How do I bind to a validation rule in XAML - validation

Question

I am following the example given here:
https://social.technet.microsoft.com/wiki/contents/articles/31422.wpf-passing-a-data-bound-value-to-a-validation-rule.aspx
I get the exception at the following line in my DataTemplateSelector:
return element.FindResource("MinParameterDataTemplateThin") as DataTemplate;
"Unexpected record in Baml stream. Trying to add to MaximumValueValidation which is not a collection or has a TypeConverter."
Code:
public class BindingProxy : System.Windows.Freezable
{
protected override Freezable CreateInstanceCore()
{
return new BindingProxy();
}
public object Data
{
get => (object)GetValue(DataProperty);
set => SetValue(DataProperty, value);
}
public static readonly DependencyProperty DataProperty =
DependencyProperty.Register("Data", typeof(object), typeof(BindingProxy), new PropertyMetadata(null));
}
public class MaxValueWrapper : DependencyObject
{
public static readonly DependencyProperty MaxValueProperty =
DependencyProperty.Register("MaxValue", typeof(double),
typeof(MaxValueWrapper), new FrameworkPropertyMetadata(double.MaxValue));
public int MaxValue
{
get => (int)GetValue(MaxValueProperty);
set => SetValue(MaxValueProperty, value);
}
}
public class MaximumValueValidation : ValidationRule
{
public MaxValueWrapper MaxValueWrapper { get; set; }
/// <summary>
/// validate whether the input value is in range
/// </summary>
public override ValidationResult Validate(object value, CultureInfo cultureInfo)
{
try
{
if (value != null && ((string)value).Length > 0)
{
double userInput = double.Parse((string)value);
if (userInput > MaxValueWrapper.MaxValue)
return new ValidationResult(false,
"Please enter a value <= Max " + MaxValueWrapper.MaxValue);
}
}
catch (Exception e)
{
return new ValidationResult(false, $"Invalid characters or {e.Message}");
}
return ValidationResult.ValidResult;
}
}
XAML:
<DataTemplate x:Key="MinParameterDataTemplateThin">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="120"/>
</Grid.ColumnDefinitions>
<TextBlock Text="{Binding DisplayName, StringFormat='{}{0}:'}" Grid.Column="0" Margin="10,5,5,10" VerticalAlignment="Top" TextWrapping="Wrap"
Visibility="{Binding Visibility}" ToolTipService.ShowDuration="20000">
<TextBlock.ToolTip>
<ToolTip DataContext="{Binding Path=PlacementTarget.DataContext, RelativeSource={x:Static RelativeSource.Self}}">
<TextBlock Text="{Binding Description}"/>
</ToolTip>
</TextBlock.ToolTip>
</TextBlock>
<StackPanel Grid.Column="1" Orientation="Horizontal">
<TextBox Margin="5" Width="50" VerticalAlignment="Top"
Visibility="{Binding Visibility}" IsEnabled="{Binding IsEnabled}">
<i:Interaction.Behaviors>
<utilities:SelectTextOfTextBox/>
</i:Interaction.Behaviors>
<TextBox.Resources>
<validations:BindingProxy x:Key="proxy" Data="{Binding}"/>
</TextBox.Resources>
<TextBox.Text>
<Binding Path="{Binding DefaultValue, StringFormat=N2}" Mode="TwoWay"
UpdateSourceTrigger="PropertyChanged"
ValidatesOnExceptions="True"
NotifyOnValidationError="True"
ValidatesOnNotifyDataErrors="True">
<Binding.ValidationRules>
<validations:MaximumValueValidation ValidatesOnTargetUpdated="True">
<validations:MaximumValueValidation.MaxValueWrapper>
<validations:MaxValueWrapper MaxValue="{Binding Data.MaxValue, Source={StaticResource proxy}}"/>
</validations:MaximumValueValidation.MaxValueWrapper>
</validations:MaximumValueValidation>
</Binding.ValidationRules>
</Binding>
</TextBox.Text>
</TextBox>
<TextBlock Text="{Binding UnitSymbol}" Margin="5" VerticalAlignment="Top" Visibility="{Binding Visibility}"/>
</StackPanel>
</Grid>
</DataTemplate>
I have several working DataTemplates like this one, but they do not use validation rules with the TextBox. Perhaps I am not following the wiki example correctly, but I am not seeing the problem, and I don't understand the exception.
EDIT:
After staring at the exception message, it occurs to me that the "outer" DataTemplate might be relevant, but I cannot be sure. The DataTemplate above is for a WorkflowParameter object, which is part of a Collection nested in a parent WorkflowParameter object. The DataTemplate for this parent is:
<DataTemplate x:Key="GroupedInversionParameterDataTemplate">
<Grid Visibility="{Binding Visibility}">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="160"/>
<ColumnDefinition Width="800"/>
</Grid.ColumnDefinitions>
<TextBlock Text="{Binding DisplayName, StringFormat='{}{0}:'}" Grid.Column="0" Margin="5" VerticalAlignment="Top" TextWrapping="Wrap"
Visibility="{Binding Visibility}" ToolTipService.ShowDuration="20000">
<TextBlock.ToolTip>
<ToolTip DataContext="{Binding Path=PlacementTarget.DataContext, RelativeSource={x:Static RelativeSource.Self}}">
<TextBlock Text="{Binding Description}"/>
</ToolTip>
</TextBlock.ToolTip>
</TextBlock>
<ItemsControl ItemsSource="{Binding GroupedWorkflowParameters}" Grid.Column="1"
ItemTemplateSelector="{StaticResource WorkflowParameterTemplateSelector}"
IsTabStop="False">
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<WrapPanel Orientation="Horizontal"/>
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.ItemContainerStyle>
<Style TargetType="ContentPresenter">
<Setter Property="Visibility" Value="{Binding Visibility}"/>
</Style>
</ItemsControl.ItemContainerStyle>
</ItemsControl>
</Grid>
</DataTemplate>
The mention of trying to add a collection, in the exception message, made me think that the ItemsControl in this outer template could be a problem. I would not expect it to be an issue, but maybe I am missing something.

Answer 1

I though the problem as with the validation XAML, so I commented it out and found there was still an exception. This snippet of XAML:
<Binding Path="{Binding DefaultValue, StringFormat=N2}"
had to change to
<Binding Path="DefaultValue" StringFormat="N2"