<Window.DataContext>
  <Binding RelativeSource="{RelativeSource Self}" />
</Window.DataContext>

<Window.Resources>
  <ControlTemplate x:Key="TextBoxErrorTemplate">
    <StackPanel>
      <Border BorderBrush="#FFdc000c" VerticalAlignment="Top">
        <Grid>
          <AdornedElementPlaceholder x:Name="adorner" Margin="-1" />
        </Grid>
      </Border>
      <Border
        x:Name="errorBorder"
        Background="#FFdc000c"
        Margin="0,2,0,0"
        MinHeight="24"
        Width="{Binding ElementName=adorner, Path=AdornedElement.ActualWidth}">
        <TextBlock
          Text="{Binding ElementName=adorner, Path=AdornedElement.(Validation.Errors)[0].ErrorContent}"
          Foreground="White"
          Margin="5,2,5,3"
          TextWrapping="Wrap"
          VerticalAlignment="Stretch"/>
      </Border>
    </StackPanel>
  </ControlTemplate>
</Window.Resources>

<Grid Margin="5">
  <Grid.RowDefinitions>
    <RowDefinition Height="Auto" />
    <RowDefinition />
  </Grid.RowDefinitions>
  <TextBox
    Margin="5"
    Validation.ErrorTemplate="{StaticResource TextBoxErrorTemplate}">
    <TextBox.Resources>
      <local:StringToIntConverter x:Key="StringToIntConverter" />
    </TextBox.Resources>
    <TextBox.Text>
      <Binding
        Path="Id"
        Mode="TwoWay"
        UpdateSourceTrigger="PropertyChanged"
        Converter="{StaticResource StringToIntConverter}">
        <Binding.ValidationRules>
          <local:IntegerValidator />
        </Binding.ValidationRules>
      </Binding>
    </TextBox.Text>
  </TextBox>
  <Button
    Grid.Row="1"
    VerticalAlignment="Center"
    HorizontalAlignment="Center"
    Content="Test"
    Padding="5"
    Command="{x:Static local:MainWindow.ShowIdCommand}">
    <Button.CommandBindings>
      <CommandBinding
        Command="{x:Static local:MainWindow.ShowIdCommand}"
        Executed="ShowIdCommandHandler" />
    </Button.CommandBindings>
  </Button>
</Grid>

public partial class MainWindow : INotifyPropertyChanged
{
  public MainWindow()
  {
    InitializeComponent();
  }

  int? _id;
  public int? Id
  {
    get => _id;
    set
    {
      _id = value;
      Console.WriteLine("Id={0}", Id?.ToString() ?? "null");
      OnPropertyChanged();
    }
  }

  public static RoutedUICommand ShowIdCommand { get; } = new RoutedUICommand("ShowId", "ShowId", typeof(MainWindow));
  void ShowIdCommandHandler(object sender, ExecutedRoutedEventArgs e)
  {
    MessageBox.Show($"Id=({Id?.ToString() ?? "null"})");
  }

  public event PropertyChangedEventHandler PropertyChanged;
  protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
  {
    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
  }
}

public class StringToIntConverter : IValueConverter
{
  public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
  {
    return ((int?)value)?.ToString(CultureInfo.InvariantCulture) ?? string.Empty;
  }

  public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
  {
    if (string.IsNullOrWhiteSpace(value as string))
      return (int?)null;
    if (int.TryParse((string)value, NumberStyles.Integer, CultureInfo.InvariantCulture, out var n))
      return (int?)n;
    return (int?)null;
  }
}

public class IntegerValidator : ValidationRule
{
  public override ValidationResult Validate(object value, CultureInfo cultureInfo)
  {
    if (string.IsNullOrWhiteSpace(value as string))
      return ValidationResult.ValidResult;
    try
    {
      int.Parse((string)value, NumberStyles.Integer, CultureInfo.InvariantCulture);
      return ValidationResult.ValidResult;
    }
    catch (Exception e)
    {
      return new ValidationResult(false, e.Message);
    }
  }
}