Error Template Design - wpf

Question

It seems like I read another question / answer on this site about this issue but I cannot recall what the answer was and now I cannot find the original post.
I am not a fan of the default error template in WPF. I understand how to change this error template. However, if I add some content to the end of, say, a textbox, the size of the textbox does not change and the added content will (potentially) get clipped. How do I alter the textbox (I believe the correct termonology is adorned element) in this scenario so that nothing gets clipped?
Here is the XAML for the error template:
<Style TargetType="{x:Type TextBox}">
<Setter Property="Validation.ErrorTemplate">
<Setter.Value>
<ControlTemplate>
<StackPanel>
<AdornedElementPlaceholder />
<TextBlock Foreground="Red" Text="Error..." />
</StackPanel>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Here is the XAML for a couple of textboxes in the form:
<StackPanel>
<TextBox Text="{Binding...}" />
<TextBox />
</StackPanel>

Answer 1

Here's a solution adapted from Josh Smith's article on Binding to (Validation.Errors)[0] without Creating Debug Spew.
The trick is to define a DataTemplate to render the ValidationError object and then use a ContentPresenterto display the error message. If there is no error, then the ContentPresenter will not be displayed.
Below, I have shared the code of the sample app that I created.
Here is the XAML:
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
SizeToContent="WidthAndHeight"
Title="MainWindow">
<StackPanel Margin="5">
<StackPanel.Resources>
<DataTemplate DataType="{x:Type ValidationError}">
<TextBlock Text="{Binding ErrorContent}" Foreground="White" Background="Red" VerticalAlignment="Center" FontWeight="Bold"/>
</DataTemplate>
</StackPanel.Resources>
<TextBox Name="TextBox1" Text="{Binding Text1, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}"/>
<ContentPresenter Content="{Binding ElementName= TextBox1, Path=(Validation.Errors).CurrentItem}" HorizontalAlignment="Left"/>
<TextBox Name="TextBox2" Text="{Binding Text2, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}"/>
<ContentPresenter Content="{Binding ElementName= TextBox2, Path=(Validation.Errors).CurrentItem}" HorizontalAlignment="Left"/>
<Button Content="Validate" Click="Button_Click"/>
</StackPanel>
</Window>
The code behind file:
namespace WpfApplication1
{
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
private ViewModel _ViewModel = null;
public MainWindow()
{
InitializeComponent();
_ViewModel = new ViewModel();
DataContext = _ViewModel;
}
private void Button_Click(object sender, RoutedEventArgs e)
{
_ViewModel.Validate = true;
_ViewModel.OnPropertyChanged("Text1");
_ViewModel.OnPropertyChanged("Text2");
}
}
}
The ViewModel:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ComponentModel;
namespace WpfApplication1
{
public class ViewModel : INotifyPropertyChanged, IDataErrorInfo
{
private string _Text1;
public string Text1
{
get { return _Text1; }
set
{
_Text1 = value;
OnPropertyChanged("Text1");
}
}
private string _Text2;
public string Text2
{
get { return _Text2; }
set
{
_Text2 = value;
OnPropertyChanged("Text2");
}
}
public bool Validate { get; set; }
#region INotifyPropertyChanged Implemenation
public event PropertyChangedEventHandler PropertyChanged;
public void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
#endregion
#region IDataErrorInfo Implementation
public string Error
{
get { return null; }
}
public string this[string columnName]
{
get
{
string errorMessage = string.Empty;
if (Validate)
{
switch (columnName)
{
case "Text1":
if (Text1 == null)
errorMessage = "Text1 is mandatory.";
else if (Text1.Trim() == string.Empty)
errorMessage = "Text1 is not valid.";
break;
case "Text2":
if (Text2 == null)
errorMessage = "Text2 is mandatory.";
else if (Text2.Trim() == string.Empty)
errorMessage = "Text2 is not valid.";
break;
}
}
return errorMessage;
}
}
#endregion
}
}