not all code path return a value

Question

hi,

   when i use the codings i have an error like not all code path return a value.how can i solve this

public

string

try{

       if

         {

                companymailid = mailname;

                Session["cmailid"] = companymailid;

                Server.Transfer(

          }

             {

                  companymailid = "";

              }

       }

else     return companymailid;}"mail.aspx");(mailname != null)companymailid;string mailid(string mailname){