回答編集履歴

- 一応できましたが、地味に難しいですね^^;

- 普段`ReactiveProperty`使ってないので間違いや、もっといい方法があるかもしれません。

- 特にViewでのエラーをどうするか（`HasViewError`）は、考慮がいるかもしれませんね（`ReactiveProperty<string> Number1`としてしまってVM側でintにするとか）

-

- `NuGet`で`Microsoft.Xaml.Behaviors.Wpf`を入れてください。

-

- ```xaml

- <Window

- x:Class="Questions289115.MainWindow"

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

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

- xmlns:i="http://schemas.microsoft.com/xaml/behaviors"

- xmlns:local="clr-namespace:Questions289115"

- Width="800"

- Height="450">

- <Window.DataContext>

- <local:ViewModel />

- </Window.DataContext>

-

- <i:Interaction.Behaviors>

- <local:ValidationErrorBehavior HasViewError="{Binding HasViewError.Value, Mode=OneWayToSource}" />

- </i:Interaction.Behaviors>

-

- <Window.Resources>

- <ControlTemplate x:Key="ValidationTemplate">

- <StackPanel>

- <ItemsControl HorizontalAlignment="Right" ItemsSource="{Binding AdornedElement.(Validation.Errors), ElementName=validationTarget}">

- <ItemsControl.ItemsPanel>

- <ItemsPanelTemplate>

- <StackPanel Orientation="Horizontal" />

- </ItemsPanelTemplate>

- </ItemsControl.ItemsPanel>

- <ItemsControl.ItemTemplate>

- <DataTemplate>

- <TextBlock Foreground="Red" Text="{Binding ErrorContent}" />

- </DataTemplate>

- </ItemsControl.ItemTemplate>

- </ItemsControl>

- <AdornedElementPlaceholder x:Name="validationTarget" />

- </StackPanel>

- </ControlTemplate>

-

- <Style TargetType="TextBox">

- <Setter Property="Validation.ErrorTemplate" Value="{StaticResource ValidationTemplate}" />

- </Style>

- </Window.Resources>

-

- <StackPanel>

- <HeaderedContentControl Header="Text1">

- <TextBox Text="{Binding Text1.Value, UpdateSourceTrigger=PropertyChanged}" />

- </HeaderedContentControl>

- <HeaderedContentControl Header="Number1">

- <TextBox Text="{Binding Number1.Value, UpdateSourceTrigger=PropertyChanged, NotifyOnValidationError=True}" />

- </HeaderedContentControl>

- <HeaderedContentControl Header="Text2">

- <TextBox Text="{Binding Text2.Value, UpdateSourceTrigger=PropertyChanged}" />

- </HeaderedContentControl>

- <HeaderedContentControl Header="Number2">

- <TextBox Text="{Binding Number2.Value, UpdateSourceTrigger=PropertyChanged, NotifyOnValidationError=True}" />

- </HeaderedContentControl>

-

- <Button Command="{Binding Command}" Content="Command" />

- </StackPanel>

- </Window>

- ```

-

- ```C#

- using System;

- using System.ComponentModel;

- using System.Diagnostics;

- using System.Linq;

- using System.Reactive.Linq;

- using System.Windows;

- using System.Windows.Controls;

- using Microsoft.Xaml.Behaviors;

- using Reactive.Bindings;

- using Reactive.Bindings.Extensions;

-

- namespace Questions289115

- {

- public class ViewModel : INotifyPropertyChanged

- {

- public ReactiveProperty<string> Text1 { get; }

- public ReactiveProperty<string> Text2 { get; }

- public ReactiveProperty<int> Number1 { get; }

- public ReactiveProperty<int> Number2 { get; }

- public ReactiveProperty<bool> HasViewError { get; } = new ReactiveProperty<bool>();

-

- public ReactiveCommand Command { get; }

-

- public ViewModel()

- {

- Text1 = new ReactiveProperty<string>()

- .SetValidateNotifyError(v =>

- {

- // 正しく複数エラー対応するならこんなん？（もっとスッキリ書けない？^^;

- var errors = new string[] {

- string.IsNullOrEmpty(v) ? "何か入力してください。" : null,

- v == Text2?.Value ? $"{nameof(Text2)}と同一です。" : null,

- }.Where(x => x != null);

-

- if(0 < errors.Count()) return errors;

- return null;

- });

- Text1.Subscribe(_ => Text2?.ForceValidate()); // もっといいのがありそう？？

-

- Text2 = new ReactiveProperty<string>()

- // 複数エラーとかどうでもいいなら

- .SetValidateNotifyError(v => string.IsNullOrEmpty(v) ? "何か入力してください。"

- : v == Text1?.Value ? $"{nameof(Text1)}と同一です。" : null);

- Text2.Subscribe(_ => Text1?.ForceValidate());

-

- Number1 = new ReactiveProperty<int>()

- .SetValidateNotifyError(v =>

- {

- var errors = new string[] {

- v < 0 ? "0以上を入力してください。" : null,

- v == Number2?.Value ? $"{nameof(Number2)}と同一です。" : null,

- }.Where(x => x != null);

-

- if(0 < errors.Count()) return errors;

- return null;

- });

- Number1.Subscribe(_ => Number2?.ForceValidate());

-

- Number2 = new ReactiveProperty<int>()

- .SetValidateNotifyError(v =>

- {

- var errors = new string[] {

- v < 0 ? "0以上を入力してください。" : null,

- v == Number1?.Value ? $"{nameof(Number1)}と同一です。" : null,

- }.Where(x => x != null);

-

- if(0 < errors.Count()) return errors;

- return null;

- });

- Number2.Subscribe(_ => Number1?.ForceValidate());

-

- Command = new[]

- {

- Text1.ObserveHasErrors,

- Text2.ObserveHasErrors,

- Number1.ObserveHasErrors,

- Number2.ObserveHasErrors,

- HasViewError,

- }

- .CombineLatestValuesAreAllFalse()

- .ToReactiveCommand();

- Command.Subscribe(() => Debug.WriteLine("Execute"));

- }

-

- public event PropertyChangedEventHandler PropertyChanged;

- }

-

- // [MVVMにおけるView層での入力値エラーの有無をViewModelで知る方法 - かずきのBlog@hatena](https://blog.okazuki.jp/entry/20110118/1295338167)

- public class ValidationErrorBehavior : Behavior<DependencyObject>

- {

- public bool HasViewError

- {

- get => (bool)GetValue(HasViewErrorProperty);

- set => SetValue(HasViewErrorProperty, value);

- }

- public static readonly DependencyProperty HasViewErrorProperty

- = DependencyProperty.Register(nameof(HasViewError), typeof(bool),

- typeof(ValidationErrorBehavior), new UIPropertyMetadata(false));

- private int errroCount;

- protected override void OnAttached()

- {

- base.OnAttached();

- Validation.AddErrorHandler(AssociatedObject, ErrorHandler);

- }

- protected override void OnDetaching()

- {

- Validation.RemoveErrorHandler(AssociatedObject, ErrorHandler);

- base.OnDetaching();

- }

- private void ErrorHandler(object sender, ValidationErrorEventArgs e)

- {

- if(e.Action == ValidationErrorEventAction.Added) errroCount++;

- else if(e.Action == ValidationErrorEventAction.Removed) errroCount--;

- HasViewError = errroCount != 0;

- }

- }

- }

- ```

-

- 参考ページ

- [ReactiveProperty の Validation は DataAnnotation じゃないと思った？ episode: 9 | :: halation ghost ::](https://elf-mission.net/programming/wpf/episode09#ReactivePropertySetValidateNotifyError_Validation)

-

- [WPFでTextBoxに入力エラーがないときだけ押せるボタンを実現したい - かずきのBlog@hatena](https://blog.okazuki.jp/entry/2016/07/16/144319)

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