c# - 当对象从另一个类返回时无法保存 Entity Framework 中的更改

Question

我正在开发从数据库更新记录的简单 MVC 应用程序。我已经使用 BAL 和 Controller 来获取记录和更新，如下所示

Student.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace WebApplication1.DAL
{
    public class Student
    {
        DemoDatabaseEntities db = new DemoDatabaseEntities();
        public Student_Mast getStudentByID()
        {
            Student_Mast student = db.Student_Mast.Where(i => i.StudID == 1).FirstOrDefault();
            return student;
        }
    }
}

和StudentController.cs

public class UpdateController : Controller
{
    DemoDatabaseEntities db = new DemoDatabaseEntities();
    Student _bal = new Student();
    / GET: Update
    public ActionResult Index()
    {
        Student_Mast model = new Student_Mast();
        model = _bal.getStudentByID();
        model.First_Name = "Hardik";
        model.Last_Name = "Gondalia";
        db.SaveChanges();
        return View();
    }
}

但是 db.savechanges() 并没有更新记录。
如果我像下面这样更新记录，而不是这个，它会起作用
StudentController.cs

public class UpdateController : Controller
{
    DemoDatabaseEntities db = new DemoDatabaseEntities();
    Student _bal = new Student();
    / GET: Update
    public ActionResult Index()
    {
        Student_Mast model = new Student_Mast();
        model = db.Student_Mast.Where(i => i.StudID == 1).FirstOrDefault();
        model.First_Name = "Hardik";
        model.Last_Name = "Gondalia";
        db.SaveChanges();
        return View();
    }
}

Answer 1

由于您正在从 DbContext 的不同实例中检索记录，因此无法保存它。解决方案是向您的 Student 类添加一个构造函数，该类采用 DbContext 实例并将其保存到您拥有的变量中。

public class Student
{
    private DemoDatabaseEntities db = null;

    public Student(DemoDatabaseEntities dbContext){
    {
        this.db = dbContext;
    }

    public Student_Mast getStudentByID()
    {
        Student_Mast student = db.Student_Mast.Where(i => i.StudID == 1).FirstOrDefault();
        return student;
    }
}