In a C# WPF project, I have a ContentControl in the MainWindow. I bind to the whole MainViewModel which is my DataContext. Only bind on my StreamDeckViewModel StreamDeck didn’t work.

My DataTemplateSelector is just hit once, but not when I change the value of the InputMode in my KeyViewModel. The KeyViewModel is in the StreamDeckViewModel which is in the MainViewModel. I use the NuGet CommunityToolkit.mvvm, all my view models inherit from ObservableObject and I use Full-Properties with their backing fields.

Markup in my MainWindow.xaml:

<ContentControl Content="{Binding}" 
       ContentTemplateSelector="{StaticResource FromModeTemplateSelector}"  />

<Window.Resources>
    <DataTemplate x:Key="OpeningModeTemplate">
        <ScrollViewer>
            <DockPanel VerticalAlignment="Top">
                <ComboBox
                      DockPanel.Dock="Top"
                      ItemsSource="{Binding StreamDeck.SelectedKey.ActionChoices}"
                      SelectedIndex="{Binding StreamDeck.SelectedKey.SelectedActionIndex}" />

                <StackPanel DockPanel.Dock="Top" Orientation="Horizontal">
                    <Label Content="{x:Static resx:Resources.ChooseFilePath}" />
                    <Button
                            Width="30"
                            Background="Transparent"
                            BorderBrush="Transparent"
                            Command="{Binding SelectFileToOpenCommand}"
                            DockPanel.Dock="Right">
                            <Image Source="/EmbeddedResourcesIn/file.png" />
                    </Button>
                </StackPanel>

                <StackPanel DockPanel.Dock="Top" Orientation="Horizontal">
                    <Label Content="{x:Static resx:Resources.ChooseFolderPath}" />
                    <Button
                            Width="30"
                            Background="Transparent"
                            BorderBrush="Transparent"
                            Command="{Binding SelectFolderToOpenCommand}"
                            DockPanel.Dock="Right">
                            <Image Source="/EmbeddedResourcesIn/folder.png" />
                    </Button>
                </StackPanel>

                <ScrollViewer
                        DockPanel.Dock="Top"
                        HorizontalScrollBarVisibility="Visible"
                        VerticalScrollBarVisibility="Disabled">
                        <TextBlock Text="{Binding StreamDeck.SelectedKey.Path}" />
                </ScrollViewer>

                <StackPanel DockPanel.Dock="Top" Orientation="Horizontal">
                    <Label Content="{x:Static resx:Resources.ActualizeImage}" />
                    <CheckBox IsChecked="{Binding Path=StreamDeck.SelectedKey.ActualizeImage, Mode=TwoWay}" />
                </StackPanel>

                <StackPanel DockPanel.Dock="Top" Orientation="Horizontal">
                    <Label Content="{x:Static resx:Resources.ChooseImagePath}" />
                    <Button
                            Width="30"
                            Background="Transparent"
                            BorderBrush="Transparent"
                            Command="{Binding SelectImagePathCommand}"
                            DockPanel.Dock="Right">
                            <Image Source="/EmbeddedResourcesIn/image.png" />
                    </Button>
                </StackPanel>

                <ScrollViewer
                        DockPanel.Dock="Top"
                        HorizontalScrollBarVisibility="Visible"
                        VerticalScrollBarVisibility="Disabled">
                        <TextBlock Text="{Binding StreamDeck.SelectedKey.ImagePath}" />
                </ScrollViewer>

                <Label Content="{x:Static resx:Resources.TypeText}" DockPanel.Dock="Top" />
                <TextBox DockPanel.Dock="Top" Text="{Binding StreamDeck.SelectedKey.Text.Content}" />
                <Label Content="{x:Static resx:Resources.ChooseTextColor}" DockPanel.Dock="Top" />
                <ComboBox
                        DockPanel.Dock="Top"
                        ItemsSource="{Binding StreamDeck.SelectedKey.Text.ColorChoices}"
                        SelectedIndex="{Binding StreamDeck.SelectedKey.Text.SelectedColorIndex}">
                        <ComboBox.ItemTemplate>
                            <DataTemplate>
                                <StackPanel Orientation="Horizontal">
                                    <Rectangle
                                        Width="10"
                                        Height="10"
                                        Fill="{Binding Code}" />
                                    <TextBlock
                                        Margin="5"
                                        VerticalAlignment="Center"
                                        Text="{Binding Name}" />
                                </StackPanel>
                            </DataTemplate>
                        </ComboBox.ItemTemplate>
                </ComboBox>
            </DockPanel>
        </ScrollViewer>
    </DataTemplate>

    <DataTemplate x:Key="ShortcutModeTemplate">
            <ScrollViewer>
                <DockPanel VerticalAlignment="Top">
                    <ComboBox
                        DockPanel.Dock="Top"
                        ItemsSource="{Binding StreamDeck.SelectedKey.ActionChoices}"
                        SelectedIndex="{Binding StreamDeck.SelectedKey.SelectedActionIndex}" />

                    <StackPanel DockPanel.Dock="Top" Orientation="Horizontal">
                        <Label Content="{x:Static resx:Resources.EnterShortcut}" />
                        <Button
                            Width="30"
                            Background="Transparent"
                            BorderBrush="Transparent"
                            Command="{Binding EnterShortcutCommand}"
                            DockPanel.Dock="Right">
                            <Image Source="/EmbeddedResourcesIn/shortcutMiniBlack.png" />
                        </Button>
                    </StackPanel>

                    <StackPanel DockPanel.Dock="Top" Orientation="Horizontal">
                        <Label Content="{x:Static resx:Resources.ActualizeImage}" />
                        <CheckBox IsChecked="{Binding Path=StreamDeck.SelectedKey.ActualizeImage, Mode=TwoWay}" />
                    </StackPanel>

                    <StackPanel DockPanel.Dock="Top" Orientation="Horizontal">
                        <Label Content="{x:Static resx:Resources.ChooseImagePath}" />
                        <Button
                            Width="30"
                            Background="Transparent"
                            BorderBrush="Transparent"
                            Command="{Binding SelectImagePathCommand}"
                            DockPanel.Dock="Right">
                            <Image Source="/EmbeddedResourcesIn/image.png" />
                        </Button>
                    </StackPanel>

                    <ScrollViewer
                        DockPanel.Dock="Top"
                        HorizontalScrollBarVisibility="Visible"
                        VerticalScrollBarVisibility="Disabled">
                        <TextBlock Text="{Binding StreamDeck.SelectedKey.ImagePath}" />
                    </ScrollViewer>

                    <Label Content="{x:Static resx:Resources.TypeText}" DockPanel.Dock="Top" />
                    <TextBox DockPanel.Dock="Top" Text="{Binding StreamDeck.SelectedKey.Text.Content}" />
                    <Label Content="{x:Static resx:Resources.ChooseTextColor}" DockPanel.Dock="Top" />
                    <ComboBox
                        DockPanel.Dock="Top"
                        ItemsSource="{Binding StreamDeck.SelectedKey.Text.ColorChoices}"
                        SelectedIndex="{Binding StreamDeck.SelectedKey.Text.SelectedColorIndex}">
                        <ComboBox.ItemTemplate>
                            <DataTemplate>
                                <StackPanel Orientation="Horizontal">
                                    <Rectangle
                                        Width="10"
                                        Height="10"
                                        Fill="{Binding Code}" />
                                    <TextBlock
                                        Margin="5"
                                        VerticalAlignment="Center"
                                        Text="{Binding Name}" />
                                </StackPanel>
                            </DataTemplate>
                        </ComboBox.ItemTemplate>
                    </ComboBox>
                </DockPanel>
            </ScrollViewer>
   </DataTemplate>

   <dts:FromModeTemplateSelector
            x:Key="FromModeTemplateSelector"
            OpeningModeTemplate="{StaticResource OpeningModeTemplate}"
            ShortcutModeTemplate="{StaticResource ShortcutModeTemplate}" />

</Window.Resources>

Code in my KeyViewModel:

public InputModeBase InputMode { get => inputMode; set => SetProperty(ref inputMode, value); }
private InputModeBase inputMode = new OpeningMode();

public int SelectedActionIndex
{
    get => selectedActionIndex;
    set
    {
        SetProperty(ref selectedActionIndex, value);

        if (selectedActionIndex == Constants.OPEN_INDEX)
        {
            InputMode = new OpeningMode();
        }
        else if (selectedActionIndex == Constants.SHORTCUT_INDEX)
        {
            InputMode = new ShortcutMode();
        }
    }
}

private int selectedActionIndex = 0;

My DataTemplateSelector:

public class FromModeTemplateSelector : DataTemplateSelector
{
     public System.Windows.DataTemplate OpeningModeTemplate { get; set; } = new();
     public System.Windows.DataTemplate ShortcutModeTemplate { get; set; } = new();

     public override System.Windows.DataTemplate SelectTemplate(object item, System.Windows.DependencyObject container)
     {
         if (container is System.Windows.FrameworkElement && item != null && item is MainViewModel mainViewModel)
         {
             if (mainViewModel.StreamDeck.SelectedKey.InputMode is OpeningMode)
             {
                 return OpeningModeTemplate;
             }
             else if (mainViewModel.StreamDeck.SelectedKey.InputMode is ShortcutMode)
             {
                 return ShortcutModeTemplate;
             }
         }
         return base.SelectTemplate(item, container);
     }
}