Can someone explain why use ViewModels in MVC3?

Question

In researching MVC 3 archetectures, one thing I've seen consistently is the concept of breaking the models out into their own project and calling it ViewModels.

As I'm coding, I'm having a hard time realizing what the benefit of this is. Could someone explain what the advantages of this are?

Answer 1

I'm not sure where you go thid idea that View models should be in their own project. That's typically not how it's done.

Often the business model is in a seperate project, as is the data model, but the view models are typically in the web project where they belong, since they're a part of the presentation layer.

Answer 2

Because one major thing about MVC is seperation of concern. You should create a ViewModel that has only the data your View needs, no logic.

You can, but you don't need, a seperate project (for example: a Data Access Layer)

A ViewModel is just a simple POCO class (Plain Old CLR Object, a class that only has properties, no logic)

More Information

• Wikipedia - Model–view–controller
• ASP.NET MVC
• ASP.NET MVC View Model Patterns
• ASP.NET MVC Tip #50 – Create View Models
• What is POCO?
• Use ViewModels to manage data & organize code in ASP.NET MVC applications

Answer 3

ViewModels are used to tailor a Model for the use of a particular View. When you want to pass specific information to the View, or retrieve particular information, then you would construct a ViewModel.

An example of a ViewModel would be:

public class PersonViewModel
{
    public Person NewPerson { get ; set ; }
    public Address NewAddress { get ; set ; }
    public SelectList Suffixes { get ; set ; }
}

If you had a view that created a new person, you could pass it this ViewModel class that is designed soley for the purpose of a new person View.

Answer 4

View models are a part of the UI layer so they should generally stay in the same project.