<Window x:Class="WpfApp6.MainWindow"
        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"
        xmlns:local="clr-namespace:WpfApp6"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.DataContext>
        <local:ViewModel/>
    </Window.DataContext>
    <Grid>
        <local:MyUserControl ItemsSource="{Binding Tag}" />
    </Grid>
</Window>

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

namespace WpfApp6
{
    public class ViewModel 
    {
        public ViewModel()
        {
            Tag = new string[3] { "One", "Two", "Three" };
        }

        public string[] Tag { set; internal get; }
    }

    public class MyUserControl : UserControl
    {
        public string[] ItemsSource
        {
            get { return (string[])GetValue(ItemsSourceProperty); }
            set { SetValue(ItemsSourceProperty, value); }
        }
        public static readonly DependencyProperty ItemsSourceProperty =
            DependencyProperty.Register("ItemsSource", typeof(string[]), typeof(MyUserControl), new PropertyMetadata(null, OnPropertyChanged));

        private static void OnPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (e.NewValue is string[] arr)
            {
                Debug.Print($"Changed: {arr.Length}");
            }
        }
    }
}