Soap header not displayed in ASP.NET C#

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C#

ASP.NET

SQL-Server-2008R2

I am implementing soap header in asp.net for web api authentication i got error as
System.NullReferenceException: Object reference not set to an instance of an object
where i have to initiate the variable like username and password.

What I have tried:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Services;
using System.Web.Services.Protocols;

namespace testapp
{
    /// <summary>
    /// Summary description for Service
    /// </summary>
    [WebService(Namespace = "http://tempuri.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    [System.ComponentModel.ToolboxItem(false)]
    // To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
    // [System.Web.Script.Services.ScriptService]
    public class Service : System.Web.Services.WebService
    {
    public AuthSoapHd spAuthenticationHeader;

    public Service () {

        //Uncomment the following line if using designed components 
        //InitializeComponent(); 
    }

    [WebMethod, SoapHeader("spAuthenticationHeader")]
    public string HelloWorld() 
    {
        if (spAuthenticationHeader.strUserName == "TestUser" && spAuthenticationHeader.strPassword == "TestPassword")
        {
            return "User Name : " + spAuthenticationHeader.strUserName + " and " + "Password : " + spAuthenticationHeader.strPassword;
        }
        else
        {
            return "Access Denied";
        }
    }

    public class AuthSoapHd : SoapHeader
    {
        public string strUserName;
        public string strPassword;
    }

    }
}