An entity object cannot be referenced by multiple instances of IEntityChangeTracker 的解决方案

使用EF对建立了关系的表新增记录时出现：

An entity object cannot be referenced by multiple instances of IEntityChangeTracker 或一个实体对象不能由多个 IEntityChangeTracker 实例引用

 

在学习MVC+EF做demo时碰到的一个异常信息。在网上查了些，看得不是很明白，就自己折腾了一会儿。

先上出错的代码：

public class CollegeInfo
{
private StudentManageContext stuCtx=new StudentManageContext();
public Models.CollegeInfoModel GetModel(Guid collegeId)
{
var model = stuCtx.CollegeInfoes.SingleOrDefault(s => s.CollegeId == collegeId);
return model;
}
public int Add(Models.CollegeInfoModel model)
{
stuCtx.CollegeInfoes.Add(model);
int count = stuCtx.SaveChanges();
return count;
}
public int Update(Models.CollegeInfoModel model)
{
Models.CollegeInfoModel oldModel = stuCtx.CollegeInfoes.Find(model.CollegeId);
oldModel = model;
stuCtx.Entry(oldModel).State = System.Data.EntityState.Modified;
return stuCtx.SaveChanges();
}
}

public class DepartmentInfo
{
private StudentManageContext stuCtx=new StudentManageContext();
public Models.DepartmentInfoModel GetModel(Guid departmentId)
{
var model = stuCtx.DepartmentInfoes.SingleOrDefault(s => s.DepartmentId == departmentId);
return model;
}
public int Add(Models.DepartmentInfoModel model)
{
stuCtx.DepartmentInfoes.Add(model);
int count = stuCtx.SaveChanges();
return count;
}
public int Update(Models.DepartmentInfoModel model)
{
Models.DepartmentInfoModel oldModel = stuCtx.DepartmentInfoes.Find(model.DepartmentId);
oldModel = model;
stuCtx.Entry(oldModel).State = System.Data.EntityState.Modified;
return stuCtx.SaveChanges();
}
}

使用表关系图:

两个表是一对多的关系。DepartmentInfo 中的collegeInfoId是外键。

 

添加DepartmentInfo代码：

static void Main(string[] args)
{
Database.SetInitializer<StudentManageContext>(new DropCreateDatabaseIfModelChanges<StudentManageContext>());

CollegeInfoModel collegex = new BLL.CollegeInfo().GetModel(new Guid("B956CA6F-DD7D-4436-9099-484366A5F0B7"));

DepartmentInfoModel depart = new DepartmentInfoModel()
{
DepartmentId = Guid.NewGuid(),
DepartmentName = "depart" + DateTime.Now.ToString(),
CollegeInfoObj = collegex
};
new BLL.DepartmentInfo().Add(depart);
}

这时会出现An entity object cannot be referenced by multiple instances of IEntityChangeTracker 或一个实体对象不能由多个 IEntityChangeTracker 实例引用的异常

 

原因是  CollegeInfoModel collegex = new BLL.CollegeInfo().GetModel(new Guid("B956CA6F-DD7D-4436-9099-484366A5F0B7"));  和  new BLL.DepartmentInfo().Add(depart); 这两句代码使用的是两个不同的StudentManageContext 对象造成的，可以仔细看看出错代码，两个类都有 private StudentManageContext stuCtx=new StudentManageContext(); ，到这儿你是否明白了呢。简单的来说，你要做的操作都必须在同一DbContext上完成。注：StudentManageContext派生于DbContext.

 

解决方案一：

static void Main(string[] args)
{
Database.SetInitializer<StudentManageContext>(new DropCreateDatabaseIfModelChanges<StudentManageContext>());
CollegeInfoModel collegex = new BLL.CollegeInfo().GetModel(new Guid("B956CA6F-DD7D-4436-9099-484366A5F0B7"));
DepartmentInfoModel depart = new DepartmentInfoModel()
{
DepartmentId = Guid.NewGuid(),
DepartmentName = "depart" + DateTime.Now.ToString(),
};
//new BLL.DepartmentInfo().Add(depart);
将这句代码改成下面两句
collegex.Departments.Add(depart);
new BLL.CollegeInfo().Update(collegex);
}

原因就是：Update时与GetModel用得时同一个DBContext。

解决方案二：

public class CollegeInfo:BaseModel
{
public Models.CollegeInfoModel GetModel(Guid collegeId)
{
var model = stuCtx.CollegeInfoes.SingleOrDefault(s => s.CollegeId == collegeId);
return model;
}
public int Add(Models.CollegeInfoModel model)
{
stuCtx.CollegeInfoes.Add(model);
int count = stuCtx.SaveChanges();
return count;
}
public int Update(Models.CollegeInfoModel model)
{
Models.CollegeInfoModel oldModel = stuCtx.CollegeInfoes.Find(model.CollegeId);
oldModel = model;
stuCtx.Entry(oldModel).State = System.Data.EntityState.Modified;
return stuCtx.SaveChanges();
}
}
public class DepartmentInfo:BaseModel
{
public Models.DepartmentInfoModel GetModel(Guid departmentId)
{
var model = stuCtx.DepartmentInfoes.SingleOrDefault(s => s.DepartmentId == departmentId);
return model;
}
public int Add(Models.DepartmentInfoModel model)
{
stuCtx.DepartmentInfoes.Add(model);
int count = stuCtx.SaveChanges();
return count;
}
public int Update(Models.DepartmentInfoModel model)
{
Models.DepartmentInfoModel oldModel = stuCtx.DepartmentInfoes.Find(model.DepartmentId);
oldModel = model;
stuCtx.Entry(oldModel).State = System.Data.EntityState.Modified;
return stuCtx.SaveChanges();
}
}
public abstract class BaseModel
{
protected StudentManageContext stuCtx = null;
public BaseModel()
{
stuCtx = DataCache.GetCache("StudentManageContext") as StudentManageContext;
if (stuCtx == null)
{
stuCtx = new StudentManageContext();
DataCache.SetCache("StudentManageContext", stuCtx);
}
}
}

这样的话可以让CollegeInfo和DepartmentInfo共用同一个DBContext。