How to create a generic view model - wpf

Question

I have to create a generic viewmodel passing an entity with a one to many relationship.
I'll explain:
My WIndows:
<Window x:Class="Invoice_Example_Brux.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:invoiceExampleBrux="clr-namespace:Invoice_Example_Brux"
Title="MainWindow" Height="350" Width="525">
<Window.DataContext>
<invoiceExampleBrux:MainWindowViewModel/>
</Window.DataContext>
<Grid>
<TextBox HorizontalAlignment="Left" Height="23" Margin="174,78,0,0" TextWrapping="Wrap" Text="{Binding MyModel.Name}" VerticalAlignment="Top" Width="120"/>
<Label Content="Id" HorizontalAlignment="Left" Margin="10,53,0,0" VerticalAlignment="Top"/>
<TextBox HorizontalAlignment="Left" Height="23" Margin="10,78,0,0" TextWrapping="Wrap" Text="{Binding MyModel.Id}" VerticalAlignment="Top" Width="120" IsReadOnly="True"/>
<Label Content="Number" HorizontalAlignment="Left" Margin="322,52,0,0" VerticalAlignment="Top"/>
<TextBox HorizontalAlignment="Left" Height="23" Margin="322,78,0,0" TextWrapping="Wrap" Text="{Binding MyModel.Number}" VerticalAlignment="Top" Width="120"/>
<Label Content="Name" HorizontalAlignment="Left" Margin="174,53,0,0" VerticalAlignment="Top"/>
<Button Content="Save" HorizontalAlignment="Left" Margin="211,288,0,0" VerticalAlignment="Top" Width="75" Command="{Binding SaveCommand}"/>
<ComboBox
SelectionChanged="Selector_OnSelectionChanged"
HorizontalAlignment="Left" Margin="180,38,0,0" VerticalAlignment="Top" Width="120"
DisplayMemberPath="Name"
ItemsSource="{Binding DocumentType,UpdateSourceTrigger=PropertyChanged}"/>
<Label Content="Type Document" HorizontalAlignment="Left" Margin="192,12,0,0" VerticalAlignment="Top"/>
</Grid>
myWindows codebheind:
namespace Invoice_Example_Brux
{
public partial class MainWindow
{
public MainWindow()
{
InitializeComponent();
}
private void Selector_OnSelectionChanged(object sender, SelectionChangedEventArgs e)
{
var cmb = sender as ComboBox;
var selectedItem = cmb.SelectedValue as DocumentType;
if (selectedItem == null) return;
var code = selectedItem.Code;
switch (code)
{
case "A":
DataContext = new ViewModelGeneric<DocumentA>();
break;
case "B":
DataContext = new ViewModelGeneric<DocumentB>();
break;
case "C":
break;
}
}
}
}
My Entity DocumentA and DocumentB:
public class DocumentA : DocumentGeneral
{
public ObservableCollection<DetailDocumentA> DetailDocumentA { get; set; }
}
public class DetailDocumentA : DetailDocumentGeneral
{
}
public class DocumentB : DocumentGeneral
{
public ObservableCollection<DetailDocumentB> DetailDocumentB { get; set; }
}
public class DetailDocumentB : DetailDocumentGeneral
{
}
public class DocumentGeneral
{
public Guid Id { get; set; }
public string Name { get; set; }
public string Number { get; set; }
public string TypeDocument { get; set; }
}
public class DetailDocumentGeneral
{
public Guid Id { get; set; }
public string Quantity { get; set; }
public string Price { get; set; }
public string Total { get; set; }
}
My ViewModelGeneric:
public class ViewModelGeneric<T> : ViewModelBase
where T : DocumentGeneral, new()
{
public T MyModel { get; set; }
public RelayCommand SaveCommand { get; set; }
public ViewModelGeneric()
{
MyModel = new T();
SaveCommand = new RelayCommand(Save);
}
private void Save(object obj)
{
if (MyModel.Id == Guid.Empty)
{
MyModel.Id = Guid.NewGuid();
}
//the problme is here. how can I do to fill in the detail of my entity
/* MyModel.Detail.Add(new DetailDocumentGeneral
{
Price = "50",Quantity = "100",Total = "5000"
});*/
using (var ctx = new DocumentContext())
{
var document = ctx.Set<T>();
document.Add(MyModel);
ctx.SaveChanges();
}
}
}
the problem is that depending on the choice of my combobox I have to go a different entity.
But I can not access the respective detailDocument = (

Answer 1

Please stop what you are doing... WPF is a verbose language. That means that you'll have to write loads of code, probably even duplicating large sections. Please just do yourself a favour and accept this fact. I've seen this situation time and time again (and even wasted my own time on it when I was first learning WPF), but it always ends in tears.
In MVVM, it is customary to provide one view model for one view. This view model is responsible for providing all of the data and functionality for the related view. Unless all of your views are almost identical, then your generic view model simply won't work. Worse still, even if it did work, it would complicate the hell out of your project and make simple tasks more difficult.
If you really don't want to rewrite your view models each time, then just copy and paste from another view model and just change the differences. Having said all that, I must say that your code seems to be somewhat at odd with your words... you say that you want a generic view model, but your problems relates to data access.
It is far more usual to have your data access class(es) in a separate project to your view models. In this respect, it seems as though you are trying to implement The Repository Pattern, but in the wrong place. Making generic data access classes is far more common and has far less to go wrong than a generic view model.
Therefore, I would strongly recommend that you forget about generic view models and declare a new one for each view and then move your data access code into a separate project (or at least a separate class) and optionally make that generic instead (using the linked page on MSDN for help). This may be the first real problem that you have come across in this endeavour, but if you continue along this path, I can guarantee you that it won't be the last.