How to add custom header to ASP.NET Core Web API response

You can just hi-jack the HttpContext from the incoming Http Request and add your own custom headers to the Response object before calling return.

If you want your custom header to persist and be added in all API requests across multiple controllers, you should then consider making a Middleware component that does this for you and then add it in the Http Request Pipeline in Startup.cs

public IActionResult SendResponse()
{
    Response.Headers.Add("X-Total-Count", "20");

    return Ok();
}    

For anyone who want to add custom header to all requests, middleware is the best way. make some change in startup.cs like this:

app.Use(async (context, next) =>
{
   context.Response.Headers.Add("X-Developed-By", "Your Name");
   await next.Invoke();
});

Good luck.

There is an example for simple GET action which returns top X records from some list as well as the count in the response header X-Total-Count:

using System;
using System.Linq;
using System.Net;
using Microsoft.AspNetCore.Mvc;

namespace WebApplication.Controllers
{
    [Route("api")]
    public class ValuesController : Controller
    {
        [HttpGet]
        [Route("values/{top}")]
        public IActionResult Get(int top)
        {
            // Generate dummy values
            var list = Enumerable.Range(0, DateTime.Now.Second)
                                 .Select(i => $"Value {i}")
                                 .ToList();
            list.Reverse();

            var result = new ObjectResult(list.Take(top))
            {
                StatusCode = (int)HttpStatusCode.OK
            };

            Response.Headers.Add("X-Total-Count", list.Count.ToString());

            return result;
        }
    }
}

URL looks like http://localhost:3377/api/values/5 and results (for 19 dummy records generated, so X-Total-Count value will be 19) are like:

["Value 18","Value 17","Value 16","Value 15","Value 14"]

A custom attribute can be a good way.

https://docs.microsoft.com/en-us/aspnet/core/mvc/controllers/filters?view=aspnetcore-2.2

public class AddHeaderAttribute : ResultFilterAttribute
{
    private readonly string _name;
    private readonly string _value;

    public AddHeaderAttribute(string name, string value)
    {
        _name = name;
        _value = value;
    }

    public override void OnResultExecuting(ResultExecutingContext context)
    {
        context.HttpContext.Response.Headers.Add(_name, new string[] { _value });
        base.OnResultExecuting(context);
    }
}

Then use it like this on your API method

[AddHeader("X-MyHeader", "123")]

If you have a common header you can just extend this class :

public class MySpecialHeaderAttribute : AddHeaderAttribute
{
    public MySpecialHeaderAttribute() : base("X-MyHeader", "true")
    {
    }
}