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。