EntityFramework 学习 一 DbContext

上一节中EDM自动生成SchoolEntities类，该类继承DbContext

EntityFramework4.1之前的版本，EDM生成的类继承ObjectContext，使用ObjectContext稍微有点棘手，DbContext概念上与ObjectContext相似，它是ObjectContext的封装，DbContext是EF重要的组成部分，它是领域或实体类和数据库的桥梁

DbContext是主要的类负责数据和对象互相转化

EntitySet:  DbContext包含实体集合（DBSet<TEntity>），所有与数据库表对应的实体

Querying: DbContext将LINQ-to-Entity查询语言转化成SQL查询语言，并发送给数据库

Change Tracking: 当对象已经从数据库中查询到后，DbContext跟踪实体的变化情况

Persisting Data: DbContext执行插入、更新、删除操作

Caching: DbContext默认使用一级缓存，在一个context对象周期内，它缓存所有已经被查询的实体

Manage Relationship: DbContext管理对象间的关系

namespace EFTutorials
{
    using System;
    using System.Data.Entity;
    using System.Data.Entity.Infrastructure;
    using System.Data.Entity.Core.Objects;
    using System.Linq;

    public partial class SchoolDBEntities : DbContext
    {
        public SchoolDBEntities()
            : base("name=SchoolDBEntities")
        {
        }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            throw new UnintentionalCodeFirstException();
        }

        public virtual DbSet<Course> Courses { get; set; }
        public virtual DbSet<Standard> Standards { get; set; }
        public virtual DbSet<Student> Students { get; set; }
        public virtual DbSet<StudentAddress> StudentAddresses { get; set; }
        public virtual DbSet<Teacher> Teachers { get; set; }
        public virtual DbSet<View_StudentCourse> View_StudentCourse { get; set; }

        public virtual ObjectResult<GetCoursesByStudentId_Result> GetCoursesByStudentId(Nullable<int> studentId)
        {
            var studentIdParameter = studentId.HasValue ?
                new ObjectParameter("StudentId", studentId) :
                new ObjectParameter("StudentId", typeof(int));

            return ((IObjectContextAdapter)this).ObjectContext.ExecuteFunction<GetCoursesByStudentId_Result>("GetCoursesByStudentId", studentIdParameter);
        }

        public virtual int sp_DeleteStudent(Nullable<int> studentId)
        {
            var studentIdParameter = studentId.HasValue ?
                new ObjectParameter("StudentId", studentId) :
                new ObjectParameter("StudentId", typeof(int));

            return ((IObjectContextAdapter)this).ObjectContext.ExecuteFunction("sp_DeleteStudent", studentIdParameter);
        }

        public virtual ObjectResult<Nullable<decimal>> sp_InsertStudentInfo(Nullable<int> standardId, string studentName)
        {
            var standardIdParameter = standardId.HasValue ?
                new ObjectParameter("StandardId", standardId) :
                new ("StandardId", typeof(int));

            var studentNameParameter = studentName != null ?
                new ObjectParameter("StudentName", studentName) :
                new ObjectParameter("StudentName", typeof(string));

            return ((IObjectContextAdapter)this).ObjectContext.ExecuteFunction<Nullable<decimal>>("sp_InsertStudentInfo", standardIdParameter, studentNameParameter);
        }

        public virtual int sp_UpdateStudent(Nullable<int> studentId, Nullable<int> standardId, string studentName)
        {
            var studentIdParameter = studentId.HasValue ?
                new ObjectParameter("StudentId", studentId) :
                new ObjectParameter("StudentId", typeof(int));

            var standardIdParameter = standardId.HasValue ?
                new ObjectParameter("StandardId", standardId) :
                new ObjectParameter("StandardId", typeof(int));

            var studentNameParameter = studentName != null ?
                new ObjectParameter("StudentName", studentName) :
                new ObjectParameter("StudentName", typeof(string));

            return ((IObjectContextAdapter)this).ObjectContext.ExecuteFunction("sp_UpdateStudent", studentIdParameter, standardIdParameter, studentNameParameter);
        }
    }
}     

DbContext实例化，用来执行GRUD操作

using (var ctx = new SchoolDBEntities())
{

    //Can perform CRUD operation using ctx here..
}

从DbContext中获取ObjectContext

DbContext API比ObjectContext用起来简单些，然而，你可以从DbContext中得到ObjectContext的引用，

using (var ctx = new SchoolDBEntities())
{
    var objectContext = (ctx as System.Data.Entity.Infrastructure.IObjectContextAdapter).ObjectContext;

    //use objectContext here..
}