C#

<UserControl x:Class="Gestione_WPF.LabelledTextBox"
             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" 
             xmlns:local="clr-namespace:Gestione_WPF"
             mc:Ignorable="d" 
             d:DesignHeight="50" d:DesignWidth="200">
    <Grid Background="White" FlowDirection="RightToLeft">
       <Label x:Name="lbl" Content="{Binding Label, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"  HorizontalAlignment="Left" Background="{StaticResource gradientBrushTabControlHeader}">
            <Label.Style>
                <Style TargetType="Label">
                    <Setter Property="Template">
                        <Setter.Value>
                            <ControlTemplate TargetType="Label">
                                <StackPanel Orientation="Horizontal">
                                    <Border Background="{StaticResource ResourceKey=gradientBrushSegmentedTabItem}" Width="200" BorderThickness="0,0,0,0">
                                        <StackPanel Orientation="Horizontal">
                                            <Viewbox StretchDirection="DownOnly" Stretch="Uniform">
                                            <ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center" RecognizesAccessKey="True" TextBlock.FontSize="14" TextBlock.Foreground="#FFFFFF" Margin="5">
                                                <ContentPresenter.Effect>
                                                    <DropShadowEffect BlurRadius="0.0"
                                                  Color="#032A6B"
                                                  Direction="90"
                                                  Opacity="1"
                                                  ShadowDepth="1" />
                                                </ContentPresenter.Effect>
                                            </ContentPresenter>
                                            </Viewbox>
                                        </StackPanel>
                                    </Border>
                                </StackPanel>
                            </ControlTemplate>
                        </Setter.Value>
                    </Setter>
                </Style>
            </Label.Style>
        </Label>
        <TextBox x:Name="txt" Text="{Binding Text, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Width="120" HorizontalAlignment="Right">
            <TextBox.Style>
                <Style TargetType="TextBox">
                    <Setter Property="Template">
                        <Setter.Value>
                            <ControlTemplate TargetType="TextBox">
                                <Border CornerRadius="0,0,0,50" BorderBrush="Black" Background="White" BorderThickness="0">
                                    <ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center" RecognizesAccessKey="True" TextBlock.FontSize="14" TextBlock.Foreground="#FFFFFF" Margin="5">
                                        <ContentPresenter.Effect>
                                            <DropShadowEffect BlurRadius="0.0"
                                                  Color="#032A6B"
                                                  Direction="90"
                                                  Opacity="1"
                                                  ShadowDepth="1" />
                                        </ContentPresenter.Effect>
                                    </ContentPresenter>
                                </Border>
                            </ControlTemplate>
                        </Setter.Value>
                    </Setter>
                </Style>
            </TextBox.Style>
        </TextBox>
    </Grid>
</UserControl>

public partial class LabelledTextBox : UserControl
   {
       public LabelledTextBox()
       {
           InitializeComponent();
           this.DataContext = this;
       }

       public override void OnApplyTemplate()
       {
           base.OnApplyTemplate();
       }

       public static readonly DependencyProperty LabelProperty = DependencyProperty.Register("Label", typeof(string), typeof(LabelledTextBox), new PropertyMetadata(null));
       public string Label
       {
           get { return (string)this.GetValue(LabelProperty); }
           set { this.SetValue(LabelProperty, value);}
       }

       public static readonly DependencyProperty TextProperty = DependencyProperty.Register("Text", typeof(string), typeof(LabelledTextBox), new PropertyMetadata(null));
       public string Text
       {
           get { return (string)this.GetValue(TextProperty); }
           set { this.SetValue(TextProperty, value); }
       }
   }

<controls:LabelledTextBox Label="{Binding Hello}" Text="{Binding World}"/>

public class BusinessObjectBase
{
}

public class BusinessObjectList<T> : List<T> where T : BusinessObjectBase
{}

public class BusinessObjectDataSet : DataSet
{
    protected BusinessObjectList<BusinessObjectBase>[] _BusinessObjectListArray = null;

    public virtual void AddBusinessObjectList(BusinessObjectList<BusinessObjectBase> List)
    {

    }


}

private class MyBusinessObject : BusinessObjectBase
{
    public MyBusinessObject()
    {

    }
}

private class MyBusinessObjectList<MyBusinessObject> : BusinessObjectList<T> where T : BusinessObjectBase
{

}


 BusinessObjectDataSet Set = new BusinessObjectDataSet();

 MyBusinessObject Bo = new MyBusinessObject();                                                 // Derives from BusinessObjectBase

 MyBusinessObjectList<MyBusinessObject> BoList = new MyBusinessObjectList<MyBusinessObject>(); // Derives from BusinessObjectList<T>

 Set.AddBusinessObjectList(BoList);  // Here it says "Error CS1503  Argument 1: cannot convert from 'MyBusinessObjectList<MyBusinessObject>' to 'BusinessObjectList<BusinessObjectBase>'