Meta-Me

Tip 40 – How to materialize presentation models via L2E

Problem:

Imagine that you have these entities

public class Product
{
    public int ID { get; set; }
    public string Name { get; set; }
    public virtual Category Category { get; set; }
}

But for your UI you want to display the product id, product name and the product’s category’s name.

You might try passing this query to the presentation layer.

var displayData = from product in ctx.Products.Include("Category")
                  select product;

But then you are passing more than is required around, and binding the UI tightly to the conceptual model, which is probably a bad idea.

Your next try might be a query like this:

var displayData = from product in ctx.Products
                  select new {
                     ID = product.ID, 
                     Name = product.Name, 
                     CategoryName = product.Category.Name 
                  };

But then you’ll quickly discover you can’t pass the anonymous types to another method, at least not without a nasty hack.

Solution: 

Most people think LINQ to Entities can only materialize Entities and Anonymous types.

But in fact it turns out it can materialize any non-generic class that has a default constructor.

Anyway what this means is you can create a view class like this:

public class ProductView
{
    public int ID { get; set; }
    public string Name { get; set; }
    public string CategoryName { get; set; }
}

And then write this:

And then pass this to your view without any problems at all.

I only just found out about this myself, I had always assumed this would fail.

So this was a nice pleasant surprise.