ASP.NET ResultFilter

last modified April 3, 2025

In this article, we explore the ResultFilter in ASP.NET 8. Result filters allow you to run code before or after action results are executed in ASP.NET Core.

ASP.NET filters provide a way to run code at specific stages in the request processing pipeline. Result filters are particularly useful for modifying responses or adding headers.

Basic Definition

A ResultFilter in ASP.NET Core is a type of action filter that runs code before or after an action result executes. It implements either IResultFilter or IAsyncResultFilter interface.

Result filters are executed after the action method completes but before the result is processed. They can modify the response or short-circuit execution.

Common use cases include response formatting, adding headers, logging, or modifying the response content. Result filters run for both successful and failed action results.

ASP.NET ResultFilter Example

The following example demonstrates creating and using a custom ResultFilter in ASP.NET 8 to add response headers.

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllers(options =>
{
    options.Filters.Add<AddCustomHeadersFilter>();
});

var app = builder.Build();

app.MapControllers();
app.Run();

This configures the application to use our custom ResultFilter globally. The filter will be applied to all controller actions.

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

public class AddCustomHeadersFilter : IResultFilter
{
    public void OnResultExecuting(ResultExecutingContext context)
    {
        // Runs before the action result executes
        if (context.HttpContext.Response.Headers.ContainsKey("X-Custom-Header"))
        {
            context.HttpContext.Response.Headers.Remove("X-Custom-Header");
        }
        
        context.HttpContext.Response.Headers.Add(
            "X-Custom-Header", "Custom-Value");
            
        context.HttpContext.Response.Headers.Add(
            "X-Response-Time", DateTime.UtcNow.ToString());
    }

    public void OnResultExecuted(ResultExecutedContext context)
    {
        // Runs after the action result executes
        // Can be used for cleanup or logging
    }
}

This filter adds two custom headers to every response. The OnResultExecuting method runs before the result executes, while OnResultExecuted runs after.

using Microsoft.AspNetCore.Mvc;

[ApiController]
[Route("api/[controller]")]
public class ProductsController : ControllerBase
{
    [HttpGet]
    public IActionResult Get()
    {
        return Ok(new { Name = "Product 1", Price = 19.99 });
    }
}

When you call the /api/products endpoint, the response will include the custom headers added by our filter. This demonstrates how ResultFilters can modify responses.

The example shows a global filter, but you can also apply ResultFilters to specific controllers or actions using the [ServiceFilter] or [TypeFilter] attributes.

Source

Microsoft ASP.NET Filters Documentation

In this article, we have explored the ResultFilter in ASP.NET 8. This powerful feature allows you to intercept and modify responses in your web applications.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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