I have a central domain assembly which contains various rich domain models. Lots of business logic, etc. To keep this example simple, here's probably the simplest one:
public class Location
{
private int _id;
public int ID
{
get { return _id; }
private set
{
if (value == default(int))
throw new ArgumentNullException("ID");
_id = value;
}
}
private string _name;
public string Name
{
get { return _name; }
set
{
if (string.IsNullOrWhiteSpace(value))
throw new ArgumentNullException("Name");
_name = value;
}
}
public string Description { get; set; }
private string _address;
public string Address
{
get { return _address; }
set
{
if (string.IsNullOrWhiteSpace(value))
throw new ArgumentNullException("Address");
_address = value;
GeoCoordinates = IoCContainerFactory.Current.GetInstance<Geocoder>().ConvertAddressToCoordinates(Address);
}
}
public Coordinates GeoCoordinates { get; private set; }
private Location() { }
public Location(string name, string address)
{
Name = name;
Description = string.Empty;
Address = address;
}
public Location(int id, string name, string description, string address, Coordinates coordinates)
{
if (coordinates == null)
throw new ArgumentNullException("GeoCoordinates");
ID = id;
Name = name;
Description = description;
Address = address;
}
public class Coordinates
{
public decimal Latitude { get; private set; }
public decimal Longitude { get; private set; }
private Coordinates() { }
public Coordinates(decimal latitude, decimal longitude)
: this()
{
Latitude = latitude;
Longitude = longitude;
}
public override bool Equals(object obj)
{
if (obj == null)
return false;
if (!(obj is Coordinates))
return false;
var coord = obj as Coordinates;
return ((coord.Latitude == this.Latitude) &&
(coord.Longitude == this.Longitude));
}
public override string ToString()
{
return string.Format("Latitude: {0}, Longitude: {1}", Latitude.ToString(), Longitude.ToString());
}
}
}
For a number of reasons, I don't want to use these domain models as my presentation models in my MVC application. At first I was just creating very similar DTOs for the models to use as presentation models. Something like this:
public class LocationViewModel
{
public int ID { get; set; }
public string Name { get; set; }
public string Description { get; set; }
public string Address { get; set; }
public decimal Latitude { get; set; }
public decimal Longitude { get; set; }
}
However, that doesn't make sense for every view situation. A Create action, for example, shouldn't have an ID property. A Delete action doesn't need all of that information. And so on.
So now I'm ending up with presentation models that are one-to-one with the presentations themselves. Something like this:
public class LocationCreateViewModel
{
public string Name { get; set; }
public string Description { get; set; }
public string Address { get; set; }
}
public class LocationDetailsVieWModel
{
public int ID { get; set; }
public string Name { get; set; }
public string Description { get; set; }
public string Address { get; set; }
public decimal Latitude { get; set; }
public decimal Longitude { get; set; }
}
And so on, customized for the views that bind to them. This became further useful as I could use data annotations to make cleaner use of the ASP.NET MVC tooling. Something like this:
public class LocationCreateViewModel
{
[Required]
public string Name { get; set; }
public string Description { get; set; }
[Required]
public string Address { get; set; }
}
These can get more complex, but the point is that I'm keeping them on the presentation models because I don't feel they have a place in the business domain. I think it would be misleading to have a [Required] annotation on a class property if it doesn't actually make it required unless interpreted by a very specific set of tools. And since lots of other things use these domain models, not just this one MVC website, then I want to make sure the logic is really baked in to the models and not loosely applied for an assumed set of tools.
A recurring piece of functionality in this setup is to convert between presentation models and domain models. So presentation models which need to convert to domain models have instance methods on them:
public Location ToDomainModel();
And presentations models which need to be built from domain models have static methods on them:
public static ConstructFromDomainModel(int id);
public static ConstructFromDomainModel(Location location);
The original goal in all of this was to separate concerns. A lot. But I wonder if I've taken a wrong turn in that effort. This isn't necessarily an unmanageable amount of code, but I don't want it to become unmanageable. There's sort of a "class explosion" going on as more and more gets added. And the unit tests are increasing at an even faster rate (which will become a question all its own once I sort out this question).
Is there a "better" way? Are there known patterns which would be better followed and still maintain the separation of concerns in a tool-agnostic approach?
LocationViewModels from theLocationin yet another way. The view models are coupled to the views they populate, and can be highly customized for those views. So a single view model might be a composite of a handful of models for a more complex view. (For example, say a Location-based view also needs some Event data or User data or something else. Rather than send all the models, I make a lighter composite view model.) – David Sep 7 '12 at 12:06