<Window x:Class="RadHighlightTextBlock_Issue.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:RadHighlightTextBlock_Issue"
        xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.Resources>
        <local:MyViewModel x:Key="MyViewModel"/>
    </Window.Resources>
    <StackPanel>
        <telerik:RadHighlightTextBlock x:Name="highlightTextBlock" 
                                       Text="{Binding Source={StaticResource MyViewModel}, Path=SelectedItem.Description}"
                                      HighlightText="{Binding Source={StaticResource MyViewModel}, Path=SelectedItem.HighlightText}"
                                      HighlightForeground="Red"
                                      HighlightBackground="Yellow"
                                      FontSize="16"
                                      Margin="10"/>
        <telerik:RadButton Content="Content0" Command="{Binding Source={StaticResource MyViewModel}, Path=C0Command}"/>
        <telerik:RadButton Content="Content1" Command="{Binding Source={StaticResource MyViewModel}, Path=C1Command}"/>
    </StackPanel>
</Window>

using System;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Input;

namespace RadHighlightTextBlock_Issue
{
    /// <summary>
    /// MainWindow.xaml 的互動邏輯
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void RadButton_Click(object sender, RoutedEventArgs e)
        {
            highlightTextBlock.Text = "This is a sample text to demonstrate the RadHighlightTextBlock control.";
        }

        private void RadButton_Click_1(object sender, RoutedEventArgs e)
        {
            highlightTextBlock.Text = "This is another sample text for the second button.";
        }
    }

    public class MyViewModel : INotifyPropertyChanged
    {
        public ObservableCollection<DataItem> Items { get; set; }
        public MyViewModel()
        {
            Items = new ObservableCollection<DataItem>
            {
                new DataItem { HighlightText = "sample", Description = "This is a sample description." },
                new DataItem { HighlightText = "second", Description = "This is the second item description." },
                new DataItem { HighlightText = "text", Description = "This item contains the word text." }
            };

            SelectedItem = Items.FirstOrDefault();
        }

        public ICommand C0Command => new RelayCommand<object>((obj) =>
        {
            SelectedItem = Items.FirstOrDefault();
        });

        public ICommand C1Command => new RelayCommand<object>((obj) =>
        {
            SelectedItem = Items.LastOrDefault();
        });

        private DataItem _selectedItem;
        public DataItem SelectedItem
        {
            get { return _selectedItem; }
            set
            {
                if (_selectedItem != value)
                {
                    _selectedItem = value;
                    OnPropertyChanged();
                }
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = "")
        {
            var handler = PropertyChanged;
            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }

    public class DataItem
    {
        public string HighlightText { get; set; }
        public string Description { get; set; }
    }

    /*Implement RelayCommand...*/
}