<!--XAML for NumericUpDown that inherits from UserControl.-->
<UserControl x:Class="MyUserControl.NumericUpDown"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:MyUserControl">
    <Grid Margin="3">
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition/>
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>

        <Border BorderThickness="1" BorderBrush="Gray" Margin="2" 
                Grid.RowSpan="2" VerticalAlignment="Center" HorizontalAlignment="Stretch">

            <!--Bind the TextBlock to the Value property-->
            <TextBlock 
                Width="60" TextAlignment="Right" Padding="5"
                Text="{Binding RelativeSource={RelativeSource FindAncestor, 
                               AncestorType={x:Type local:NumericUpDown}}, 
                               Path=Value}"/>

        </Border>

        <RepeatButton Name="upButton" Click="upButton_Click" 
                      Grid.Column="1" Grid.Row="0">Up</RepeatButton>

        <RepeatButton Name="downButton" Click="downButton_Click" 
                      Grid.Column="1" Grid.Row="1">Down</RepeatButton>

    </Grid>
</UserControl>

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

namespace MyUserControl
{
    public partial class NumericUpDown : System.Windows.Controls.UserControl
    {
        /// <summary>
        /// Initializes a new instance of the NumericUpDownControl.
        /// </summary>
        public NumericUpDown()
        {
            InitializeComponent();

        }

        /// <summary>
        /// Identifies the Value dependency property.
        /// </summary>
        public static readonly DependencyProperty ValueProperty =
            DependencyProperty.Register(
                "Value", typeof(decimal), typeof(NumericUpDown),
                new FrameworkPropertyMetadata(MinValue, new PropertyChangedCallback(OnValueChanged),
                                              new CoerceValueCallback(CoerceValue)));

        /// <summary>
        /// Gets or sets the value assigned to the control.
        /// </summary>
        public decimal Value
        {          
            get { return (decimal)GetValue(ValueProperty); }
            set { SetValue(ValueProperty, value); }
        }

        private static object CoerceValue(DependencyObject element, object value)
        {
            decimal newValue = (decimal)value;
            NumericUpDown control = (NumericUpDown)element;

            newValue = Math.Max(MinValue, Math.Min(MaxValue, newValue));

            return newValue;
        }

        private static void OnValueChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
        {
            NumericUpDown control = (NumericUpDown)obj;            

            RoutedPropertyChangedEventArgs<decimal> e = new RoutedPropertyChangedEventArgs<decimal>(
                (decimal)args.OldValue, (decimal)args.NewValue, ValueChangedEvent);
            control.OnValueChanged(e);
        }

        /// <summary>
        /// Identifies the ValueChanged routed event.
        /// </summary>
        public static readonly RoutedEvent ValueChangedEvent = EventManager.RegisterRoutedEvent(
            "ValueChanged", RoutingStrategy.Bubble, 
            typeof(RoutedPropertyChangedEventHandler<decimal>), typeof(NumericUpDown));

        /// <summary>
        /// Occurs when the Value property changes.
        /// </summary>
        public event RoutedPropertyChangedEventHandler<decimal> ValueChanged
        {
            add { AddHandler(ValueChangedEvent, value); }
            remove { RemoveHandler(ValueChangedEvent, value); }
        }

        /// <summary>
        /// Raises the ValueChanged event.
        /// </summary>
        /// <param name="args">Arguments associated with the ValueChanged event.</param>
        protected virtual void OnValueChanged(RoutedPropertyChangedEventArgs<decimal> args)
        {
            RaiseEvent(args);
        }

        private void upButton_Click(object sender, EventArgs e)
        {
            Value++;
        }

        private void downButton_Click(object sender, EventArgs e)
        {
            Value--;
        }

        private const decimal MinValue = 0, MaxValue = 100;
    }
}