把这段代码改写成输出一个txt文件，写入读出的信息

可以使用C#中的File类来实现将数据写入txt文件的操作。具体实现代码如下：

using System.IO;
using System.Text;

public static void WriteToFile(string text)
{
    string filePath = "output.txt";
    using (StreamWriter writer = new StreamWriter(filePath, true, Encoding.UTF8))
    {
        writer.WriteLine(text);
    }
}

public static List<Ty_Station> GetStationInfo()
{
    var Db = GetDB();
    var stations = Db.Queryable<Ty_Station>().ToList();
    StringBuilder sb = new StringBuilder();
    foreach (var station in stations)
    {
        sb.AppendLine($"StationId: {station.Id}, StationName: {station.Name}, StationAddress: {station.Address}");
    }
    WriteToFile(sb.ToString());
    return stations;
}

public static void InsertMonitorData(List<Ty_Monitor> monitors,DateTime dt)
{
    var Db=GetDB();
    Db.Deleteable<Ty_Monitor>().Where(it=>it.observtime==dt).ExecuteCommand();
    Db.Insertable<Ty_Monitor>(monitors).ExecuteCommand();
    StringBuilder sb = new StringBuilder();
    foreach (var monitor in monitors)
    {
        sb.AppendLine($"MonitorId: {monitor.Id}, StationId: {monitor.StationId}, ObservTime: {monitor.ObservTime}");
    }
    WriteToFile(sb.ToString());
}

public static string SendSms(string content,string phones)
{
    SMSService.SMSWebServiceSoapClient client = new SMSService.SMSWebServiceSoapClient();
    return client.SendMultiSmsByPhones("【天元监控】"+content, phones,"X","X");
}

public static SqlSugarClient GetDB()
{
    SqlSugarClient Db = new SqlSugarClient(new ConnectionConfig()
    {
        ConnectionString = "Server=ip;Database=MonitorDB;User=name;Password=m;MultipleActiveResultSets=True",
        DbType = DbType.SqlServer,
        IsAutoCloseConnection = true
    }, db =>
    {
        db.Aop.OnLogExecuting = (sql, pars) =>
        {
            WriteToFile(sql);
        };
    });
    return Db;
}

以上代码中，我们新增了一个`WriteToFile`方法，用于将文本数据写入到指定的文件中。我们在`GetStationInfo`和`InsertMonitorData`方法中，将需要输出的信息先存储到`StringBuilder`中，然后再调用`WriteToFile`方法将数据写入到文件中。