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
    }
}