Question

If I am using the MVVM pattern to populate my views, i.e. I am using a View Model in addition to the model classes. The model classes do not talk to the view; the view model populates the view.

In such a scenario, if I need to use data annotations for validation, etc. I need to annotate my view model classes and not my model classes, right?

For e.g. should the [Required] attribute be annotated on:

namespace MyApp.ViewModels
{
    public class Category
    {
        [Required]
        [StringLength(50)]
        public string CategoryName { get; set; }

        [StringLength(255)]
        [AllowHtml]
        public string Description { get; set; }
    }
}

Or should it be this way:

namespace MyApp.Models
{
    public partial class Category
    {
        [Required]
        public virtual string CategoryName
        {
            get;
            set;
        }
    }
}

Answer 1

Correct, your viewmodel should not use Models(entity) but can contain model properties, example:

//wrong
public class Person{
     public string FirstName{get;set;}
     public string LatsName{get;set;}
}

public class ViewModel{
     public Person Person{get;set;}
}

this is a better use of viewmodel:

public class ViewModel{
     public string FirstName{get;set;}
     public string LastName{get;set;}
}

and yes, your annotations should be on the viewmodel.

---

You can set annotations to complex type in the viewmodel too, but please note that complex type should not be a entity/domain class, but a data model class specific only to viewmodel, example:

public class InsertPerson{
     [Required]
     public string FirstName{get;set;}
}

public class EditPerson{
     [Required]
     public string Id{get;set;}

     [Required]
     public string FirstName{get;set;}
}

public class ViewModel{
     public InsertPerson insertPerson{get;set;}
}