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

具体详细错误信息：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.

代码片段：

                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();
                }

介绍下业务场景：导入时excel中存在重复数据。更新数据用的UnitWork。

上面的报错信息很明显了，它告诉我们一条数据已经被trace了，不能重复trace，只要我们把这条被trace的数据取消就行了。

下面是修改后的代码：

                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，修改当前数据的Entry状态为Detached。