Adding arbitrary properties to a strongly typed list

Question

I'm looking for a good way to add arbitrary properties to the objects in a strongly typed list, based on the principle that I shouldn't pass a DataTable from my business layer to my presentation layer.

For example, I might have a Category class with the properties CategoryId and Title . On one page I would like to fetch a list of all categories (ie. List<Category> ) together with the most expensive product in each category.

A while ago, I would have just returned a DataTable with some additional columns in it with the product data in, but I'm trying not to do that -- it would be trivial to set up it's not good practice.

One option is to add a MostExpensiveProduct property to my Category class, but I might want to display the most recently added product in another case, or the cheapest product, so I'd end up adding a lot of properties to cover all the options. This just doesn't feel right to me.

Am I missing a trick here? What is the best way of doing this? Or should I just be returning a DataTable to which I can add as many columns as I need and not worry about it?

Answer 1

The issue seems to be you have a lot of different views you'd like to offer the user. The options I see are:

• You could construct separate classes for each view that inherit from the Category class. Code gen would be a good solution here.
• You could store an Attributes property, which has an IDictionary interface, and refer to items by key. I'm becoming a fan of this approach.
• You could generate a data table only for binding purposes, for these views... or develop a data table like component where you can refer to fields by Key...
• For fields that you compute (say you store sales tax and net price, and compute gross cost), you could store as a method of the Category object, or as an extension method.

I'm sure there are other options that I haven't thought about...

HTH.

Answer 2

You should create a specialized class (a view model) for each view you have containing only the properties you are interested in using in the view. This may seem like unnecessary duplication for the simplest cases, but pays off in terms of consistency and separation of layers. You can construct the view models manually, or if that gets tedious, use an object-object mapping framework like AutoMapper .

Answer 3

There are several things to consider here IMHO. First, it seems that the only reference from Category to Product should be Category.Products, meaning you should never have something like Category.MostExpensiveProdcut etc. As far as your business layer, I would do something like this:

From your code behind in the presentation layer:

call CategoryManager.GetCategories();
call List<Product>ProductManager.GetMostExpensiveProducts(List<Category>);

Now that you have a list of Categories, and a list of Products (assuming your Product has a reference back to its Category) you have all the necessary information to work with. Using this setup your entities (Category, Product) are not polluted.

Another thing to consider is introducing a services layer. If you find that you don't want (for whatever reason) to make two calls to the business managers, rather you want to make a single call and get all your information in one shot I would consider introducing a services layer sometimes aka "application facade". This facade would be responsible for making the individual calls to the business managers and combining results into one response before shipping it back to the UI layer. Someone mentioned that that custom object would be a "ViewModel", which is correct but often used in reference to MVC. Another name for it would be a DTO (Data Transfer Object), which designed for use with service layers/application facade.