public bool Authenticate()
  {
    String username = "......";
    String password = "......";
    Cookie _authCookie;
    CookieContainer _cookieContainer;


    AuthService.Authentication authentication = new AuthService.Authentication();
    authentication.Url = "<web_url>/_vti_bin/Authentication.asmx";
    authentication.CookieContainer = new CookieContainer();
    authentication.AllowAutoRedirect = true;
    authentication.PreAuthenticate = true;
    AuthService.LoginResult loginResult = authentication.Login(username, password);

    _authCookie = new Cookie();

    if (loginResult.ErrorCode == AuthService.LoginErrorCode.NoError)
    {
        CookieCollection cookies = authentication.CookieContainer.GetCookies(new Uri(authentication.Url));
        _authCookie = cookies[loginResult.CookieName];
        _cookieContainer = new CookieContainer();
        _cookieContainer.Add(_authCookie);
        return true;
    }
    else
        return false;
  }

AuthService.Authentication authentication = new AuthService.Authentication();
authentication.Url = "<web_url>/_vti_bin/Authentication.asmx";
authentication.CookieContainer = new CookieContainer();
authentication.AllowAutoRedirect = true;
authentication.PreAuthenticate = true;
AuthenticationSoap authenticationSoap = null;
authenticationSoap = authentication.getAuthenticationSoap();
AuthServiceLoginResult loginResult = authenticationSoap.Login(username, password);