Calculated columns should be where in MVVM model? - mvvm

Question

I have a WPF DataGrid displaying Products. I have two fields price and mass in that which are actually the properties of Product class. I need to show a seperate column in grid name MultipliedValue = price * mass. As per MVVM model where should i do it ?
1) In model by making a readonly property.
2) In converter so that only my UI will be aware of that?
3) or in View model?
Please suggest which option i should choose and why?
Thanks.

Answer 1

I would disregard option #2 from the beginning -- converters should be used only to account for implementation details of the UI, and specifically in MVVM perhaps not even then (as you can do the conversion inside the ViewModel, which is option #3 and more convenient).
Between #1 and #3, in this case IMHO it's best to go with #1 -- price is not something that's only relevant for your UI and of course the concept of price (and how it is derived) is going to stay fixed throughout your application. Both the UI and your backend may choose to use this property or not.

Answer 2

I would argue differently (than #jon). I put in the model only properties that I would like to serialize (say, from the server). Computed properties don't serialize, and hence they aren't in the model.
Recently, my favorite Model/View Model paradigm as as follow: Product is a class in the Model, that has nothing but the simplest getters and setters. ProductVm is a class in the VM, which contains Product, and has the additional VM logic. Most importantly, the property changed notification - which in my opinion is also part of the VM and not the model.
// Model:
class Product {
public double Price { get; set; }
public double Mass { get; set; }
}
// View Model:
class ProductVM : INotifyPropertyChanged
{
Product _product;
public event PropertyChangedEventHandler PropertyChanged;
public double Price {
get { return _product.Price; }
set { _product.Price = value; raise("Price"); raise("Total"); }
}
public double Mass {
get { return _product.Mass; }
set { _product.Mass = value; raise("Mass"); raise("Total"); }
}
public double total {
get { return Price * Mass; }
}
private void raise(string name) {
if( PropertyChanged ) {
PropertyChanged( this, new PropertyChangedEventArgs(name) );
}
}
public ProductVm( Product p ) {
_product = p;
}
public ProductVm() {
// in case you need this
_product = new Product();
}
}
Yes, there is a lot of boilerplate here, but once you do all the typing, you'll find this separation between Model and ViewModel very helpful. My 2 cents.
Note: I think #Jon approach is also correct, and is reasons are valid. I don't think there is one answer.