<Window x:Class="WpfZoom.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="WPF局部放大效果" Height="370" Width="700">
  <Canvas x:Name="RootCanvas">
    <!--左側小圖-->
    <Canvas x:Name="SmallBox" Width="320" Height="180" Canvas.Left="20" Canvas.Top="20">
      <Canvas.Background>
        <ImageBrush ImageSource="Images/dog.jpg" Stretch="UniformToFill" />
      </Canvas.Background>
      <!--半透明矩形框-->
      <Rectangle x:Name="MoveRect" Fill="White" Opacity="0.3" Stroke="Red" Width="50" Height="50" Canvas.Top="78" Canvas.Left="202"
          MouseMove="MoveRect_MouseMove"
          MouseLeftButtonDown="MoveRect_MouseLeftButtonDown"
          MouseLeftButtonUp="MoveRect_MouseLeftButtonUp"/>
    </Canvas>
    <!--右側大圖-->
    <Canvas x:Name="BigBox" Width="300" Height="300" Canvas.Left="360" Canvas.Top="20">
      <!--右側原圖片 注意尺寸-->
      <Image x:Name="bigImg" Width="1920" Height="1080" Canvas.Left="0" Canvas.Top="-780" Source="Images/dog.jpg" />
      <Canvas.Clip>
        <RectangleGeometry Rect="0,0,300,300" />
      </Canvas.Clip>
    </Canvas>
  </Canvas>
</Window>

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
namespace WpfZoom
{
  /// <summary>
  /// MainWindow.xaml 的交互邏輯
  /// </summary>
  public partial class MainWindow : Window
  {
    //移動標志
    bool trackingMouseMove = false;
    //鼠標按下去的位置
    Point mousePosition;
    public MainWindow()
    {
      InitializeComponent();
      this.Loaded += new RoutedEventHandler(MainWindow_Loaded);
    }
    void MainWindow_Loaded(object sender, RoutedEventArgs e)
    {
      AdjustBigImage();
    }
    /// <summary>
    /// 半透明矩形框鼠標左鍵按下
    /// </summary>
    private void MoveRect_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {
      FrameworkElement element = sender as FrameworkElement;
      mousePosition = e.GetPosition(element);
      trackingMouseMove = true;
      if (null != element)
      {
        //強制獲取此元素
        element.CaptureMouse();
        element.Cursor = Cursors.Hand;
      }
    }
    /// <summary>
    /// 半透明矩形框鼠標左鍵彈起
    /// </summary>
    private void MoveRect_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
    {
      FrameworkElement element = sender as FrameworkElement;
      trackingMouseMove = false;
      element.ReleaseMouseCapture();
      mousePosition.X = mousePosition.Y = 0;
      element.Cursor = null;
    }
    /// <summary>
    /// 半透明矩形框移動
    /// </summary>
    private void MoveRect_MouseMove(object sender, MouseEventArgs e)
    {
      FrameworkElement element = sender as FrameworkElement;
      if (trackingMouseMove)
      {
        //計算鼠標在X軸的移動距離
        double deltaV = e.GetPosition(element).Y - mousePosition.Y;
        //計算鼠標在Y軸的移動距離
        double deltaH = e.GetPosition(element).X - mousePosition.X;
        //得到圖片Top新位置
        double newTop = deltaV + (double)element.GetValue(Canvas.TopProperty);
        //得到圖片Left新位置
        double newLeft = deltaH + (double)element.GetValue(Canvas.LeftProperty);
        //邊界的判斷
        if (newLeft <= 0)
        {
          newLeft = 0;
        }
        //左側圖片框寬度 - 半透明矩形框寬度
        if (newLeft >= (this.SmallBox.Width - this.MoveRect.Width))
        {
          newLeft = this.SmallBox.Width - this.MoveRect.Width;
        }
        if (newTop <= 0)
        {
          newTop = 0;
        }
        //左側圖片框高度度 - 半透明矩形框高度度
        if (newTop >= this.SmallBox.Height - this.MoveRect.Height)
        {
          newTop = this.SmallBox.Height - this.MoveRect.Height;
        }
        element.SetValue(Canvas.TopProperty, newTop);
        element.SetValue(Canvas.LeftProperty, newLeft);
        AdjustBigImage();
        if (mousePosition.X <= 0 || mousePosition.Y <= 0) { return; }
      }
    }
    /// <summary>
    /// 調整右側大圖的位置
    /// </summary>
    void AdjustBigImage()
    {
      //獲取右側大圖框與透明矩形框的尺寸比率
      double n = this.BigBox.Width / this.MoveRect.Width;
      //獲取半透明矩形框在左側小圖中的位置
      double left = (double)this.MoveRect.GetValue(Canvas.LeftProperty);
      double top = (double)this.MoveRect.GetValue(Canvas.TopProperty);
      //計算和設置原圖在右側大圖框中的Canvas.Left 和 Canvas.Top
      double newLeft = -left * n;
      double newTop = -top * n;
      bigImg.SetValue(Canvas.LeftProperty, newLeft);
      bigImg.SetValue(Canvas.TopProperty, newTop);
    }
  }
}