Initialise View Model that inherits from base model with values from base model

Question

I have a function that returns Class1Model

Class1Model model = GetClass1Model();

I also have a Class1ViewModel that inherits from Class1Model to which I have added some get only properties for formatting in a view.

public class Class1ViewModel : Class1Model  
{
    public string MyFormattedProperty => $"{base.Height:###.00} x {base.Width:###.00}" 
}

So I was hoping that I could do this:

Class1Model model = GetClass1Model();
var viewModel = model as Class1ViewModel;

But this doesn't work

So how would this usually be done?

Your object isn't a `Class1ViewModel`. It's a `Class1Model`. If you want a `Class1ViewModel`, you will need to return one from your method. — , Nov 10 '17 at 14:14
Sounds like cloning to me. Serialize base class, deserialize **as** inherited. — Sinatr, Nov 10 '17 at 14:19
Composition ? Does that mean passing the model to the viewmodel via constructor and initialising a property of type model on the viewmodel? — wingyip, Nov 10 '17 at 14:26

score 2 · Accepted Answer · answered Nov 10 '17 at 14:32

I suggest a composition approach instead of inheritance.

class Class1Model
{
    // model properties
}

class Class1ViewModel
{
    public Class1ViewModel(Class1Model model)
    {
        _Model = model;
    }

    private Class1Model _Model;
    public Class1Model Model { get { return _Model; } }

    // viewmodel specific extensions

    public string MyFormattedProperty => $"{Model.Height:###.00} x {Model.Width:###.00}"
}

If you expect your model properties to change, you should subscribe to model property changes with some weak event listener and issue appropriate property changed events for dependent viewmodel properties.

Skyuppercut · Answer 2 · 2017-11-10T14:26:33.370

0

You are trying to cast an instance of the base type to the sub-type which is not allowed.

An Class1ViewModel is a Class1Model. Not the other way around.

So this would work:

Class1ViewModel viewModel = new Class1ViewModel();
Class1Model model = viewModel;

edited Nov 10 '17 at 14:26

answered Nov 10 '17 at 14:20

Skyuppercut

332
3
14

It's true you can cast to base types, but this doesn't solve the problem in question, where a model is given first. – grek40 Nov 10 '17 at 14:22
The word "necessarily" isn't well chosen IMO. A base class is never one of its subclasses. There may be a conversion operator, but that's a distinct special case (and usually not considered a good use of a conversion operator). – 15ee8f99-57ff-4f92-890c-b56153 Nov 10 '17 at 14:23
well.. he needs to create a new instance of the sub type. I thought that was self-explanatory. – Skyuppercut Nov 10 '17 at 14:25

score 0 · Answer 3 · answered Nov 10 '17 at 14:51

In general it makes no sense (read this answer), so I am not sure if providing general solution is a good idea.

You can do surely have one, e.g. via cloning via serialization trick:

using Newtonsoft.Json;

class Base
{
    public string A;
    public string B;
}

class Inherited : Base
{
    public string C = "c";
}

// in some method
var @base = new Base { A = "a", B = "b" };
var inherited = JsonConvert.DeserializeObject<Inherited>(JsonConvert.SerializeObject(@base));

inherited will have @base values (I used fields, should work for properties too) copied and own values will be default.

This is very slow approach. Consider to use manual copy instead:

var @base = new Base { A = "a", B = "b" };
var inherited = new Inherited { A = @base.A, B = @base.B, C = "whatever" };

Initialise View Model that inherits from base model with values from base model

3 Answers3