ASP.Net MVC Linq to Sql how to handle a null datetime

Question

I have a Topic parent table, and a Post table childed to the Topic table.

What I'm trying to do within the Linq query is return the last post date, from the linked Post table, however, if there are no Posts, then the query below fails, as DateTime is not nullable:

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.

The query is:

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();

My ViewModel is:

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; }
}

Is there anyway of changing this line:

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

...so that it doesn't error if DateOfPost is null?

Use nullable type: DateTime?

MarcinJuraszek
– MarcinJuraszek

2013-06-20 07:35:23 +00:00
Commented Jun 20, 2013 at 7:35 — MarcinJuraszek
– MarcinJuraszek, Commented Jun 20, 2013 at 7:35

Tikkes · Accepted Answer · 2013-06-20 07:37:20Z

6

You can make your property Nullable

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

This should fix your issue. The questionmark makes sure you can have Null in the nullableDate property

answered Jun 20, 2013 at 7:37

Tikkes

4,6874 gold badges38 silver badges66 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Mark Over a year ago

I thought I had tried that!! - Thank you. (I'll mark as answer when SO allows)

Oliver · Accepted Answer · 2013-06-20 07:37:45Z

1

You could use .GetValueOrDefault() to specify a default value if there is a null value:

LastPostDate = t.Posts
    .OrderByDescending(x => x.DateOfPost)
    .AsEnumerable()
    .FirstOrDefault()
    .DateOfPost.GetValueOrDefault(DateTime.MinValue);

Alternatively, you could make LastPostDate nullable in your model:

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; }
}

I normally don't use nullable types in my view models, and set default values where possible.

answered Jun 20, 2013 at 7:37

Oliver

9,0222 gold badges45 silver badges63 bronze badges

2 Comments

Mark Over a year ago

Hi - that gives the error 'System.DateTime' does not contain a definition for 'GetValueOrDefault' and no extension method 'GetValueOrDefault' accepting a first argument of type 'System.DateTime' could be found' - thanks

Oliver Over a year ago

Hi, sorry about that, I have made a change which should stop this error.

Dave Williams · Accepted Answer · 2013-06-20 07:41:11Z

1

eIf the DateOfPost column in the databse is nullable then the DateTime in your entity should also be nullable as should your viewmodel Property. Alternatively if you don't want null in your view model you can us null coalescer

t.Posts.OrderByDescending(x => x.DateOfPost).FirstOrDefault().DateOfPost ?? DefaultDate

answered Jun 20, 2013 at 7:41

Dave Williams

2,24621 silver badges26 bronze badges

Collectives™ on Stack Overflow

ASP.Net MVC Linq to Sql how to handle a null datetime

3 Answers 3

1 Comment

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related