无法使用类型为“System.Data.Entity.Core.Objects.ObjectQuery”的实例进行调用

我想通过 userId 查找用户名

这个代码片段工作

Discussion_CreateBy = db.AspNetUsers.Find(discussion.CreatedBy).UserName,

这一次在以下控制器类中不起作用

Comment_CreateBy = db.AspNetUsers.Find(c.CreatedBy).UserName,

这是我的模型课程

public class DiscussionVM
{
    public int Disussion_ID { get; set; }
    public string Discussion_Title { get; set; }
    public string Discussion_Description { get; set; }
    public Nullable<System.DateTime> Discussion_CreateDate { get; set; }
    public string Discussion_CreateBy { get; set; }

    public string Comment_User { get; set; }

    public IEnumerable<CommentVM> Comments { get; set; }

}

public class CommentVM
{

    public int Comment_ID { get; set; }
    public Nullable<System.DateTime> Comment_CreateDate { get; set; }
    public string Comment_CreateBy { get; set; }
    public string Comment_Description { get; set; }

}

这是整个控制器类

    public ActionResult Discussion_Preview()
    {
        int Discussion_ID = 1;

        var discussion = db.AB_Discussion.Where(d => d.Discussion_ID == Discussion_ID).FirstOrDefault();
        var comments = db.AB_DiscussionComments.Where(c => c.Discussion_ID == Discussion_ID);



        DiscussionVM model = new DiscussionVM()
        {

            Disussion_ID = discussion.Discussion_ID,
            Discussion_Title = discussion.Discussion_Name,
            Discussion_Description = discussion.Discussion_Name,
            Discussion_CreateBy = db.AspNetUsers.Find(discussion.CreatedBy).UserName,
            Discussion_CreateDate = discussion.CreatedDate,

            Comments = comments.Select(c => new CommentVM()

            {
                Comment_ID = c.Comment_ID,
                Comment_Description = c.Comment_Discription,
                Comment_CreateBy = db.AspNetUsers.Find(c.CreatedBy).UserName,                    
                Comment_CreateDate = c.CreatedDate

            })

        };



        return View(model);
    }

出现以下错误

在类型“System.Data.Entity.DbSet”上声明的方法“Project.Models.AspNetUser Find(System.Object[])”1[Project.Models.AspNetUser]' cannot be called with instance of type 'System.Data.Entity.Core.Objects.ObjectQuery1[项目.模型.AspNetUser]'

Discussion_CreateBy = db.AspNetUsers.Find(discussion.CreatedBy).UserName

之所以有效，是因为讨论是内存中的对象，因为您正在通过调用执行查询FirstOrDefault on it:

var discussion = db.AB_Discussion.Where(d => d.Discussion_ID == Discussion_ID).FirstOrDefault();

另一方面，在以下声明中：

db.AspNetUsers.Find(c.CreatedBy).UserName

c尚未查询，因为

db.AB_DiscussionComments.Where(c => c.Discussion_ID == Discussion_ID)

返回一个IQueriable而不是实际的评论集合

解决这个问题的最简单方法是将您的所有评论放入内存中（因为无论如何您都需要它们）：

var comments = db.AB_DiscussionComments.Where(c => c.Discussion_ID == Discussion_ID).ToList();