How to pass values by reference in a web service operation?

how can I pass values by reference in a web service operation of Silverlight?
can anyone help me with this?

this is my code...
 

string sCompanyID = txtCompanyID.Text;
                string sUserName = txtUserName.Text;
                string sPassword = txtPassword.Password;
                string sToken = string.Empty;               

                ServiceReference1.udvUserProfile oUserProfile = new ServiceReference1.udvUserProfile();
                ServiceReference1.udvCompanyProfile oCompanyProfile = new ServiceReference1.udvCompanyProfile();

                //convert del saldecimal
                int iSingInReturn = Convert.ToInt32(client.SignInAsync(sCompanyID, sUserName, sPassword, ref sToken, ref oUserProfile, ref oCompanyProfile).Value);

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Error:      12      Argument 4 should not be passed with the 'ref' keyword      

thanks in advance...