The instance of entity type 'xxx' cannot be tracked because another instance with the same key value for {'Id'} is already being tracked.

具體詳細錯誤信息：The instance of entity type 'xxx' cannot be tracked because another instance with the same key value for {'Id'} is already being tracked. When attaching existing entities, ensure that only one entity instance with a given key value is attached. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see the conflicting key values.

代碼片段：

                foreach (var item in list)
                {
                    var func = this.GetCodeCheckFunction(item);
                    var gauge = UnitWork.FindSingle(func);

                    if (gauge == null)
                    {
                        //新增
                        UnitWork.Add(item);
                    }
                    else
                    {
                        //修改
                        gauge.GaugeType = item.GaugeType;
                        gauge.ModifiedBy = _auth.GetCurrentUser().User.Id;
                        gauge.ModifiedTime = DateTime.Now;
                        UnitWork.Update(gauge);
                    }
                    UnitWork.Save();
                }

介紹下業務場景：導入時excel中存在重復數據。更新數據用的UnitWork。

上面的報錯信息很明顯了，它告訴我們一條數據已經被trace了，不能重復trace，只要我們把這條被trace的數據取消就行了。

下面是修改后的代碼：

                foreach (var item in list)
                {
                    var func = this.GetCodeCheckFunction(item);
                    var gauge = _dbContext.MesInfoGauges.FirstOrDefault(func);

                    if (gauge == null)
                    {
                        //新增
                        _dbContext.MesInfoGauges.Add(item);
                        _dbContext.SaveChanges();
                        _dbContext.Entry(item).State = EntityState.Detached;
                    }
                    else
                    {
                        //修改
                        gauge.GaugeType = item.GaugeType;
                        gauge.ModifiedBy = user.Id;
                        gauge.ModifiedTime = DateTime.Now;
                        _dbContext.MesInfoGauges.Update(gauge);
                        _dbContext.SaveChanges();
                        _dbContext.Entry(gauge).State = EntityState.Detached;
                    }
                }

保存后，修改當前數據的Entry狀態為Detached。