回答編集履歴

  結構悩ましいですね（`double`なとこも地味にいじわる^^;

  takapi_csさんの回答が真っ当という気がしますが、手抜きならこんなのもあるかなと。

 

  コンバータは適当です^^; お好きな実装をどうぞ。

  `KeepTextBoxDisplaySynchronizedWithTextProperty`の影響範囲はよくわかりません（使ったことないので^^;

 

  <Window

  x:Class="Questions295331.MainWindow"

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

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

  xmlns:local="clr-namespace:Questions295331"

  Width="800"

  Height="450">

  <StackPanel>

  <TextBox

  x:Name="textBox"

  Width="100"

  Margin="10"

  Text="{Binding ValidateParameter.Value, UpdateSourceTrigger=PropertyChanged}" />

  <Button

  Width="100"

  Command="{Binding ValidCommand}"

  Content="Valid"

  IsEnabled="{Binding (Validation.HasError), Converter={local:InverseBooleanConverter}, ElementName=textBox}" />

  </StackPanel>

  </Window>

  ```

 

  using System;

  using System.ComponentModel.DataAnnotations;

  using System.Diagnostics;

  using System.Globalization;

  using System.Windows;

  using System.Windows.Data;

  using System.Windows.Markup;

  using Reactive.Bindings;

 

  namespace Questions295331

  {

  public class InverseBooleanConverter : MarkupExtension, IValueConverter

  {

  public object Convert(object value, Type targetType, object parameter, CultureInfo culture)

  => !(bool)value;

  public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)

  => throw new NotImplementedException();

  public override object ProvideValue(IServiceProvider serviceProvider) => this;

  }

 

  class MainWindowsViewModel

  {

  [Range(0, 100d)]

  public ReactiveProperty<double> ValidateParameter { get; }

 

  public ReactiveCommand ValidCommand { get; }

 

  public MainWindowsViewModel()

  {

  ValidateParameter = new ReactiveProperty<double>()

  .SetValidateAttribute(() => ValidateParameter);

 

  ValidCommand = new ReactiveCommand()

  .WithSubscribe(() => Debug.WriteLine(ValidateParameter.Value));

  }

  }

 

  public partial class MainWindow : Window

  {

  public MainWindow()

  {

  FrameworkCompatibilityPreferences.KeepTextBoxDisplaySynchronizedWithTextProperty = false;

 

  InitializeComponent();

  DataContext = new MainWindowsViewModel();

  }

  }

  }

  ```