How do you switch your .net app between connecting to Produciton and Sandbox web services?

I have code working on my Developer orgI now want to  move to my Sandbox.

 private const string _url = "https://test.salesforce.com/services/Soap/u/10.0";
    

        protected void Button1_Click(object sender, EventArgs e)
        {
            if (!isConnected())
            {
                getSessionInfo();
            }
            MyService ws = new MyService();
            ws.Url = Application["_serverUrl"].ToString();
            ws.Proxy = getProxy();       
            ws.SessionHeaderValue = new SessionHeader();
            ws.SessionHeaderValue.sessionId = Application["_sessionId"].ToString();
            string foo= ws.HelloWolrd("bar");


        }

        public WebProxy getProxy()
        {
            System.Net.WebProxy proxy = System.Net.WebProxy.GetDefaultProxy();
            proxy.Credentials = CredentialCache.DefaultCredentials;
            return proxy;
        }


        public bool isConnected()
        {
            bool functionReturnValue = false;
            if (Application["_sessionId"] != null )
            {
                if (DateTime.Now < (DateTime)(Application["_nextLoginTime"]) )
                {
                   return true;
                }
            }
           return false;
        }

        public void getSessionInfo()
        {
            SforceService ss = new SforceService();
            ss.Proxy = getProxy();
            ss.Url = _url;
            LoginResult lr = ss.login(_userId, _password);
            if (!lr.passwordExpired)
            {
                  Application["SforceService"] = ss;
                Application["_sessionId"] = lr.sessionId;
                Application["_serverUrl"] = lr.serverUrl;
                Application["_nextLoginTime"] = DateTime.Now.AddMinutes(_sessionTime);
            }
        }

this code works fine on my developer edition, but I cannot get it working on my sandbox.

The value for 

serverUrl = "https://tapp0-api.salesforce.com/services/Soap/u/10.0/00DT0000000Gg4C"

The error I get is:

 No operation available for request {http://soap.sforce.com/schemas/class/MyService}HelloWolrd