MVC4 Code First - IList is not filled with data / NullPointer Exception in View

Question

I try to write a Blog with MVC4 and have problems with the Post - Comment (1 to n), as well as with the Post - Tag (n to m) relationship, using code first.

Tag:

public class Tag
{
    [Key]
    [Required]
    public int ID { get; set; }

    [Required]
    public string Name { get; set; }

    public IList<Post> Posts { get; set; }
}

Comment:

public class Comment
{
    [Key]
    [Required]
    public int ID { get; set; }

    [Required]
    public string Name { get; set; }

    [Required]
    [EmailAddress]
    public string Email { get; set; }

    [Required]
    public DateTime DateTime { get; set; }

    [Required]
    [DataType(DataType.MultilineText)]
    public string Body { get; set; }
}

Post:

public class Post
{
    [Key]
    [Required]
    public int ID { get; set; }

    [Required]
    public string Title { get; set; }

    [Required]
    public DateTime DateTime { get; set; }

    [Required]
    [DataType(DataType.MultilineText)]
    public string Body { get; set; }

    public IList<Tag> Tags { get; set; }
    public IList<Comment> Comments { get; set; }
}

Using this code first, I get a database which looks like:

Comment n to 1 Post n to m Tag

Where for the n to 1 the foreign key to Post is in Comment and for the n to m a Cross Table was created.

Now, I created a Controller as well as a View to create a new Post with Tags.

When I look into my database, everything is filled correctly. The Post and Tags exist, and the Cross Table is filled with the IDs.

But now, when I want to render the Post in my View I want to get the data with @post.Tags, where @post represents my currently selected Post which has the correct Data (Title, Name etc), but for Tags, which is the IList<Tag> from my Model I get a NullPointer Exception. (The same goes for @post.Comments)

Although, when creating the Post:

new Post() { ID = -1, Tags=new List<Tag>(), Comments=new List<Comment>(), DateTime = DateTime.Now };

Edit:
When using virtual, I get a System.Data.EntityCommandExecutionException. There is already an open DataReader associated with this Command which must be closed first

Solution I was missing the virtual Keyword as well as to invoke the Include Method. To get the Method I had to use System.Data.Entity

Now it works, thank you both

Did you try including the child entities. e.g. dbContext.Post.where(predicate).Include(post=>post.Comments) ? — Siva Gopal
– Siva Gopal, Commented Sep 15, 2014 at 14:02

Peter · Accepted Answer · 2014-09-15 13:55:48Z

1

You probably need to make the Tags and Comments properties of your Post class virtual so they can be lazy-loaded.

public virtual IList<Tag> Tags { get; set; }
public virtual IList<Comment> Comments { get; set; }

answered Sep 15, 2014 at 13:55

Peter

12.8k4 gold badges36 silver badges39 bronze badges

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

3 Comments

Loki Over a year ago

When using virtual, I get a System.Data.EntityCommandExecutionException. There is already an open DataReader associated with this Command which must be closed first

Carles Company Over a year ago

@Loki Add MultipleActiveResultSets=true to your connection string if you want to use lazy loading. Otherwise, use Include to load the related entities (eager loading).

Loki Over a year ago

@CarlesCompany Also a nice way to do it. I probably go with this one

Collectives™ on Stack Overflow

MVC4 Code First - IList is not filled with data / NullPointer Exception in View

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related