public class GridViewMultiSelect : Behavior<RadGridView>
{
	protected override void OnAttached()
	{
		base.OnAttached();
		AssociatedObject.DataContextChanged += OnDataContextChanged;
		AssociatedObject.SelectedItems.CollectionChanged += OnSelectedItemsChanged;
		AssociatedObject.SelectionChanging += OnSelectionChanged;
		AssociatedObject.Items.CollectionChanged += OnItemsChanged;
	}

	private void OnSelectionChanged(object? sender, SelectionChangingEventArgs e)
	{
		
	}

	private void OnItemsChanged(object? sender, NotifyCollectionChangedEventArgs e)
	{
	}

	private void Select(IEnumerable<NsMisuraViewWithType> misure)
	{
		foreach (var item in misure)
		{
			AssociatedObject.SelectedItems.Add(item);
		}
	}

	private void OnDataContextChanged(object? sender, DependencyPropertyChangedEventArgs e)
	{
		if (AssociatedObject.DataContext is LoadedMeasurmentsVM viewModel)
		{
			viewModel.SelectAction = Select;
			SetSelectMeasurements();
		}
	}

	private void SetSelectMeasurements()
	{
		if (AssociatedObject.DataContext is LoadedMeasurmentsVM viewModel)
		{
			foreach (var item in AssociatedObject.SelectedItems)
			{
				if (viewModel.SelectedMeasurements.All(m => m.Misura.Id != ((NsMisuraViewWithType)item).Misura.Id))
				{
					viewModel.SelectedMeasurements.Add((NsMisuraViewWithType)item);
				}
			}
		}
	}

	private void OnSelectedItemsChanged(object? sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
	{
		SetSelectMeasurements();
	}

	protected override void OnDetaching()
	{
		base.OnDetaching();
		AssociatedObject.DataContextChanged -= OnDataContextChanged;
		AssociatedObject.SelectedItems.CollectionChanged -= OnSelectedItemsChanged;
	}
}

        <telerik:HtmlDataProvider RichTextBox="{Binding ElementName=radRichTextBox}" Html="{Binding MyRichText, Mode=TwoWay, NotifyOnValidationError=true, TargetNullValue='', ValidatesOnExceptions=true}">
            <telerik:HtmlDataProvider.FormatProvider>
                <telerik:HtmlFormatProvider>
                    <telerik:HtmlFormatProvider.ExportSettings>
                        <telerik:HtmlExportSettings DocumentExportLevel="Fragment"
ImageExportMode="AutomaticInline"
StyleRepositoryExportMode="DontExportStyles"
StylesExportMode="Inline"
ExportEmptyDocumentAsEmptyString="True"      
/>
                    </telerik:HtmlFormatProvider.ExportSettings>
                </telerik:HtmlFormatProvider>
            </telerik:HtmlDataProvider.FormatProvider>
        </telerik:HtmlDataProvider>

<telerik:RadRichTextBox x:Name="radRichTextBox" DocumentInheritsDefaultStyleSettings="True" Foreground="black" FontFamily="Times New Roman" FontSize="12"   Padding="10">
</telerik:RadRichTextBox>