ASP.NET WebApi：不允许使用（405）方法

我有一个Web Api控制器。它的行为非常奇怪。当我使用PostMan时，我可以在Web Api上访问POST方法，但是当我从.net使用HttpWebRequest时，它将返回（405）不允许的方法。我将Web Api代码放在这里：

using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Web;
using System.Web.Http;

namespace MyProject.Controllers
{
    public class TestController : ApiController
    {
        [HttpPost]
        public int buy(OrderResult value)
        {
            try
            {
                if (value.InvoiceDate != null && value.Result == 1)
                {
                    return 0;
                }
            }
            catch{}

            return -1;
        }
    }

    public class OrderResult
    {
        public int Result { get; set; }
        public long InvoiceNumber { get; set; }
        public string InvoiceDate { get; set; }
        public long TimeStamp { get; set; }
    }
}

这是我的WebApiConfig.cs：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web.Http;

namespace MyProject
{
    public static class WebApiConfig
    {
        public static void Register(HttpConfiguration config)
        {

            config.Routes.MapHttpRoute(
                name: "DefaultApi",
                routeTemplate: "api/{controller}/{action}/{id}",
                defaults: new { action = RouteParameter.Optional, id = RouteParameter.Optional }
            );

        }
    }
}

这就是我从另一个.NET项目发送POST请求的方式：

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net;
using System.Text;
using System.Web;

namespace MyProject.Controllers
{
    public static class WebReq
    {
        public static string PostRequest(string Url, string postParameters)
        {
            try
            {
                HttpWebRequest myReq = (HttpWebRequest)WebRequest.Create(Url);
                myReq.Method = "POST";
                myReq.Timeout = 30000;
                myReq.Proxy = null;

                byte[] postData = Encoding.UTF8.GetBytes(postParameters);
                myReq.ContentLength = postData.Length;
                myReq.ContentType = "application/x-www-form-urlencoded";
                using (Stream requestWrite = myReq.GetRequestStream())
                {
                    requestWrite.Write(postData, 0, postData.Length);
                    requestWrite.Close();
                    using (HttpWebResponse webResponse = (HttpWebResponse)myReq.GetResponse())
                    {
                        if (webResponse.StatusCode == HttpStatusCode.OK)
                        {
                            using (Stream str = webResponse.GetResponseStream())
                            {
                                using (StreamReader sr = new StreamReader(str))
                                {
                                    return sr.ReadToEnd();
                                }
                            }
                        }
                        return null;
                    }
                }
            }
            catch (Exception e)
            {
                var message = e.Message;
                return null;
            }
        }

    }
}

我已经在web.config中添加了以下代码：

<modules runAllManagedModulesForAllRequests="true">
  <remove name="WebDAVModule" />
</modules>

很奇怪，因为我可以从PostMan成功发送POST请求。邮递员将此代码发送到API。

POST /api/Test/buy HTTP/1.1
Host: domain.com
Cache-Control: no-cache
Postman-Token: 9220a4e6-e707-5c5f-ea61-55171a5dd95f
Content-Type: application/x-www-form-urlencoded

InvoiceDate=28012016&Result=1

对于解决问题的任何建议，我将不胜感激。