string retVal = "";
string sql = "select wname from tablename where wno='xxx'";

using (SqlConnection conn = new SqlConnection(connectionString))
{
    SqlCommand cmd = new SqlCommand();
    cmd.CommandText = strSql;
    cmd.CommandType = CommandType.Text;
    cmd.Connection = conn;
    conn.Open();
    retVal = cmd.ExecuteScalar().ToString();
    conn.Close();
}
return retVal;