c# - 如何解决 ASP.NET C# 中的类型转换错误？

Question

这是我的模型

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

namespace TheFoody.Models
{
    public class ManageViewModel
    {
        public string FirstName { get; set; }

        public string LastName { get; set; }

        public string Email { get; set; }

        public string Phone { get; set; }

        public string Photo { get; set; }

        public string Address { get; set; }

        public string City { get; set; }

        public int PostCode { get; set; }

        public string District { get; set; }

        public string UserType { get; set; }

        public string Status { get; set; }
    }
}

这是我的背景

    namespace TheFoody.DataAccess
{
    using System;
    using System.Data.Entity;
    using System.Data.Entity.Infrastructure;

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

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

        public virtual DbSet<Menu> Menus { get; set; }
        public virtual DbSet<User> Users { get; set; }
        public virtual DbSet<Category> Categories { get; set; }
        public virtual DbSet<Restaurant> Restaurants { get; set; }
        public virtual DbSet<Restaurant_Type> Restaurant_Type { get; set; }
    }
}

这是我的 Controller

 using System;
using System.Collections.Generic;
using System.Data.Entity;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using TheFoody.DataAccess;
using TheFoody.Models;

namespace TheFoody.Controllers
{
    public class ManageController : Controller
    {
        public ActionResult Manage()
        {
            return View();
        }

        [HttpPost]
        public ActionResult Manage(ManageViewModel manageviewmodel)
        {
            TheFoodyContext db = new TheFoodyContext();
            ManageViewModel user_to_update = db.Users.SingleOrDefault(s => s.email == manageviewmodel.Email);

            return View(manageviewmodel);
        }
    }
}

但是在这里，当我尝试编写这部分代码时“ManageViewModel user_to_update = db.Users.SingleOrDefault(s => s.email == manageviewmodel.Email);”它会给我突出显示名为“无法将类型‘TheFoody.DataAccess.User’隐式转换为‘TheFoody.Models.ManageViewModel’”的错误

我对这个环境很陌生，我不知道为什么会出现这样的错误。

Answer 1

db.Users是 DbSet<TheFoody.DataAccess.User> , 和 SingleOrDefault()返回一个这样的用户，如果该电子邮件地址未找到任何用户，则返回 null。

该类的实例不能隐式转换为请求的 ManageViewModel ，你需要映射它:

DataAccess.User user = db.Users.SingleOrDefault(...);
if (user == null)
{
    // show error page telling that the POSTed email address is not known
}

ManageViewModel user_to_update = new ManageViewModel 
{
    FirstName = user.FirstName,
    LastName = user.LastName,
    // ...
}
return View(user_to_update);

您可以使用AutoMapper 以更方便的方式进行映射。

虽然，您似乎想用通过 POST 发送的模型更新数据库，然后您需要更改逻辑:您需要分配 user收到的属性 manageviewmodel :

// read from database
DataAccess.User user = db.Users.SingleOrDefault(...);

if (user == null)
{
    // show error page telling that the POSTed email address is not known
}

// update from POST
user.FirstName = manageviewmodel.FirstName;
user.LastName = manageviewmodel.LastName;
// ...

// persist to database
db.SaveChanges();