Bindinggroup validation function not called - wpf

Question

I have set a BindingGroup on my CustomControl and implemented a Validator function. Below is a code snippet from the XAML. My problem is that the validator never gets called. When I create a small sample program things are working. How could I trace the reason down?
XAML:
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfApplication1"
Title="MainWindow" Height="350" Width="525">
<Window.BindingGroup>
<BindingGroup>
<BindingGroup.ValidationRules>
<local:DurationValidator/>
</BindingGroup.ValidationRules>
</BindingGroup>
</Window.BindingGroup>
<StackPanel>
<TextBox Text="{Binding SomeString, UpdateSourceTrigger=PropertyChanged}"></TextBox>
<Button>add</Button>
</StackPanel>
</Window>
Code behind:
namespace WpfApplication1
{
public partial class MainWindow : Window, INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public MainWindow()
{
InitializeComponent();
this.DataContext = this;
SomeString = "Some string";
}
private string _someString;
public string SomeString
{
get { return _someString; }
set
{
if (_someString == value) return;
_someString = value;
if (this.PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs("SomeString"));
}
}
}
public class DurationValidator : ValidationRule
{
public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)
{
BindingGroup bindingGroup = (BindingGroup)value;
return new ValidationResult(false, "Whatever");
}
}
}

Answer 1

Refer to the below code.
XAML
<StackPanel x:Name="stk">
<StackPanel.BindingGroup>
<BindingGroup Name="myBindingGroup">
<BindingGroup.ValidationRules>
<local:DurationValidator ValidatesOnTargetUpdated="True" />
</BindingGroup.ValidationRules>
</BindingGroup>
</StackPanel.BindingGroup>
<TextBox Text="{Binding BindingGroupName=myBindingGroup,UpdateSourceTrigger=PropertyChanged,Path=SomeString,Mode=TwoWay}"></TextBox>
<Button>add</Button>
</StackPanel>
C#
public partial class MainWindow : Window, INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public MainWindow()
{
InitializeComponent();
this.DataContext = this;
SomeString = "Some string";
}
private string _someString;
public string SomeString
{
get { return _someString; }
set
{
if (_someString == value) return;
_someString = value;
if (this.PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs("SomeString"));
this.stk.BindingGroup.CommitEdit();
}
}
}
public class DurationValidator : ValidationRule
{
public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)
{
BindingGroup bindingGroup = (BindingGroup)value;
return new ValidationResult(false, "Whatever");
}
}