用自定义的联接表创建多对多关系是不可能的。在多对多关系中，EF内部管理和隐藏联接表。这是一个在模型中没有Entity类的表。要使用具有其他属性的联接表，您实际上必须创建两个一对多关系。它可能看起来像这样：

public class Member
{
    public int MemberID { get; set; }

    public string FirstName { get; set; }
    public string LastName { get; set; }

    public virtual ICollection<MemberComment> MemberComments { get; set; }
}

public class Comment
{
    public int CommentID { get; set; }
    public string Message { get; set; }

    public virtual ICollection<MemberComment> MemberComments { get; set; }
}

public class MemberComment
{
    [Key, Column(Order = 0)]
    public int MemberID { get; set; }
    [Key, Column(Order = 1)]
    public int CommentID { get; set; }

    public virtual Member Member { get; set; }
    public virtual Comment Comment { get; set; }

    public int Something { get; set; }
    public string SomethingElse { get; set; }
}

例如，如果现在要查找带有LastName=“ Smith” 的成员的所有注释，则可以编写如下查询：

var commentsOfMembers = context.Members
    .Where(m => m.LastName == "Smith")
    .SelectMany(m => m.MemberComments.Select(mc => mc.Comment))
    .ToList();

… 要么 …

var commentsOfMembers = context.MemberComments
    .Where(mc => mc.Member.LastName == "Smith")
    .Select(mc => mc.Comment)
    .ToList();

或创建一个名称为“ Smith”（假设有多个）的成员及其注释的列表，您可以使用投影：

var membersWithComments = context.Members
    .Where(m => m.LastName == "Smith")
    .Select(m => new
    {
        Member = m,
        Comments = m.MemberComments.Select(mc => mc.Comment)
    })
    .ToList();

如果要查找成员MemberId= 1的所有注释：

var commentsOfMember = context.MemberComments
    .Where(mc => mc.MemberId == 1)
    .Select(mc => mc.Comment)
    .ToList();

现在，您还可以按联接表中的属性进行过滤（在多对多关系中这是不可能的），例如：过滤成员1中属性为99的所有注释Something：

var filteredCommentsOfMember = context.MemberComments
    .Where(mc => mc.MemberId == 1 && mc.Something == 99)
    .Select(mc => mc.Comment)
    .ToList();

由于延迟加载，事情可能会变得更容易。如果您已加载Member，则无需显式查询即可获得注释：

var commentsOfMember = member.MemberComments.Select(mc => mc.Comment);

我想延迟加载会自动在后台获取评论。

编辑

仅出于娱乐目的，在此模型中还提供了一些示例，说明如何添加实体和关系以及如何删除它们：

1）创建一个成员和该成员的两个评论：

var member1 = new Member { FirstName = "Pete" };
var comment1 = new Comment { Message = "Good morning!" };
var comment2 = new Comment { Message = "Good evening!" };
var memberComment1 = new MemberComment { Member = member1, Comment = comment1,
                                         Something = 101 };
var memberComment2 = new MemberComment { Member = member1, Comment = comment2,
                                         Something = 102 };

context.MemberComments.Add(memberComment1); // will also add member1 and comment1
context.MemberComments.Add(memberComment2); // will also add comment2

context.SaveChanges();

2）添加member1的第三条评论：

var member1 = context.Members.Where(m => m.FirstName == "Pete")
    .SingleOrDefault();
if (member1 != null)
{
    var comment3 = new Comment { Message = "Good night!" };
    var memberComment3 = new MemberComment { Member = member1,
                                             Comment = comment3,
                                             Something = 103 };

    context.MemberComments.Add(memberComment3); // will also add comment3
    context.SaveChanges();
}

3）创建新成员并将其与现有评论2关联：

var comment2 = context.Comments.Where(c => c.Message == "Good evening!")
    .SingleOrDefault();
if (comment2 != null)
{
    var member2 = new Member { FirstName = "Paul" };
    var memberComment4 = new MemberComment { Member = member2,
                                             Comment = comment2,
                                             Something = 201 };

    context.MemberComments.Add(memberComment4);
    context.SaveChanges();
}

4）在现有的member2和comment3之间创建关系：

var member2 = context.Members.Where(m => m.FirstName == "Paul")
    .SingleOrDefault();
var comment3 = context.Comments.Where(c => c.Message == "Good night!")
    .SingleOrDefault();
if (member2 != null && comment3 != null)
{
    var memberComment5 = new MemberComment { Member = member2,
                                             Comment = comment3,
                                             Something = 202 };

    context.MemberComments.Add(memberComment5);
    context.SaveChanges();
}

5）再次删除此关系：

var memberComment5 = context.MemberComments
    .Where(mc => mc.Member.FirstName == "Paul"
        && mc.Comment.Message == "Good night!")
    .SingleOrDefault();
if (memberComment5 != null)
{
    context.MemberComments.Remove(memberComment5);
    context.SaveChanges();
}

6）删除member1及其与注释的所有关系：

var member1 = context.Members.Where(m => m.FirstName == "Pete")
    .SingleOrDefault();
if (member1 != null)
{
    context.Members.Remove(member1);
    context.SaveChanges();
}

该删除的关系MemberComments也因为之间的一个一对多的关系Member和MemberComments与之间Comment并MemberComments是建立与惯例级联删除。之所以如此，是因为将MemberId和CommentIdin
MemberComment检测为Member和Comment导航属性的外键属性，并且由于FK属性的类型为不可为空int，因此需要这种关系，最终导致级联删除设置。我认为在这种模型中是有道理的。