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?

Answer 1

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.

Answer 2

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;

Answer 3

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, eg 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" };