Add custom header to all responses in Web API

Previous answers to this question don't address what to do if your controller action throws an exception. There are two basic ways to get that to work:

Add an exception filter:

using System.Net;
using System.Net.Http;
using System.Web.Http.Filters;

public class HeaderAdderExceptionFilter : ExceptionFilterAttribute
{
    public override void OnException(HttpActionExecutedContext context)
    {
        if (context.Response == null)
            context.Response = context.Request.CreateErrorResponse(
                HttpStatusCode.InternalServerError, context.Exception);

        context.Response.Content.Headers.Add("header", "value");
    }
}

and in your WebApi setup:

configuration.Filters.Add(new HeaderAdderExceptionFilter());

This approach works because WebApi's default exception handler will send the HttpResponseMessage created in a filter instead of building its own.

Replace the default exception handler:

using System.Net;
using System.Net.Http;
using System.Web.Http.ExceptionHandling;
using System.Web.Http.Results;

public class HeaderAdderExceptionHandler : ExceptionHandler
{
    public override void Handle(ExceptionHandlerContext context)
    {
        HttpResponseMessage response = context.Request.CreateErrorResponse(
            HttpStatusCode.InternalServerError, context.Exception);
        response.Headers.Add("header", "value");

        context.Result = new ResponseMessageResult(response);
    }
}

and in your WebApi setup:

configuration.Services.Replace(typeof(IExceptionHandler), new HeaderAdderExceptionHandler());

You can't use both of these together. Okay, well, you can, but the handler will never do anything because the filter already converted the exception into a response.

Super important to note that as written, this code will send all the exception details to the client. You probably don't want to do this in production, so check out all the available overloads on CreateErrorResponse() and pick which one suits your needs.

For that you can use a custom ActionFilter (System.Web.Http.Filters)

public class AddCustomHeaderFilter : ActionFilterAttribute
{
    public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext)
    {
       actionExecutedContext.Response.Headers.Add("customHeader", "custom value date time");
    }
}

You can then apply the filter to all your controller's actions by adding this in the configuration in Global.asax for example :

GlobalConfiguration.Configuration.Filters.Add(new AddCustomHeaderFilter());

You can also apply the filter attribute to the action that you want without the global cofiguration line.

Julian's answer led me to have to create the filter but only using the the System.Web (v4) and System.Web.Http (v5) namespace (MVC packages were not part of this particular project this was used on.)

using System.Web;
using System.Web.Http.Filters;
...
public class AddCustomHeaderActionFilterAttribute : ActionFilterAttribute
{
    public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext)
    {
        base.OnActionExecuted(actionExecutedContext);
        actionExecutedContext.ActionContext.Response.Headers.Add("name", "value");
    }
}

And add it to the global.asax to have it used on every controller/action

        GlobalConfiguration.Configuration.Filters.Add(new AddCustomHeaderActionFilterAttribute());