如何在EF Codefirst中创建可为空的属性？ [英] How to make a nullable property in EF Codefirst?

问题描述

我的书架测试应用程序中有两个POCO：

I have two POCOs in my "Bookshelf" test application:

/// <summary>
/// Represents a book
/// </summary>
public class Book
{
    public int ID { get; set; }
    public string Title { get; set; }
    public string Author { get; set; }
    public string ISBN { get; set; }
    public virtual Loaner LoanedTo { get; set; }
}

/// <summary>
/// Represents a Loaner
/// </summary>
public class Loaner
{
    public int ID { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Book> Loans { get; set; }
}

我的LoanedTo是否可以为空？我的意思是说书并不总是借来的，对！我尝试过

Is there a way that my LoanedTo could be nullable? I mean a book isn't always loaned, right! I tried

public virtual Loaner? LoanedTo { get; set; }

但我得到：
类型'RebtelTests.Models.Loaner'必须为非空值类型，以便在通用类型或方法 System.Nullable中将其用作参数 T

But I get: The type 'RebtelTests.Models.Loaner' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Nullable'

因此，我在某处一定在想错，但是我不知道。

So I must be thinking wrong somewhere, but I can't figure it out. Probably easy squeeze for you guys.

推荐答案

您不需要做任何特别的事情。类始终是可为空的。

You don't need to do anything special. Classes are always nullable.

我刚刚尝试了此操作（使用MVC3）：

I just tried this (with MVC3):

在我的模型目录中：

namespace MvcApplication2.Models
{
    public class Book
    {
        public int ID { get; set; }
        public string Title { get; set; }
        public string Author { get; set; }
        public string ISBN { get; set; }
        public virtual Loaner LoanedTo { get; set; }
    }

    public class Loaner
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public virtual ICollection<Book> Loans { get; set; }
    }

    public class BookContext : System.Data.Entity.DbContext
    {
        public System.Data.Entity.DbSet<Book> Books { get; set; }
        public System.Data.Entity.DbSet<Loaner> Loaners { get; set; }
    }
}

在我的HomeController中：

In my HomeController:

namespace MvcApplication2.Controllers
{
    public class HomeController : Controller
    {
        //
        // GET: /Home/

        public ActionResult Index()
        {
            string message = "OK";

            try
            {
                var context = new Models.BookContext();
                var book = new Models.Book();
                book.Title = "New Title";
                book.Author = "New Author";
                book.ISBN = "New ISBN";
                context.Books.Add(book);
                context.SaveChanges();
            }
            catch (Exception err)
            {
                message = err.ToString();
            }

            ViewBag.Message = message;

            return View();
        }

    }
}

连接字符串在Web.Config中：

The connectionstring in Web.Config:

<add name="BookContext" connectionString="Data Source=|DataDirectory|BookContext.sdf" providerName="System.Data.SqlServerCe.4.0" />

运行应用程序时，视图显示为 OK。这意味着不会引发任何异常。当我查看App_Data文件夹时，已经创建了一个BookContext.sdf文件。该数据库包含有关帐簿和贷款人的表格。贷款人表格为空。书中的一条包含一条记录：

When I run the application, the view displays "OK". This means that no exception was thrown. When I look in my App_Data folder, a BookContext.sdf file has been created. That database contains a table for the Books and the Loaners. The table for the Loaners is empty. The one for the Books contains one record:

ID: 1; Title: "New Title"; Author: "New Author"; ISBN: "New ISBN"; LoanerID: null

这篇关于如何在EF Codefirst中创建可为空的属性？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！