在不同命名空间的基类上使用属性和元数据

Question

I have a separate Visual Studio project where I keep my data model (EF6). So, my entities are in namespace Name1 (created by EF6 database first, but simplified below, for this example):

namespace Name1
{
   public class Person
   {
      public string FName {get; set;}
      public string LName {get; set;}
   }
}

Now, I have created a new MVC 5 project, which references the data Visual Studio project so that I can access the entities. In my MVC project, I want to add some metadata to the entity like this:

namespace NameMvc 
{
   [MetadataType(typeof(PersonMetaData))]
   public class Person : Name1.Person
   {
    
   }

   public class PersonMetaData
   {
      [Display(Name = "Firstname")]
      public string FName;
   }
}

In my controller I want to get all the persons, so I have an Action like this:

using Name1;
using NameMvc;

-- controller class code
public ActionResult Index()
{
   var persons = db.Person.ToList();
   return View(persons);
}
-- controller class code

And in my view I try to access that via:

@model IEnumerable<NameMvc.Person>

Now, when I run the code I get an error:

The model item passed into the dictionary is of type 'System.Data.Entity.Infrastructure.DbQuery`1[Name1.Person]', but this dictionary requires a model item of type 'System.Collections.Generic.IEnumerable`1[Name.MvcPerson]'.

I probably did something wrong in the action, because db.Person comes from the Name1 namespace.

I want to use the metadata in my View so that when I do something like this @Html.DisplayNameFor(model => model.FName) , it displays "Firstname".

Answer 1

You are getting an error rendering your view because you are returning IEnumerable<Name1.Person> from your action, but your view is expecting IEnumerable<NameMvc.Person> . When you are using a strongly-typed model in your view, it must match the model returned from the action.

There are two possible solutions for this error:

1. Change your view to use IEnumerable<Name1.Person> as its model, or
2. Change your action to return an IEnumerable<NameMvc.Person> as the model.

When you use MetadataType to add metadata to your model, the most common way is to use the fact that the original model class is generated as a partial class. You create another "part" to the partial class and add the MetadataType attribute to it.

However, partial classes cannot be used across assemblies. So this means that if you move your model to its own assembly, then you cannot add a partial class to your MVC project to add the metadata.

To solve this, you can do one of the following:

1. Add your metadata in your model's assembly. To do this, you use the partial class solution in your model's assembly. The problem with this is that you're now mixing view logic with data logic.
2. Create a new class in your MVC project which is a pseudo-copy of your data model to act as a view model. You add your metadata to that. Your action will return this view model and your view will use that. In your action, you copy the data from your data model to your view model.

I prefer option #2 for a couple of reasons:

1. It solves the problem you're facing, and
2. How data is presented to the user is often different than how I want it represented in my databases. This mapping allows me to handle that nicely.

The drawback to option #2 is the repetitive copying of data. However you could use tools like AutoMapper to simplify the data copying.