How to save SOAP Request in a file while receiving it

Question

Hello, 

I need to log the SOAP Requests sent from a web service. How can I get a SOAP Request saved in an xml file for error handling? Here's the following example:

using System.Web.Services;

namespace TestWebService
{
    [WebService(Namespace = "http://tempuri.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    [System.ComponentModel.ToolboxItem(false)]
    public class Test : System.Web.Services.WebService
    {

        [WebMethod]
        public string HelloWorld(int a, int b)
        {
            int c = a + b;

            return "The result is: "+c;
        }
    }
}

Request (needs to be saved to file):

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:tem="http://tempuri.org/">
   <soapenv:Header/>
   <soapenv:Body>
      <tem:HelloWorld>
         <tem:a>5</tem:a>
         <tem:b>3</tem:b>
      </tem:HelloWorld>
   </soapenv:Body>
</soapenv:Envelope>

Any kind of help would be appreciated, thank you!