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。

-

- ```xaml

- <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>

- ```

-

- ```C#

- 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); }

- }

- }