回答編集履歴

  > 選択されているというのが重要な情報の場合（ファイルに選択情報も保存して次回復元する等）は、別の手のほうがいいかもしれません。

 

  > Taskクラスにbool IsSelectedプロパティを用意して、lst_TitleではListBoxItemのIsSelectedにバインドし、lst_DetailではListBoxItemのVisibilityにバインドします。

  > どちらのListBoxもTaskListがソースですが、lst_Detailでは未選択のものは無いように見えます。

  > ちょっとずるいようですが、割とポピュラーな手段だと思います。もし気になるようでしたら追記しますので言ってください。

 

  １万字に収まらなかったのでこちらで失礼します。

 

  <Window

  x:Class="Questions284783.Views.MainWindow"

  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

  xmlns:controls="clr-namespace:System.Windows.Controls;assembly=PresentationFramework"

  xmlns:prism="http://prismlibrary.com/"

  Width="525"

  Height="350"

  prism:ViewModelLocator.AutoWireViewModel="True">

  <Window.Resources>

  <BooleanToVisibilityConverter x:Key="BooleanToVisibilityConverter" />

  </Window.Resources>

  <Grid>

  <Grid.ColumnDefinitions>

  <ColumnDefinition />

  <ColumnDefinition Width="2*" />

  </Grid.ColumnDefinitions>

  <Grid.RowDefinitions>

  <RowDefinition Height="Auto" />

  <RowDefinition />

  </Grid.RowDefinitions>

  <Grid>

  <Grid.ColumnDefinitions>

  <ColumnDefinition Width="Auto" />

  <ColumnDefinition />

  </Grid.ColumnDefinitions>

  <Label

  Margin="5"

  VerticalAlignment="Center"

  Content="表題" />

  <TextBox

  Grid.Column="1"

  Margin="5"

  VerticalAlignment="Center"

  Text="{Binding Txt_Title, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"

  TextWrapping="Wrap" />

  </Grid>

  <Grid Grid.Column="1">

  <Grid.ColumnDefinitions>

  <ColumnDefinition Width="Auto" />

  <ColumnDefinition />

  <ColumnDefinition Width="Auto" />

  </Grid.ColumnDefinitions>

  <Label

  Margin="5"

  VerticalAlignment="Center"

  Content="Memo" />

  <TextBox

  Grid.Column="1"

  Margin="5"

  VerticalAlignment="Center"

  Text="{Binding Txt_Memo, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"

  TextWrapping="Wrap" />

  <Button

  Grid.Column="2"

  MinWidth="56"

  Margin="5"

  Command="{Binding ExecuteCommand}"

  Content="追加" />

  </Grid>

  <Grid Grid.Row="1">

  <Grid.RowDefinitions>

  <RowDefinition Height="Auto" />

  <RowDefinition />

  </Grid.RowDefinitions>

  <ComboBox

  x:Name="comboBox"

  Margin="5"

  SelectedIndex="0">

  <controls:SelectionMode>Single</controls:SelectionMode>

  <controls:SelectionMode>Multiple</controls:SelectionMode>

  <controls:SelectionMode>Extended</controls:SelectionMode>

  </ComboBox>

  <ListBox

  x:Name="lst_Title"

  Grid.Row="1"

  Margin="5"

  DisplayMemberPath="Title"

  IsSynchronizedWithCurrentItem="True"

  ItemsSource="{Binding TaskList}"

  SelectionMode="{Binding SelectedValue, ElementName=comboBox}">

  <ListBox.ItemContainerStyle>

  <Style TargetType="ListBoxItem">

  <Setter Property="IsSelected" Value="{Binding IsSelected}" />

  </Style>

  </ListBox.ItemContainerStyle>

  </ListBox>

  </Grid>

  <ListBox

  Grid.Row="1"

  Grid.Column="1"

  Margin="5"

  ItemsSource="{Binding TaskList}">

  <ListBox.ItemContainerStyle>

  <Style TargetType="ListBoxItem">

  <Setter Property="Visibility" Value="{Binding IsSelected, Converter={StaticResource BooleanToVisibilityConverter}}" />

  </Style>

  </ListBox.ItemContainerStyle>

  <ListBox.ItemTemplate>

  <DataTemplate>

  <StackPanel>

  <TextBlock FontWeight="Bold" Text="{Binding Title}" />

  <TextBlock Text="{Binding Detail}" />

  </StackPanel>

  </DataTemplate>

  </ListBox.ItemTemplate>

  </ListBox>

  </Grid>

  </Window>

  ```

 

  using System.Collections.ObjectModel;

  using Prism.Commands;

  using Prism.Mvvm;

 

  namespace Questions284783.ViewModels

  {

  public class MainWindowViewModel : BindableBase

  {

  private string txt_Title;

  public string Txt_Title

  {

  get => txt_Title;

  set { if(SetProperty(ref txt_Title, value)) ExecuteCommand.RaiseCanExecuteChanged(); }

  }

 

  private string txt_Memo;

  public string Txt_Memo

  {

  get => txt_Memo;

  set { if(SetProperty(ref txt_Memo, value)) ExecuteCommand.RaiseCanExecuteChanged(); }

  }

 

  public ObservableCollection<Task> TaskList { get; } = new ObservableCollection<Task>();

  public DelegateCommand ExecuteCommand { get; private set; }

 

  public MainWindowViewModel()

  {

  TaskList.Add(new Task { Title = "aaa", Detail = "111" });

  TaskList.Add(new Task { Title = "bbb", Detail = "222" });

  TaskList.Add(new Task { Title = "ccc", Detail = "333" });

  ExecuteCommand = new DelegateCommand(Execute, CanExecute);

  }

 

  private void Execute()

  {

  TaskList.Add(new Task { Title = Txt_Title, Detail = Txt_Memo });

  Txt_Title = "";

  Txt_Memo = "";

  ExecuteCommand.RaiseCanExecuteChanged();

  }

 

  private bool CanExecute() => !string.IsNullOrEmpty(Txt_Title);

  }

 

  public class Task : BindableBase

  {

  public string Title { get; set; }

  public string Detail { get; set; }

 

  private bool isSelected;

  public bool IsSelected { get => isSelected; set => SetProperty(ref isSelected, value); }

  }

  }

  ```