DAL, BLL and Web layer - create the same class for each layer? Is it correct?

Question

For example: I need to display comments from Comments table in database. So in DAL project I have POCO class:

public class Comments
{
    public int CommentId { get; set; }
    public string Author { get; set; }
    public string Content { get; set; }
}

In BLL project I have class (DTO - data transfer object):

public class CommentsDTO
{
    public int CommentId { get; set; }
    public string Author { get; set; }
    public string Content { get; set; }
}

So I get IEnumerable from DAL, convert to IEnumerable and return it to Web project.

In Web project I have class:

public class CommentsViewModel
{
    public int CommentId { get; set; }
    public string Author { get; set; }
    public string Content { get; set; }
}

So in Web project I get IEnumerable from BLL, convert to IEnumerable and return it to view.

Is it correct? Because these classes have only different names.

score 1 · Answer 1 · answered Jul 12 '14 at 19:52

It is certainly incorrect and you should not do this generally ad this just duplicates the code. You need different classes only when they differ. Suppose that you want to display a Comment in your web application and Comment is associated with User entity. Then, instead of sending 2 object separately you just create DTO class that combines properties of the two (like comment text and user name). Then sometimes you need to change the model in your web application to display data properly. For instance you want to display them in grid, and to do that properly you need to assign some attributes over properties and this is a good reason to create separate model in your web application. If you do not need to change your classes you should reuse existing ones. However even in your case changing CommentsViewModel a bit might be beneficial - usually you do not want user to see CommentId value (this is db internal thing) - so you can this field in [HiddenInput(DisplayValue = false)] so you could use Html.EditorForModel just to display editor panel. But this is your choice.

score 1 · Answer 2 · edited May 23 '17 at 12:29

Creating each class for every layer is certainly beneficial for large projects. When I say large, I mean exceeding 5 developers with a application lifetime of 10 years. On large projects, the disadvantage of duplication becomes small compare to the benefit it provides.

For small projects, it is certainly overkill. The extra weight will slow you down with little or no benefit. Its like packing 7-day clothes on a 2 day hiking trip.

The larger the project, the more formal is your architecture.

With that said, here is my advice:

The Comment should be shared by the DAL and Business Layer.
If you don't have a "service layer", loose the DTO classes and map the Comment straight to the CommentViewModel. More about the service layer
I say Comment without "s" because only classes that represents collections should be named plural.

DAL, BLL and Web layer - create the same class for each layer? Is it correct?

2 Answers2