Are data templates obsolete in MVVM?

Question

I have created the following model (the code is simplified to illustrate the situation):

public abstract class Account
{
    public string Name { get; set; }
}

public class Person : Account
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

public class Company : Account
{
    public string Owner { get; set; }
}

Next I have created a view model:

public class ViewModel
{
    public Account Model { ... }
    public string Name { ... }
    public string FirstName { ... }
    public string LastName { ... }
    public string Owner { ... }   
    ...
}

And finally, the view:

<UserControl>
  <UserControl.Resources>

    <!-- Person data template -->
    <DataTemplate x:Key="personTemplate" DataType="{x:Type model:Person}">
      <Grid DataContext="{Binding ElementName=rootLayout, Path=DataContext}">
        <TextBlock Text="{Binding Path=Name}" />
        <TextBlock Text="{Binding Path=FirstName}" />
        <TextBlock Text="{Binding Path=LastName}" />
      </Grid>
    </DataTemplate>

    <!-- Company data template -->
    <DataTemplate x:Key="companyTemplate" DataType="{x:Type model:Company}">
      <Grid DataContext="{Binding ElementName=rootLayout, Path=DataContext}">
        <TextBlock Text="{Binding Path=Name}" />
        <TextBlock Text="{Binding Path=Owner}" />
      </Grid>
    </DataTemplate>

    <!-- Data template selector for different account types -->
    <local:AccountTemplateSelector x:Key="templateSelector" 
        PersonTemplate="{StaticResource personTemplate}" 
        CompanyTemplate="{StaticResource companyTemplate}" />

  </UserControl.Resources>

  <StackPanel Name="rootLayout" DataContext="{Binding Path=viewModel}">
    <ContentControl Content="{Binding Path=Model}" 
        ContentTemplateSelector="{StaticResource templateSelector}"/>
    <Button Content="Save" />
    <Button Content="Close" />
  </StackPanel>

</UserControl>

So, when the model that is loaded is of type Person the personTemplate is shown; vice versa, when the model is Company the companyTemplate is shown.

My questions are:

1. Does this approach make sense at all? Would it be smarter to delete the Model property in the ViewModel class and to introduce an enum or just a simple bool which would show person if true, or company if `false?
2. While defining the data templates, I specified DataTypes to Person and Company types (it was natural to me to do it this way). Do I need it at all because in the very next line I am setting a new data context to be the one from the UserControl?
3. Should the DataTypes of the data templates be different view models, something like PersonViewModel and CompanyViewModel? Does it make sense to create them?
4. How can I, and can I at all, make data template inherit the data context from the ContentControl automatically?

I know that all this is a matter of a personal choice in the end, but since I am learning MVVM (I am using MVVM Light), I am wondering which approach would be the most recommendable one? I still do not fully understand when should the classes from models be used as data types for data templates and when should view models be used for that purpose. Should the assembly that represents the model even be referenced in the view assembly (assuming that view, model and view model all reside in separate assemblies)?

Thanks for all the clarifications!

UPDATE:

This update should explain the problem of having classes of the model as DataTypes in the data templates when the property of the model class is not directly binded to just one control in the view.

There is an enum and a new property in the Person, so now it looks like this:

public class Person : Account
{
    public enum GenderType { Female, Male, NotSpecified }

    public string FirstName { get; set; }
    public string LastName { get; set; }
    public GenderType Gender {get; set; }
}

And in the view, the data template of the person is changed as well of course:

<!-- Person data template -->
<DataTemplate x:Key="personTemplate" DataType="{x:Type model:Person}">
  <Grid DataContext="{Binding ElementName=rootLayout, Path=DataContext}">
    <TextBlock Text="{Binding Path=Name}" />
    <TextBlock Text="{Binding Path=FirstName}" />
    <TextBlock Text="{Binding Path=LastName}" />
    <RadioButton Name="Female" />
    <RadioButton Name="Male" />
    <RadioButton Name="NotSpecified" />
  </Grid>
</DataTemplate>

If the Content of the ContentControl is set to Model property of the ViewModel, how would I resolve the gender/radio buttons situation; because, now they do not match in the way one control/one property?

Answer 1

I would change it to this:

<UserControl>
  <UserControl.Resources>
    <!-- Person data template -->
    <DataTemplate DataType="{x:Type model:Person}">
      <Grid>
        <TextBlock Text="{Binding Path=Name}" />
        <TextBlock Text="{Binding Path=FirstName}" />
        <TextBlock Text="{Binding Path=LastName}" />
        <RadioButton Name="Female"       IsChecked="{Binding Gender , Converter={StaticResource enumBooleanConverter}, ConverterParameter=Female}" />
        <RadioButton Name="Male"         IsChecked="{Binding Gender , Converter={StaticResource enumBooleanConverter}, ConverterParameter=Male}" />
        <RadioButton Name="NotSpecified" IsChecked="{Binding Gender , Converter={StaticResource enumBooleanConverter}, ConverterParameter=NotSpecified }" />
      </Grid>
    </DataTemplate>

    <!-- Company data template -->
    <DataTemplate DataType="{x:Type model:Company}">
      <Grid>
        <TextBlock Text="{Binding Path=Name}" />
        <TextBlock Text="{Binding Path=Owner}" />
      </Grid>
    </DataTemplate>
  </UserControl.Resources>

  <StackPanel DataContext="{Binding viewModel}">
    <ContentControl Content="{Binding Model}" />
    <Button Content="Save" />
    <Button Content="Save" />
    <Button Content="Close" />
  </StackPanel>

</UserControl>

like this you define implicit styles for your classes and you don't have to use a templateselector. Also then you don't need all your string properties in the ViewModel class:

public class ViewModel
{
    public Account Model { ... } 
    ...
}

Disclaimer, the binding in the RadioButtons uses a Converter from here.

Answer 2

Absolutely positively not (are they obsolete).

1. Yes, it makes perfect sense, however your bindings, not so much. Those issues can be handled in a number of different ways. For instance, the Account could have a Parent property which exposes the ViewModel it is contained in (I didn't say it was the best approach).
2. This is an issue with your ViewModel design, where you must bind not against the Account but the ViewModel. It might be possible to change the design so that you don't need to do this; hard to tell with the snapshot you have provided.
3. I don't think that will help you at this point. I'd see if there was a way to keep your ViewModel related UI out of the DataTemplate.
4. Here's one good solution: Create a DataTemplateSelector that chooses the template based on the Account property. That way, you can bind the ItemsSource directly to the DataContext, and it will be available within the DataTemplate.