Appropriate way to set default values for ViewModel in MVC4? - asp.net-mvc

Question

I have a viewModel that contains the following:
public class CreateCardViewModel
{
[HiddenInput(DisplayValue = false)]
public int SetId { get; set; }
[Required]
public IList<Side> Sides { get; set; }
[Required]
public int Stage { get; set; }
[Required]
[DataType(DataType.Date)]
[HiddenInput(DisplayValue = false)]
public DateTime DateCreated { get; set; }
[Required]
public bool IsReady { get; set; }
}
The model is as follows:
public class Card
{
public virtual int CardId { get; set; }
// belongs to a Set
public virtual int SetId { get; set; }
public virtual Set Set { get; set; }
// has Sides
public virtual IList<Side> Sides { get; set; }
// is in a certain Stage
public virtual int Stage { get; set; }
// is ready to study
public virtual bool IsReady { get; set; }
public virtual DateTime DateCreated { get; set; }
}
How can I set a default value for DateCreated?
Would the method change if I want to insert a blank Side into Sides upon Card creation?

Answer 1

You could set defaults in the constructor:
public CreateCardViewModel()
{
DateCreated = DateTime.Now;
Sides = new List<Side> { new Side() };
}
Caveat: There is an issue with using DateTime.Now from a unit testing perspective. If you're unit testing your ViewModel creation and need to be able to verify that the created date is set to a known value, you can look at creating a separate concept for time, as detailed in Ayende's blog. You basically create a static func, SystemTime, that you can set to a known value in your tests. If you don't set it, it defaults to DateTime.Now:
public static class SystemTime
{
public static Func<DateTime> Now = () => DateTime.Now;
}
Your constructor code then becomes:
public CreateCardViewModel()
{
DateCreated = SystemTime.Now();
Sides = new List<Side> { new Side() };
}
If you need to actually set the time to a known value, you do this:
SystemTime.Now = () => new DateTime(2013, 2, 11, 17, 41, 12);

Answer 2

I agree on The SystemTime approach.
Although, I personally don't like setting the CreatedDate on the constructor, since there can be a short time lapse since you instantiate the object and when you persist it to the database. (And here I am assuming you definitely are)
You could make all your domain objects inherit from an interface like this one:
public interface ITimeStamped
{
DateTime DateCreated { get; set; }
}
And then on the Commit method int the Context class I would do something like this to set the date for all entities that implement the interface:
foreach (var entry in ChangeTracker.Entries<ITimeStamped>()
.Where(entry => entry.State == EntityState.Added))
{
entry.Entity.DateCreated = SystemTime.Now();
}
This way you're totally certain that the entity is stored with the correct DateTime when it was persisted on the database.