Appropriate way to set default values for ViewModel in MVC4?

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.