WPF

public class _ViewModelBase : BindableBase, INotifyDataErrorInfo
{
    #region Events
    public event EventHandler<DataErrorsChangedEventArgs> ErrorsChanged;
    #endregion

    #region Private Fields
    public readonly Dictionary<string, List<string>> _validationErrors = new Dictionary<string, List<string>>();
    #endregion

    #region Properties
    public bool HasErrors => _validationErrors.Any();
    #endregion

    #region Public Methods
    public IEnumerable GetErrors(string propertyName)
    {
        return _validationErrors.ContainsKey(propertyName) ? _validationErrors[propertyName] : null;
    }
    #endregion

    #region Protected Methods
    protected void AddError(string propertyName, string error)
    {
        if (!_validationErrors.ContainsKey(propertyName))
        {
            _validationErrors[propertyName] = new List<string>();
        }

        if (!_validationErrors[propertyName].Contains(error))
        {
            _validationErrors[propertyName].Add(error);
            RaiseErrorsChanged(propertyName);
        }
    }

    protected void ClearErrors(string propertyName)
    {
        if (_validationErrors.ContainsKey(propertyName))
        {
            _validationErrors.Remove(propertyName);
            {
                RaiseErrorsChanged(propertyName);
            }
        }
    }
    #endregion

    #region Private Methods
    private void RaiseErrorsChanged(string propertyName)
    {
        ErrorsChanged?.Invoke(this, new DataErrorsChangedEventArgs(propertyName));
    }
    #endregion
}

<TextBlock Grid.Row="0"
           Text="Customer Name"/>
<TextBox Grid.Row="1"
         Grid.Column="0"
         Text="{Binding CustomerName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}"
         IsEnabled="{Binding AreFieldsEnabled}">

    <Validation.ErrorTemplate>
        <ControlTemplate>
            <StackPanel>
                <AdornedElementPlaceholder x:Name="textBox" />
                <ItemsControl ItemsSource="{Binding}">
                    <ItemsControl.ItemTemplate>
                        <DataTemplate>
                            <TextBlock Text="{Binding ErrorContent}" Foreground="Red" />
                        </DataTemplate>
                    </ItemsControl.ItemTemplate>
                </ItemsControl>
            </StackPanel>
        </ControlTemplate>
    </Validation.ErrorTemplate>

</TextBox>

private void Validate()
{
    ClearErrors(nameof(Customer.CustomerName));

    if (string.IsNullOrWhiteSpace(Customer.CustomerName))
    {
        AddError(nameof(Customer.CustomerName), "The Customer Name cannot be empty.");
    }

}

private void Customer_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    IsChanged = true;

    Validate();
    RaiseChangedEvent();
}

1. How would you validate that a list has items in it?
2. I have 2 comboboxes with the same collection of items in it. How can I validate that they both have differt items selected?

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:local="clr-namespace:Marois.Framework.WPF.Controls">

<pre>
<SolidColorBrush x:Key="disabledBackgroundBrush" Color="Gray"/>

<Style TargetType="{x:Type local:MaroisFilePicker}">

    <Setter Property="Template">

        <Setter.Value>

            <ControlTemplate>

                <Border x:Name="border"
                        Background="{TemplateBinding Background}" 
                        BorderBrush="{TemplateBinding BorderBrush}" 
                        BorderThickness="{TemplateBinding BorderThickness}" 
                        CornerRadius="{TemplateBinding BorderThickness}">

                    <Grid>

                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="Auto"/>
                            <ColumnDefinition Width="*"/>
                            <ColumnDefinition Width="Auto"/>
                        </Grid.ColumnDefinitions>

                        <TextBlock Text="{Binding Caption}"
                                   x:Name="caption"
                                   Margin="2"/>

                        <TextBox Grid.Column="1"
                                 x:Name="textbox"
                                 Text="{Binding File}"/>

                        <Button Grid.Column="2"
                                x:Name="button"
                                Command="{Binding SelectedFileCommand}"/>

                    </Grid>

                </Border>

                <ControlTemplate.Triggers>
                    <Trigger Property="IsEnabled" Value="False">
                        <Setter TargetName="button" Property="Background" Value="{StaticResource disabledBackgroundBrush}" />
                    </Trigger>

                </ControlTemplate.Triggers>
            </ControlTemplate>

        </Setter.Value>

    </Setter>

</Style>

glfw

<ListView x:Name="ProgressListView" Grid.Row="1" Grid.RowSpan="2" ItemsSource="{Binding ProgressCollection}" Background="Transparent" Panel.ZIndex="10" IsHitTestVisible="False">
<!-- Omitted WPF for the progress units -->
</ListView>

<Button Grid.Row="0" Grid.Column="1" 
Content="{wpf:MaterialIconExt Kind=WindowClose}"
Command="{Binding CloseProgressCommand}" 
Style="{StaticResource ProgressButtonStyle}" />

• I have experimented with overriding the hittest of the listview, so I can ignore hittest in code but allow the button to be pressed. This did not work, but I am still looking into this.
• I have tried fiddling with the ZIndex on the Button, but if the listview ZIndex is lower that the rest, then buttons inside it will of cause also be behind it, no matter the buttons ZIndex. Its a part of the ListView.
• I have also looked into the Adorner layer. I find that this could probably work, but it will require a complete rewrite of the logic and as I understand it is not really what it is meant for and will be very complicated.

<Canvas Grid.Row="1" Grid.RowSpan="2" VerticalAlignment="Bottom">
    <ListView x:Name="ProgressListView" ItemsSource="{Binding ProgressCollection}" Background="Transparent" Canvas.Bottom="0" BorderThickness="0">
    <!-- Content -->
</ListView>
</Canvas>

public App()
{
    AppHost = Host.CreateDefaultBuilder()
                    .ConfigureServices((hostContext, services) =>
                    {
                        services.AddSingleton<IEventAggregator, EventAggregator>();
                        services.AddSingleton<IApplicationSecurity, ApplicationSecurity>();
                        services.AddSingleton<MainWindowViewModel>();
                        services.AddSingleton<MainWindowView>();
                        services.AddSingleton<LoginViewModel>();
                        services.AddSingleton<LoginView>();
                    })
                    .Build();
}