using System.Windows.Controls;

    namespace TestControls.Controls
    {
        public class МyTextBox : TextBox
        {

            public МyTextBox()
            {
                Text = "MyTextBox";
            }
        }
    }

<UserControl x:Class="TestControls.Controls.MyText"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400">

    <Grid x:Name="LayoutRoot" Background="White">
        <TextBlock Name="Text"/>
    </Grid>
</UserControl>

using System.Windows.Controls;

namespace TestControls.Controls
{
    public partial class MyText : UserControl
    {
        public MyText()
        {
            InitializeComponent();
            Text.Text = "MyTextControl";
        }
    }
}

<UserControl x:Class="TestControlsApplication.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400">

    <Grid x:Name="LayoutRoot" Background="White">
        <StackPanel Name="Panel"/>
    </Grid>
</UserControl>

using System.Windows.Controls;
using TestControls.Controls;

namespace TestControlsApplication
{
    public partial class MainPage : UserControl
    {
        public MainPage()
        {
            InitializeComponent();

            for (int i = 0; i < 1000; i++)
            {
                MyText t = new MyText();
                МyTextBox tb = new МyTextBox();
                Panel.Children.Add(t);
                Panel.Children.Add(tb);
            }
        }
    }
}