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how to make Validation work on ContentControl ?
There is a class that is responsible for storing a value with units of measure, it is displayed in the content control through a dataset, I would like to check for correctness when displaying and editing it, for example, units cannot have a value of 2, or the value should be less than 10,000. solve this with multibinding, the validation rule is not satisfied when editing values.

Xaml file:

 <Window x:Class="DELETE1.MainWindow"
 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
 xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
 xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
 xmlns:local="clr-namespace:DELETE1"
 xmlns:diag="clr-namespace:System.Diagnostics;assembly=WindowsBase"
 mc:Ignorable="d"
 x:Name="m_win"
 Title="MainWindow" Height="450" Width="800">
 <Window.Resources>
 <DataTemplate DataType="x:Type local:ValWithUnits">
 <Border>
 <DockPanel>
 <ComboBox DockPanel.Dock="Right" Width="60" IsEnabled="True" SelectedIndex="Binding Unit" ItemsSource="Binding Src, ElementName=m_win" />
 <TextBox Text="Binding Val, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged" IsEnabled="True"/>
 </DockPanel>
 </Border>
 </DataTemplate>
 <local:MultiBindingConverter x:Key="MultiBindingConverter" />
 </Window.Resources>
 <Grid>
 <ContentControl x:Name="m_contentControl">
 <ContentControl.Content>
 <MultiBinding Mode="TwoWay" NotifyOnValidationError="True" 
 UpdateSourceTrigger="PropertyChanged"
 diag:PresentationTraceSources.TraceLevel="High"
 Converter="StaticResource MultiBindingConverter" >
 <Binding Path="Val" Mode="TwoWay" UpdateSourceTrigger="PropertyChanged" diag:PresentationTraceSources.TraceLevel="High"></Binding>
 <Binding Path="Unit" Mode="TwoWay" UpdateSourceTrigger="PropertyChanged" diag:PresentationTraceSources.TraceLevel="High"></Binding>
 <MultiBinding.ValidationRules>
 <local:ContentControlValidationRule ValidatesOnTargetUpdated="True" x:Name="MValidationRule"/>
 </MultiBinding.ValidationRules>
 </MultiBinding>
 </ContentControl.Content>
 </ContentControl>
 </Grid>
 </Window>

And code file:

public class ValWithUnits:INotifyPropertyChanged
 
 private double m_val;
 private int m_unit;
 public double Val
 
 get => m_val;
 set
 
 m_val = value;
 OnPropertyChanged(nameof(Val));
 
 
 public int Unit
 
 get => m_unit;
 set
 
 m_unit = value;
 OnPropertyChanged(nameof(Unit));
 
 
 public event PropertyChangedEventHandler PropertyChanged;

 [NotifyPropertyChangedInvocator]
 protected virtual void OnPropertyChanged(string propertyName)
 
 PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
 
 
 /// <summary>
 /// Interaction logic for MainWindow.xaml
 /// </summary>
 public partial class MainWindow : Window
 
 public ValWithUnits MyValueWithUnits get; set; = new ValWithUnits();

 public MainWindow()
 
 InitializeComponent();
 var a = new Binding("MyValueWithUnits");
 a.Source = this;
 a.ValidatesOnDataErrors = true;
 a.ValidatesOnExceptions = true;
 a.NotifyOnValidationError = true;
 m_contentControl.SetBinding(ContentControl.ContentProperty, a);

 

 public IEnumerable<int> Src => new int[] 1,2,3;
 

 public class ContentControlValidationRule : ValidationRule
 
 public override ValidationResult Validate(object value, CultureInfo cultureInfo)
 
 //Debug.Fail("validate");

 return ValidationResult.ValidResult;
 

 private static ValidationResult BadValidation(string msg) =>
 new ValidationResult(false, msg);
 


 public class MultiBindingConverter : IMultiValueConverter
 
 public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
 
 return string.Format("0-1", values[0], values[1]);
 

 public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
 
 return new object[] 1, 1 ;
 
 

Validation 'ContentControlValidationRule' which I install through multi-binding does not work.