一个实体对象不能由多个 IEntityChangeTracker 实例引用

错误：一个实体对象不能由多个 IEntityChangeTracker 实例引用

原因：

之前的代码：db.T_DeviceVarDataCurrent.Add(current);

SaveChanges执行后current对象中的一些值（或引用）会发生改变，导致catch 中obj发生改变。

修改：try中list不参与数据库操作，或者catch中重新赋值给新对象current2。

如下：

/// <summary>
///  最新数据
/// </summary>
/// <param name="str"></param>
/// <param name="minuteInterval"></param>
public void AddDeviceVarDataCurrent(string str)
{
    var waterSensor = JsonConvert.DeserializeObject<WaterSensorRealTimeModel>(str);
    if (waterSensor.CP != null && waterSensor.CP.data != null && waterSensor.CP.data.Count() > 0)
    {
       
        //如果不报错，一下插入多条。
        using (var db = new FarmSensorDataWaterEntities())
        {
            var tran = db.Database.BeginTransaction();//开启事务
            var list = new List<T_DeviceVarDataCurrent>() { };
            var nowTime = DateTime.Now;
            try
            {
                var varList = db.T_Var.ToList();
                var deviceList= db.T_Device.ToList();
                var varTypeList = db.T_VarType.ToList();
                var deviceVarList = db.T_DeviceVar.ToList();
                foreach (var item in waterSensor.CP.data)
                {
                    var dateTime = GetDateTime(item.SampleTime);
                    //如果是正常值
                    if (item.Flag == "N" && dateTime != null)
                    {
                        var varId = varList.Where(d => d.Tag == item.code).Select(d => d.VarID).SingleOrDefault();
                        var deviceId = deviceList.Where(d => d.Tag == waterSensor.MN&& d.ProviderID==1).Select(d => d.DeviceID).SingleOrDefault();//中科山东东营地理研究院
                        var typeList = varTypeList.Where(d => d.TypeID ==1).ToList();// 实时值
                        if (varId != 0 && deviceId != 0)
                        {
                            foreach (var type in typeList)
                            {
                                var deviceVarID = deviceVarList.Where(d => d.VarID == varId && d.DeviceID == deviceId && d.TypeID == type.TypeID).Select(d => d.DeviceVarID).SingleOrDefault();
                                if (deviceVarID != 0)
                                {
                                    //存储list
                                    var current = new T_DeviceVarDataCurrent
                                    {
                                        DeviceVarID = deviceVarID,
                                        Value = item.Rtd,
                                        VarDate = (DateTime)dateTime,
                                        UpdateTime = nowTime,
                                    };
                                    list.Add(current);
 
                                    var currentInfo = db.T_DeviceVarDataCurrent.Where(d => d.DeviceVarID == deviceVarID).SingleOrDefault();
                                    if (currentInfo != null)                                         
                                    {   //修改
                                        //currentInfo.Value = item.Rtd;
                                        //currentInfo.VarDate = (DateTime)dateTime;
                                        //currentInfo.UpdateTime = nowTime; 
                                        currentInfo.Value = current.Value;
                                        currentInfo.VarDate = current.VarDate;
                                        currentInfo.UpdateTime = current.UpdateTime;
                                    }
                                    else
                                    {  //添加
                                        var current2 = new T_DeviceVarDataCurrent
                                        {
                                            DeviceVarID = deviceVarID,
                                            Value = item.Rtd,
                                            VarDate = (DateTime)dateTime,
                                            UpdateTime = nowTime,
                                        };
                                        db.T_DeviceVarDataCurrent.Add(current2);
                                    }
                                }
                            }
                        }
                    }
                }
                if (list.Count > 0)
                {
                    db.SaveChanges();
                     //必须调用Commit()，不然数据不会保存
                    tran.Commit();
 
                }
 
            }
            catch (Exception ex)
            {
                tran.Rollback(); //出错就回滚
                while (ex.InnerException != null)
                {
                    ex = ex.InnerException;
                }
                LogManage.Error(ex);
                //若重复错误，则逐条遍历。
                if (ex.Message.Contains("IX_"))
                {
                    foreach (var obj in list)
                    {
                        try
                        {
                            using (var db2 = new FarmSensorDataWaterEntities())
                            {
                                var currentInfo = db2.T_DeviceVarDataCurrent.Where(d => d.DeviceVarID == obj.DeviceVarID).SingleOrDefault();
                         
                                if (currentInfo != null)
                                {
                                    currentInfo.Value = obj.Value;
                                    currentInfo.VarDate = obj.VarDate;
                                    currentInfo.UpdateTime = nowTime;
                                }
                                else
                                {
                                    var current = new T_DeviceVarDataCurrent
                                    {
                                        DeviceVarID = obj.DeviceVarID,
                                        Value = obj.Value,
                                        VarDate = obj.VarDate,
                                        UpdateTime = obj.UpdateTime,
                                    };
                                    db2.T_DeviceVarDataCurrent.Add(current);
                                }
                                db2.SaveChanges();
 
                            }
                        }
                        catch (Exception ex2)
                        {
                            //LogManage.Error(ex2);
                        }
                    }
               }
            }
        }
    }
}