the user is authorized to execute the request.
    • Example: [Authorize] attribute.
  2. Resource Filters
    • Run after authorization but before model binding, useful for caching or short-circuiting the request.
    • Example: [ResponseCache] attribute.
  3. Action Filters
    • Execute before and after an action method is called, often used for logging or modifying the result.
    • Example: [ServiceFilter] or [TypeFilter] attribute.
  4. Exception Filters
    • Handle exceptions thrown by the action methods or other filters, allowing custom error responses.
    • Example: [HandleError] attribute in MVC (built-in exception filter).
  5. Result Filters
    • Run before and after the execution of the action result, used to modify the response or the result.
    • Example: Custom result filters for response formatting.

Creating a Custom Filter in .NET Core

Let’s create a custom action filter that logs the execution time of a controller action.

Step 1. Implement the Filter

First, create a class that implements the IActionFilter interface:

using Microsoft.AspNetCore.Mvc.Filters;
using System.Diagnostics;

public class ExecutionTimeLoggerAttribute : Attribute, IActionFilter
{
    private Stopwatch _stopwatch;

    public void OnActionExecuting(ActionExecutingContext context)
    {
        _stopwatch = Stopwatch.StartNew();
    }

    public void OnActionExecuted(ActionExecutedContext context)
    {
        _stopwatch.Stop();
        var executionTime = _stopwatch.ElapsedMilliseconds;
        Debug.WriteLine($"Action {context.ActionDescriptor.DisplayName} executed in {executionTime} ms.");
    }
}

Step 2. Apply the Filter

You can apply this filter to a specific action, a controller, or globally across all controllers.

Applying to a Single Action

[ExecutionTimeLogger]
public IActionResult MyAction()
{
    // Action logic here
    return View();
}

Applying to a Controller

[ExecutionTimeLogger]
public class MyController : Controller
{
    public IActionResult MyAction()
    {
        // Action logic here
        return View();
    }
}

Applying Globally

To apply the filter globally, register it in the Startup.cs file:

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllersWithViews(options =>
    {
        options.Filters.Add<ExecutionTimeLoggerAttribute>();
    });
}

Using Built-in Filters

ASP.NET Core includes several built-in filters that can be used without writing custom code. Here’s an example of using the [Authorize] and [ResponseCache] filters.

Example

[Authorize]
[ResponseCache(Duration = 60, Location = ResponseCacheLocation.Client)]
public class MyController : Controller
{
    public IActionResult SecureAction()
    {
        // Action logic for authorized users
        return View();
    }
}

Exception Filters Example

To handle exceptions in a custom way, you can create an exception filter:

Step 1. Create the Exception Filter

using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Filters;

public class CustomExceptionFilter : IExceptionFilter
{
    public void OnException(ExceptionContext context)
    {
        context.Result = new ObjectResult("An error occurred.")
        {
            StatusCode = 500
        };
    }
}

Step 2. Register the Exception Filter

Register the filter globally or apply it to specific actions or controllers:

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllersWithViews(options =>
    {
        options.Filters.Add<CustomExceptionFilter>();
    });
}

Conclusion

With.NET Core, filters offer a potent way to introduce cross-cutting issues into your program. You can write code that is more readable, reusable, and maintainable by learning how to use and recognizing the many sorts of filters, including authorization, resource, action, result, and exception filters. They are an essential part of any ASP.NET Core application’s request processing pipeline, whether you use pre-built ones or make your own.

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