c# - Bind button in DataTemplate to command in the form's ViewModel

Question

My problem is similar to the one described in this question:
WPF MVVM Button Control Binding in DataTemplate

Here is my XAML:

<Window x:Class="MissileSharp.Launcher.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MissileSharp Launcher" Height="350" Width="525">
    <Grid>
        <!-- when I put the button here (outside the list), the binding works -->
        <!--<Button Content="test" Command="{Binding Path=FireCommand}" />-->
        <ListBox ItemsSource="{Binding CommandSets}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <!-- I need the button here (inside the list), and here the binding does NOT work -->
                    <Button Content="{Binding}" Command="{Binding Path=FireCommand}" />
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
    </Grid>
</Window>

It's just a ListBox, bound to an ObservableCollection<string> named CommandSets (which is in the ViewModel).
This binding works (it displays a button for each item in the collection).

Now I want to bind the button to a command (FireCommand), which is also in the ViewModel.
Here's the relevant part of the ViewModel:

public class MainWindowViewModel : INotifyPropertyChanged
{
    public ICommand FireCommand { get; set; }
    public ObservableCollection<string> CommandSets { get; set; }

    public MainWindowViewModel()
    {
        this.FireCommand = new RelayCommand(new Action<object>(this.FireMissile));
    }

    private void FireMissile(Object obj)
    {
        System.Windows.MessageBox.Show("fire");
    }
}

The binding of this button does NOT work.
From what I've understood from the question I linked above, the binding doesn't work because:
(correct me if I'm wrong)

• The button is inside the ListBox, so it only "knows" the binding of the ListBox (the ObservableCollection, in this case), but not the binding of the main window
• I'm trying to bind to a command in the main ViewModel of the main window (which the button doesn't "know")

The command itself is definitely correct, because when I put the button outside the ListBox (see the XAML above for an example), the binding works and the command is executed.

Apparently, I "just" need to tell the button to bind to the main ViewModel of the form.
But I'm not able to figure out the right XAML syntax.

I tried several approaches that I found after some googling, but none of them worked for me:

<Button Content="{Binding}" Command="{Binding RelativeSource={RelativeSource Window}, Path=DataContext.FireCommand}" />

<Button Content="{Binding}" Command="{Binding Path=FireCommand, Source={StaticResource MainWindow}}" />

<Button Content="{Binding}" Command="{Binding Path=FireCommand, RelativeSource={RelativeSource AncestorType={x:Type Window}}}" />

Could someone please:

1. give me the proper XAML to bind the button inside the ListBox to a command in the form's MainViewModel?
2. point me to a link where this advanced binding stuff is explained in a way that a WPF/MVVM beginner can understand?
   I'm feeling like I'm just copying and pasting arcane XAML incantations, and so far I don't have any clue (and can't find any good documentation) how I would figure out by myself in which cases I'd need RelativeSource or StaticResource or whatever instead of a "normal" binding.

Answer 1

It's:

{Binding DataContext.FireCommand,
         RelativeSource={RelativeSource AncestorType=ListBox}}

No need to walk up to the root unless you actually change the DataContext along the way, but as the ListBox seems to bind to a property on the main VM this should be enough.

The only thing i recommend reading is the Data Binding Overview, and the Binding class documentation (including its properties).

---

Also here is a short explanation on how bindings are constructed: A binding consists of a source and a Path relative to that source, by default the source is the current DataContext. Sources that can be set explicitly are: Source, ElementName & RelativeSource. Setting any of those will override the DataContext as source.

So if you use a source like RelativeSource and want to access something in the DataContext on that level the DataContext needs to appear in the Path.