ASP.Net MVC Linq to Sql如何处理null datetime

Question

我有一个Topic父表,一个Post表childed到Topic表.

我在Linq查询中尝试做的是从链接的Post表返回最后一个发布日期,但是,如果没有Posts,则下面的查询失败,因为DateTime不可为空:

The cast to value type 'DateTime' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type.

查询是:

var topic = db.Topics.Include(x => x.Posts).Include(x => x.Forum).Where(x => x.ForumId==id)
           .Select(t => new TopicViewModel
             {
                 TopicId =t.TopicId,
                 ForumId=t.ForumId,
                 Title=t.Title,
                 DateOfTopic=t.DateOfPost,
                 Replies=t.Posts.Count()-1,
                 Author=t.Author,
                 Views = t.Views,
                 LastPost = t.Posts.OrderByDescending(x => x.DateOfPost).FirstOrDefault().Author,
                 LastPostDate = t.Posts.OrderByDescending(x => x.DateOfPost).FirstOrDefault().DateOfPost
             }).OrderByDescending(x=> x.DateOfTopic).ToList();

我的ViewModel是:

public class TopicViewModel
{
    public int TopicId { get; set; }
    [Required]
    public string Title { get; set; }
    public string Author { get; set; }
    public DateTime DateOfTopic { get; set; }
    public int Replies { get; set; }
    public int Views { get; set; }
    public string LastPost { get; set; }
    public DateTime LastPostDate { get; set; }
    public int ForumId { get; set; }
}

无论如何改变这条线:

LastPostDate = t.Posts.OrderByDescending(x => x.DateOfPost).FirstOrDefault().DateOfPost

...如果DateOfPost为null,它不会出错？

Answer 1

你可以做你的财产 Nullable

public class x {
public DateTime? nullableDate {get; set;}
}

这应该可以解决您的问题.该问号确保您可以拥有Null该nullableDate物业