INotifyDataErrorInfo is ignored when opening a window with an user control a second time

Question

I've got quite a confusing situation:

I open a dialogue that shows a view with an INotifyDataErrorInfo that immediately returns an error (when the text field is not empty), I see the the red border error notifier:

Opening #1:

I do nothing and close the window, then click the open button again:

Opening #2:

What the heck? I've checked the error flag, it is set. The error border re-appears when I remove the text and write something back, since the error condition checks for string empty? error: no error

Here is a small reproduction case:

Edit: I added the ViewModel back, which is created on every show, causing the INCP change event

MainWindow.xaml.cs

using System;
using System.Collections;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Controls;

namespace LayoutBreakerMinimal
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void ButtonBase_OnClick( object sender, RoutedEventArgs e )
        {
            var w = new Window();
            var v = Resources["OneInstanceView"] as View; // new View(); <-- would work
            w.Content = v;
            v.DataContext = new ViewModel();
            w.ShowDialog();
        }
    }


    public partial class View : UserControl
    {
        public View()
        {
            InitializeComponent();
        }
    }


    public partial class ViewModel : INotifyDataErrorInfo, INotifyPropertyChanged
    {
        private string _myTextField;

        public ViewModel()
        {
            MyTextField = "Error field";
        }

        public string MyTextField
        {
            get { return _myTextField; }
            set
            {
                _myTextField = value;
                OnPropertyChanged();
                if ( ErrorsChanged != null ) ErrorsChanged( this, new DataErrorsChangedEventArgs( "MyTextField" ) );
            }
        }

        public IEnumerable GetErrors( string propertyName )
        {
            yield return "Field is null";
        }

        public bool HasErrors
        {
            get { return MyTextField != ""; }
        }

        public event EventHandler< DataErrorsChangedEventArgs > ErrorsChanged;

        protected virtual void OnPropertyChanged( [CallerMemberName] string propertyName = null )
        {
            var handler = PropertyChanged;
            if ( handler != null ) handler( this, new PropertyChangedEventArgs( propertyName ) );
        }

        public event PropertyChangedEventHandler PropertyChanged;
    }
}

MainWindow.xaml

<Window x:Class="LayoutBreakerMinimal.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:layoutBreakerMinimal="clr-namespace:LayoutBreakerMinimal"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <layoutBreakerMinimal:View x:Key="OneInstanceView" />
    </Window.Resources>
    <Grid>

        <Button Click="ButtonBase_OnClick" Margin="40">Open Dialog, then open it again</Button>

    </Grid>
</Window>

View.xaml

<UserControl x:Class="LayoutBreakerMinimal.View"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
 <StackPanel>
    <TextBox Text="{Binding MyTextField, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, NotifyOnValidationError=True}" Height="30" Width="100"></TextBox>
    <Label Content="{Binding HasErrors}" Height="30" Width="100"></Label>
 </StackPanel>
</UserControl>

I can't get my head around why the border vanishes.

What I've found out: If I create the view every time new (and not the single resource instance), then the red border is available right from the start every time.

I tested moving the INotifyDataErrorInfo into a separate ViewModel, which is instantiated every time new -> No luck.

Edit 2: I added the HasError label to the View to indicate that it keeps displaying error

Answer 1

Solution:

Add x:Shared="False" as shown below: I have tested the fix and it's working as expected.

<Window.Resources>
        <layoutBreakerMinimal:View x:Key="OneInstanceView" x:Shared="False" />
</Window.Resources>

When x:Shared="False", modifies WPF resource-retrieval behavior so that requests for the attributed resource create a new instance for each request instead of sharing the same instance for all requests.

Here is the Modified MainWindow.xaml

<Window x:Class="LayoutBreakerMinimal.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:layoutBreakerMinimal="clr-namespace:LayoutBreakerMinimal"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <layoutBreakerMinimal:View x:Key="OneInstanceView" x:Shared="False" />
    </Window.Resources>

    <Grid>
        <Button Click="ButtonBase_OnClick" Margin="40">Open Dialog, then open it again</Button>
    </Grid>
</Window>