Is there a way to return an XML file contents using an ASP.NET API Controller?

Question

I am trying to emulate a server I don't have access to for testing. The server returns the contents of XML files that the client I am testing reads and loads into XmlDocument classes. It seems like it should be easy to read an xml file and return its contents but I can't seem to figure it out. If I load the file into an XmlDocument and return the InnerXML, the controller wraps the result in a root and the XmlDocument on the client balks at it.

<string xmlns="http://schemas.microsoft.com/2003/10/Serialization/"><?xml version="1.0" encoding="utf-8"?><credential xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"><user>User</user><password>Password</password><cookie>Cookie Text</cookie><site xmlns="vs">VS Site</site><funcList>func1, func2, func3</funcList></credential></string>

If I try to return the XmlDocument the controller throw an exception: 'System.Xml.XmlDocument' is an invalid collection type since it does not have a valid Add method with parameter of type 'System.Object'.

using Newtonsoft.Json;
using POSComWebTest.Models;
using System;
using System.Collections.Generic;
using System.IO;
using System.Web;
using System.Web.Http;
using System.Xml;
using System.Xml.Serialization;

namespace WshhWebApi.Controllers
{
    [RoutePrefix("SiteController")]
    public class AuthorizeController : ApiController
    {
        [Route("cgi-bin/CGILink")]
        [HttpGet]
        public IHttpActionResult CGILink(string cmd, string user, string passwd)
        {
            try
            {
                string file = Path.Combine(HttpContext.Current.Server.MapPath("~"), "App_Data", "credential.xml");
                XmlDocument xmlDoc = new XmlDocument();
                xmlDoc.Load(file);
                HttpContext.Current.Response.ContentType = "application/xml";
                //return Content(System.Net.HttpStatusCode.OK, xmlDoc.InnerText);  // garbage
                return Content(System.Net.HttpStatusCode.OK, xmlDoc.InnerXml); // <string root> client can't read as xml
                //return Content(System.Net.HttpStatusCode.OK, xmlDoc); // 'System.Xml.XmlDocument' is an invalid collection type since it does not have a valid Add method with parameter of type 'System.Object'.
            }
            catch (Exception ex)
            {
                return BadRequest($"CGILink Failed: {ex.Message}");
            }
        }
    }
}

It seems like this should be easy and I hope I just haven't Googled the wrong questions. Thanks!

Oh, here is my XML file credential.xml:

<?xml version="1.0" encoding="utf-8"?>
<credential xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <user>User</user>
  <password>Password</password>
  <cookie>Cookie Text</cookie>
  <site xmlns="vs">VS Site</site>
  <funcList>func1, func2, func3</funcList>
</credential>

Answer 1

Firstly I want to thank everybody for their answers! I think I figured it out. If I just download the file as 'application/octet-stream' the client is able to turn this into an XmlDocument. I really did not think it would work but I have spent too much time on this and am getting desperate. Below was my solution:

[Route("cgi-bin/CGILink")]
[HttpGet]
public IHttpActionResult CGILink(string cmd, string user, string passwd)
{
    try
    {
        string file = Path.Combine(HttpContext.Current.Server.MapPath("~"), "App_Data", "credential.xml");
        using (FileStream fileStream = File.OpenRead(file))
        {
            MemoryStream memStream = new MemoryStream();
            memStream.SetLength(fileStream.Length);
            fileStream.Read(memStream.GetBuffer(), 0, (int)fileStream.Length);
            var result = new HttpResponseMessage(HttpStatusCode.OK)
            {
                Content = new ByteArrayContent(memStream.GetBuffer())
            };

            result.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");
            var response = ResponseMessage(result);

            return response;
        }
    }
    catch (Exception ex)
    {
        return BadRequest($"CGILink Failed: {ex.Message}");
    }
}

Thanks again to everyone who responded.