Combobox.SelectedItem binding to nested property - wpf

Question

I am doing a simple WPF application using MVVM and I am having trouble binding to the SelectedItem property of the combobox.
The setter of the bound property does not get called, and there is no output in the debug windows telling me it is not able to bind (I assume it is able to).
This is .NET 3.5, I made a small example that has the same problem.
In XAML:
<Window x:Class="Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Window1" Height="300" Width="300">
<StackPanel>
<ComboBox IsDropDownOpen="False" IsReadOnly="False" ItemsSource="{Binding Path=Printers}" SelectedIndex="0" SelectedItem="{Binding Path=Printer.SelectedPrinter, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}" Name="cmbPrinters" />
</StackPanel>
</Window>
View code behind:
using System.Windows;
public partial class Window1 : Window
{
ViewModel viewmodel;
public Window1()
{
viewmodel = new ViewModel();
this.DataContext = viewmodel;
InitializeComponent();
}
}
View model:
using System;
using System.Collections.ObjectModel;
public class ViewModel
{
public ViewModel()
{
Printers = new ObservableCollection<string>() { "test", "test2" };
Printer = new PrinterViewModel();
}
public PrinterViewModel Printer { get; set; }
public ObservableCollection<string> Printers { get; set; }
}
PrinterViewModel:
using System.Windows;
using System.Diagnostics;
public class PrinterViewModel : DependencyObject
{
public string SelectedPrinter
{
get { return (string)GetValue(SelectedPrinterProperty); }
set
{
SetValue(SelectedPrinterProperty, value);
Debug.WriteLine("!!!!!! SelectedPrinter setter called");
}
}
public readonly DependencyProperty SelectedPrinterProperty =
DependencyProperty.Register("SelectedPrinter", typeof(string), typeof(PrinterViewModel), new UIPropertyMetadata());
}
Can anyone see what I am doing wrong here?

Answer 1

The problem here is that you have a misunderstanding about how the Silverlight dependency property system works.
When the value of a dependency property changes, Silverlight doesn't go through the properties you've defined (such as SelectedPrinter) to set the value of the dependency property. The Silverlight dependency property mechanism keeps track of all of the values of dependency properties, and when the value of one of these properties changes, Silverlight changes its value directly without calling your code to do so. In particular, it will not call your property setter. This should explain why your debugging message wasn't appearing.
The getter in a property that uses a dependency property, such as your SelectedPrinter property, should contain only a call to GetValue, and the setter should contain only a call to SetValue. You shouldn't add any code to the getter or setter, as doing this will not achieve what you want.
Furthermore, you are using dependency properties in the view-model layer. This is not where they are intended to be used. Dependency properties are only intended to be used in the view-layer. Your view-model classes should instead be implementing INotifyPropertyChanged rather than extending DependencyObject.
It is possible to bind two dependency properties together. This is permitted, and occasionally it comes in useful to wire together two dependency properties in the view-layer. In fact, the bindings in your example were working, which explains why you weren't getting any messages about problems with bindings.

Answer 2

why inherit from DependencyObject when doing mvvm?`
public class PrinterViewModel : INotifyPropertyChanged
{
private string _selected;
public string SelectedPrinter
{
get { return this._selected; }
set
{
_selected= value;
OnPropertyChanged("SelectedPrinter");
}
}
}
now your code should work