WPF

public class TestControl : Label 
{
    public static readonly DependencyProperty dependencyPropertyTestInfo =
        DependencyProperty.Register("TestInfo", typeof(string), typeof(TestControl),
            new PropertyMetadata("", new PropertyChangedCallback(ChangeTestInfo)));

    public string TestInfo
    {
        get => (string)GetValue(dependencyPropertyTestInfo);
        set => SetValue(dependencyPropertyTestInfo, value);
    }
    private static void ChangeTestInfo(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var testControl = (TestControl)d;
        testControl.TestInfo = (string)e.NewValue;
    }
}

<StackPanel>
    <local:TestControl Content="TestControl" x:Name="tct" TestInfo="{Binding TestInfoView}"/>
    <Button Content="show TestInfo by Button-Click in xaml.cs" x:Name="BtnShow" Click="BtnShow_Click"/>
    <Button Content="set new TestInfo" Name="BtnNewTestInfo" Command="{Binding NewTestInfoCommand}"/>
</StackPanel>

public class MainWindowViewModel : BindableBase
{
    private int counter;
    private string _testInfoView;
    public string TestInfoView
    {
        get { return _testInfoView; }
        set { SetProperty(ref _testInfoView, value); }
    }
    public DelegateCommand NewTestInfoCommand { get; }
    public MainWindowViewModel()
    {
        NewTestInfoCommand = new DelegateCommand(OnNewTestInfoCommand);
        TestInfoView = "Info from ctor";
    }
    private void OnNewTestInfoCommand()
    {
        string newInfo = $"Info No. {++counter}";
        Debug.WriteLine($"{MethodBase.GetCurrentMethod().Name}: {nameof(TestInfoView)}=«{TestInfoView}» -> «{newInfo}»");
        TestInfoView = newInfo;
    }
}

private void BtnShow_Click(object sender, RoutedEventArgs e)
{
   Debug.WriteLine($"{MethodBase.GetCurrentMethod().Name}: {nameof(tct.TestInfo)}={tct.TestInfo}");
}

<!Tab Item Header Template>
<DataTemplate x:Key="CustomHeaderTemplate">

    <DockPanel LastChildFill="True">

        <TextBlock Text="{Binding FileName}"
                   VerticalAlignment="Center"
                   FontSize="14"/>

        <Button Command="{Binding DataContext.CloseTabCommand, RelativeSource={RelativeSource AncestorType={x:Type Window}}}"
                CommandParameter="{Binding}"
                VerticalAlignment="Center"
                Height="22"
                Width="22"
                Background="SteelBlue"
                Margin="3,0,0,0">

            <Path Data="M1,9 L9,1 M1,1 L9,9"
                  Stroke="White"
                  StrokeThickness="2" />

        </Button>

    </DockPanel>

</DataTemplate>

<!Tab Item Style>
<Style TargetType="TabItem">

    <Setter Property="ContextMenu">
        <Setter.Value>
            <ContextMenu DataContext="{Binding Path=PlacementTarget.DataContext, RelativeSource={RelativeSource Self}}" >

                <MenuItem Header="Close Tab"
                          Command="{Binding Source={StaticResource Proxy}, Path=Data.CloseTabCommand}"
                          CommandParameter="{Binding}"/>

                <MenuItem Header="Close All Tabs"
                          Command="{Binding Source={StaticResource Proxy}, Path=Data.CloseAllTabsCommand}"
                          CommandParameter="{Binding}"/>

                <MenuItem Header="Close All But This"
                          Command="{Binding Source={StaticResource Proxy}, Path=Data.CloseAllButThisTabCommand}"
                          CommandParameter="{Binding}"/>
            </ContextMenu>

        </Setter.Value>
    </Setter>

</Style>

<Window.Resources>

    <DataTemplate DataType="{x:Type vms:PlayerHostViewModel}">
        <vws:PlayerHostView/>
    </DataTemplate>

</Window.Resources>

<Border Grid.Row="1"
        Grid.Column="0"
        BorderThickness="1"
        BorderBrush="SteelBlue"
        Margin="5,0,0,5">

    <ItemsControl ItemsSource="{Binding Players}"
                    Margin="2">

        <ItemsControl.ItemsPanel>
            <ItemsPanelTemplate>
                <UniformGrid IsItemsHost="True"
                                Margin="2"/>
            </ItemsPanelTemplate>
        </ItemsControl.ItemsPanel>

    </ItemsControl>

</Border>

private void LoadPlayers()
{
    // If a player type and count are selected...
    if (SelectedPlayerInfo != null && SelectedPlayerCount > -1)
    {
        // For each player to add...
        for (int i = 0; i < SelectedPlayerCount + 1; i++)
        {
            // Create the player ActiveX control
            var playerAssembly = (IPlayer)Activator.CreateInstance(SelectedPlayerInfo.PlayerType);

            // Create the player host
            var player = new PlayerHostViewModel(playerAssembly, $"Player {i + 1}", LiveStream, DVRStream);

            // Add the player host to the Players list
            Players.Add(player);
        }
    }
}

public PlayerHostViewModel()
{
}
public PlayerHostViewModel(IPlayer player, string playerName, string liveStream, string dvrStream)
{
    Player = player;
    PlayerName = playerName;
    LiveStream = LiveStream;
    DVRStream = dvrStream;  
}

var player = new PlayerHostViewModel(playerAssembly, $"Player {i + 1}", LiveStream, DVRStream);

// Add the player host to the Players list
Players.Add(player);

<i:Interaction.Triggers>
    <i:EventTrigger EventName="AddingNewItem">
        <i:InvokeCommandAction Command="{Binding DataContext.NewItemAddedCommand, 
            RelativeSource={
                RelativeSource FindAncestor, AncestorType=Window}}"/>
    </i:EventTrigger>
</i:Interaction.Triggers>

<DataGrid Grid.Row="2"
          Grid.Column="0"
          ItemsSource="{Binding ColumnInfos}"
          HorizontalGridLinesBrush="LightGray"
          VerticalGridLinesBrush="LightGray"
          AutoGenerateColumns="False"
          CanUserAddRows="True"
          CanUserDeleteRows="True"
          CanUserReorderColumns="False"
          CanUserResizeColumns="False"
          CanUserSortColumns="False"
          BorderBrush="SteelBlue"
          BorderThickness="1"
          HeadersVisibility="Column"
          HorizontalScrollBarVisibility="Auto"
          VerticalScrollBarVisibility="Auto"
          VerticalAlignment="Stretch"
          Margin="5,0,5,5">

    <DataGrid.Resources>
        <local:BindingProxy x:Key="proxy" Data="{Binding IsColumnWidthVisibile}"/>
    </DataGrid.Resources>

    <DataGrid.Columns>

        <DataGridTemplateColumn>
            <DataGridTemplateColumn.CellTemplate>
                <DataTemplate>
                    <Button Command="{Binding DeleteColumnCommand, RelativeSource={RelativeSource FindAncestor,
                                                                    AncestorLevel=1, AncestorType={x:Type Window}}}"
                            CommandParameter="{Binding}"
                            Margin="2"
                            Height="18"
                            Width="18"/>
                </DataTemplate>
            </DataGridTemplateColumn.CellTemplate>
        </DataGridTemplateColumn>

        <DataGridTextColumn Header="Column Name"
                            Binding="{Binding ColumnName}"
                            Width="*"/>

        <DataGridTextColumn Header="Width"
                            Binding="{Binding ColumnWidth}"
                            Width="65"
                            Visibility="{Binding Data, Converter={StaticResource boolToVisibilityConverter}, Source={StaticResource proxy}}"/>

    </DataGrid.Columns>

</DataGrid>

System.InvalidCastException: 'Unable to cast object of type 'MS.Internal.NamedObject' to type 'WPFDataGrid.ColumnInfoEntity'.'

<pre><ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:local="clr-namespace:MyTextBoxEx">

<pre>
<SolidColorBrush x:Key="CaptionNormalColor" Color="Green"/>
<SolidColorBrush x:Key="CaptionMouseOverColor" Color="Purple"/>

<SolidColorBrush x:Key="TextNormalColor" Color="Red"/>
<SolidColorBrush x:Key="TextMouseOverColor" Color="Blue"/>

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

    <Setter Property="Padding" Value="1"/>

    <Setter Property="Template">

        <Setter.Value>

            <ControlTemplate TargetType="{x:Type local:TextBoxEx}">

                <StackPanel Orientation="Vertical">

                    <Border Padding="{Binding Padding, RelativeSource={RelativeSource TemplatedParent}}"
                            Margin="{Binding Margin, RelativeSource={RelativeSource TemplatedParent}}"
                            Background="{TemplateBinding Background}"
                            BorderBrush="{TemplateBinding BorderBrush}"
                            BorderThickness="{TemplateBinding BorderThickness}">

                        <StackPanel Orientation="Vertical">

                            <TextBlock x:Name="caption"
                                       Foreground="{StaticResource CaptionNormalColor}"
                                       Text="{TemplateBinding Caption}"/>

                            <TextBox x:Name="text"
                                     Foreground="{StaticResource TextNormalColor}"
                                     Text="{TemplateBinding Text}"/>

                        </StackPanel>

                    </Border>

                </StackPanel>

                <ControlTemplate.Triggers>

                    <Trigger Property="IsMouseOver" Value="true">
                        <Setter TargetName="caption" Property="TextElement.Foreground" Value="{StaticResource CaptionMouseOverColor}"/>
                        <Setter TargetName="caption" Property="TextElement.FontWeight" Value="Bold"/>

                        <Setter TargetName="text" Property="TextElement.Foreground" Value="{StaticResource TextMouseOverColor}"/>
                        <Setter TargetName="text" Property="TextElement.FontStyle" Value="Italic"/>
                    </Trigger>

                </ControlTemplate.Triggers>

            </ControlTemplate>

        </Setter.Value>

    </Setter>

</Style>

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();
}