How to move Drawingvisual by mouse?

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WPF

drawing

Here is my code.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Diagnostics;
namespace Visualexample
{
    /// <summary>
    /// Логика взаимодействия для MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public bool letsmove = false;
        
        DrawingVisual b = null;
        DrawingVisual selectedvisual = null;
        Size somesize = new Size(50,50);
        public MainWindow()
        {
            InitializeComponent();
            
            Tablet.Style = (Style)Grid1.FindResource("Canvasstyle");
            Tablet.MouseLeftButtonDown += drawmethod;
           

        }
        public void drawrectangle(DrawingVisual a,Point somepoint)
        {
           
            Pen somepen = new Pen(Brushes.Green,5);
            Brush somebrush = Brushes.Red;
            
            using(DrawingContext dc=a.RenderOpen())
            {
                dc.DrawRectangle(somebrush,somepen,new Rect(somepoint,somesize));
            }
        }
      
        private void drawmethod(object sender, MouseButtonEventArgs e)
        {
         
           
            Point clickedpoint = e.GetPosition(Tablet);
            if (RadADD.IsChecked == true)
            {
                b = new DrawingVisual();
                drawrectangle(b, clickedpoint);
                Tablet.Addvisual(b);
            }
            else if (Radrem.IsChecked == true)
            {
                b = Tablet.getvisual(ref Tablet, clickedpoint);
                if (b != null)
                {
                    Tablet.Removevisual(b);
                }
            }
            else if (Radsel.IsChecked == true)
            {
                b = Tablet.getvisual(ref Tablet, clickedpoint);
                
                letsmove = true;
            }
          
            
        }
        [DebuggerStepThrough]
        private void Move(object sender,MouseEventArgs e)
        {
            if (letsmove == true)
            {
                Point pointdragger = e.GetPosition(Tablet);
                if(b!=null)
                drawrectangle(b, pointdragger);
               
            }
           
        }
        private void MouseUP(object sender,MouseButtonEventArgs e)
        {
             letsmove=false;
        }
      
     
    }
    [DebuggerStepThrough]
    public class someconvass : Panel
    {
        List<Visual> thing = new List<Visual>();
        public DrawingVisual getvisual(ref someconvass k,Point p)
        {
            HitTestResult hitresult = VisualTreeHelper.HitTest(k, p);
            return hitresult.VisualHit as DrawingVisual;
        }
        public someconvass()
        {
            
   
        }
        
        public void Addvisual(Visual a)
        {
            AddVisualChild(a);
            AddLogicalChild(a);
            thing.Add(a);
        }
      
        public void Removevisual(Visual a)
        {
            RemoveVisualChild(a);
            RemoveLogicalChild(a);
            thing.Remove(a);
        }
        protected override int VisualChildrenCount
        {
            get
            {
                return thing.Count;
            }
        }
        protected override Visual GetVisualChild(int index)
        {
           
            return thing[index];
        }
       
    }
    
}

<Window x:Class="Visualexample.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="500" Width="500" xmlns:local="clr-namespace:Visualexample" ResizeMode="NoResize"> 
<Window.Resources>
        <Style x:Key="Canvasstyle">
            <Setter Property="Canvas.Width" Value="362"/>
            <Setter Property="Canvas.Height" Value="500"/>
            <Setter Property="Canvas.Background" Value="White"/>
            <Setter Property="Canvas.HorizontalAlignment" Value="Right"/>
        </Style>
    </Window.Resources>
    <Grid Name="Grid1">
       
            <ToolBarTray Orientation="Vertical"  HorizontalAlignment="Left" Width="130">
        <ToolBar Width="130" HorizontalAlignment="Left" Margin="0,0,0,-476">
            <RadioButton Content="DrawRectangle" Name="RadADD"/>
        <RadioButton Margin="0,20,0,0" Content="RemoveRectangle" Name="Radrem"/>
        <RadioButton Margin="0,40,0,0" Content="Select/Move" Name="Radsel"/>
       
        </ToolBar>
        </ToolBarTray>
        <Border HorizontalAlignment="Right" Width="370" ToolTip="Hello" >
            <local:someconvass x:Name="Tablet" MouseLeftButtonDown="drawmethod"  MouseLeftButtonUp="MouseUP"  MouseMove="Move"/>
        </Border>
        
    </Grid>
    
</Window>

Explain me please how works MouseMove event handler.When i drag rectangle,there are creates new rectangle and drags to another place of Panel container.