TwoWay Binding of a ComboBox to a static property

Question

-------EDIT------

So, i figured that my code is correct and so are the code snippets from all of your answers. Thanks for that. My problem is that my dev-maschine runs .NET4.5 which behaves differently! The very same program (compiled against .NET4.0) runs correct on a maschine with .NET4.0 but not on a maschine with .NET4.5!

So here is my revised question.

-------EDIT------

First, the simple example how i two-way bind my combobox to my data context:

View model:

public class MainWindowViewModel
{
    public List<String> MyElements { get; set; }
    public string SelectedElement { get; set; }

    public MainWindowViewModel()
    {
        MyElements = new List<string>() {"a", "b", "c"};
        SelectedElement = "a";
    }
}

and code-behind

private readonly MainWindowViewModel _viewModel = new MainWindowViewModel();
public MainWindow()
{
    InitializeComponent();
    DataContext = _viewModel;
}

and my xaml

<ComboBox
        ItemsSource="{Binding MyElements, Mode=OneWay}"
        SelectedItem="{Binding SelectedElement}" />

This works fine and if i select an item whith the combobox, it is bound to my view model.

OK, now i want to make my viewModel static but still two-way bind the selectedItem. I try this:

public class MainWindowViewModel
{
    public static List<String> MyElements { get; set; }
    public static string SelectedElement { get; set; }

    static MainWindowViewModel()
    {
        MyElements = new List<string>() {"a", "b", "c"};
        SelectedElement = "a";
    }
}

I do not need to set the datacontext in the Code-behind anymore and i know, that xaml needs an instance for two-way binding, so i have still the default constructor. I then bind the combobox

<Window.Resources>
    <me:MainWindowViewModel x:Key="model"/>
</Window.Resources>

<StackPanel>
    <ComboBox
        ItemsSource="{Binding Source={x:Static me:MainWindowViewModel.MyElements}, Mode=OneWay}"
        SelectedItem="{Binding Source={StaticResource model}, Path=SelectedElement}" />
</StackPanel>

The initial value is properly bound, but if i select another item with the combobox it it not reflected in my viewModel. What am i doing wrong?

EDIT:

If I use the exact same binding string for a TextBox and change the text in the box, it is reflected in the property.

<TextBox Text="{Binding Source={StaticResource model}, Path=SelectedElement}"/>

So obviously my binding string is correct but the way i use the combobox seems to be wrong. I also tried to bind SelectedValue instead... no change either.

Answer 1

Checked just in a sample project, works fine

public class ViewModel
{
    static ViewModel()
    {
        Items=new ObservableCollection<string>();
        SelectedItem = "222";
        Items.Add("111");
        Items.Add("222");
        Items.Add("333");
        Items.Add("444");
        Items.Add("555");
    }
    private static string _selectedItem;
    public static string SelectedItem
    {
        get { return _selectedItem; }
        set { _selectedItem = value;
            MessageBox.Show("Item " + value + " was selected");
        }
    }

    private static ObservableCollection<string> _items;
    public static ObservableCollection<string> Items
    {
        get { return _items; }
        set { _items = value; }
    }
}

and xaml

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:my="clr-namespace:WpfApplication1"
    Title="MainWindow" Height="200" Width="300">
<Grid>
    <Grid.Resources>
        <my:ViewModel x:Key="viewM"/>
    </Grid.Resources>
    <ComboBox Height="23" HorizontalAlignment="Left" Margin="101,12,0,0" Name="comboBox1" VerticalAlignment="Top" Width="146" 
               ItemsSource="{Binding Source={x:Static my:ViewModel.Items}, Mode=OneWay}"
              SelectedItem="{Binding Source={StaticResource viewM}, Path=SelectedItem}" />
    </Grid>
</Window>

I have uploaded sample.

Answer 2

You're being confused between instances and static properties: you don't need to bind a static object.

<ComboBox
        ItemsSource="{x:Static me:MainWindowViewModel.MyElements}"
        SelectedItem="{x:Static me:MainWindowViewModel.SelectedElement}" />

And you should implement INotifyPropertyChanged nevertheless.

Binding is about resolving the right instance from which you want to fetch data.
If there is no meaning of instance, there is no need for binding.