Error converting data type varchar to bigint.

 protected void Button1_Click(object sender, EventArgs e)     {         string diaryno = getvalue("select max(letter_current_diary_no)+1 as letter_current_diary_no from offices where office_code = '" + Session["user_code"].ToString() + "'");                 string offcode = GetOfficecode("select OFFICE_CODE from USERS where USER_CODE='" + Session["user_code"].ToString() + "'");                // string diaryno = getdiaryno("select max(letter_current_diary_no)+1 as letter_current_diary_no  from OFFICES where OFFICE_CODE in(select OFFICE_CODE from USERS where USER_CODE='" + Session["user_code"].ToString() + "')"); ...