摘自这里 https://blog.csdn.net/u013113678/article/details/121719278
调试效果如下

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Shapes;

namespace Wpf
{
    public class GridAdorner : Adorner
    { //4条边
        Thumb _leftThumb, _topThumb, _rightThumb, _bottomThumb;
        //4个角
        Thumb _leftTopThumb, _rightTopThumb, _rightBottomThumb, _leftbottomThumb;
        //布局容器，如果不使用布局容器，则需要给上述8个控件布局，实现和Grid布局定位是一样的，会比较繁琐且意义不大。
        Grid _grid;
        UIElement _adornedElement;

        public GridAdorner(UIElement adornedEleemnt) : base(adornedEleemnt)
        {
            _adornedElement = adornedEleemnt;
            //初始化thumb
            _leftThumb = new Thumb();
            _leftThumb.HorizontalAlignment = HorizontalAlignment.Left;
            _leftThumb.VerticalAlignment = VerticalAlignment.Center;
            _leftThumb.Cursor = Cursors.SizeWE;
            _topThumb = new Thumb();
            _topThumb.HorizontalAlignment = HorizontalAlignment.Center;
            _topThumb.VerticalAlignment = VerticalAlignment.Top;
            _topThumb.Cursor = Cursors.SizeNS;
            _rightThumb = new Thumb();
            _rightThumb.HorizontalAlignment = HorizontalAlignment.Right;
            _rightThumb.VerticalAlignment = VerticalAlignment.Center;
            _rightThumb.Cursor = Cursors.SizeWE;
            _bottomThumb = new Thumb();
            _bottomThumb.HorizontalAlignment = HorizontalAlignment.Center;
            _bottomThumb.VerticalAlignment = VerticalAlignment.Bottom;
            _bottomThumb.Cursor = Cursors.SizeNS;
            _leftTopThumb = new Thumb();
            _leftTopThumb.HorizontalAlignment = HorizontalAlignment.Left;
            _leftTopThumb.VerticalAlignment = VerticalAlignment.Top;
            _leftTopThumb.Cursor = Cursors.SizeNWSE;
            _rightTopThumb = new Thumb();
            _rightTopThumb.HorizontalAlignment = HorizontalAlignment.Right;
            _rightTopThumb.VerticalAlignment = VerticalAlignment.Top;
            _rightTopThumb.Cursor = Cursors.SizeNESW;
            _rightBottomThumb = new Thumb();
            _rightBottomThumb.HorizontalAlignment = HorizontalAlignment.Right;
            _rightBottomThumb.VerticalAlignment = VerticalAlignment.Bottom;
            _rightBottomThumb.Cursor = Cursors.SizeNWSE;
            _leftbottomThumb = new Thumb();
            _leftbottomThumb.HorizontalAlignment = HorizontalAlignment.Left;
            _leftbottomThumb.VerticalAlignment = VerticalAlignment.Bottom;
            _leftbottomThumb.Cursor = Cursors.SizeNESW;
            _grid = new Grid();
            _grid.Children.Add(_leftThumb);
            _grid.Children.Add(_topThumb);
            _grid.Children.Add(_rightThumb);
            _grid.Children.Add(_bottomThumb);
            _grid.Children.Add(_leftTopThumb);
            _grid.Children.Add(_rightTopThumb);
            _grid.Children.Add(_rightBottomThumb);
            _grid.Children.Add(_leftbottomThumb);
            AddVisualChild(_grid);
            foreach (Thumb thumb in _grid.Children)
            {
                thumb.Width = 16;
                thumb.Height = 16;
                thumb.Background = Brushes.Red;
                thumb.Template = new ControlTemplate(typeof(Thumb))
                {
                    VisualTree = GetFactory(new SolidColorBrush(Colors.Red))
                };
                thumb.DragDelta += Thumb_DragDelta;
            }
        }

        protected override Visual GetVisualChild(int index)
        {
            return _grid;
        }

        protected override int VisualChildrenCount
        {
            get { return 1; }
        }

        protected override Size ArrangeOverride(Size finalSize)
        {
            Debug.WriteLine(finalSize.Width + ":" + finalSize.Height);
            //直接给grid布局，grid内部的thumb会自动布局。
            _grid.Arrange(new Rect(new Point(-_leftThumb.Width / 2, -_leftThumb.Height / 2), new Size(finalSize.Width + _leftThumb.Width, finalSize.Height + _leftThumb.Height)));
            Debug.WriteLine(finalSize.Width + ":1      " + finalSize.Height);

            return finalSize;
        }

        private FrameworkElementFactory GetFactory(SolidColorBrush back)
        {
            var fef = new FrameworkElementFactory(typeof(Ellipse));
            fef.SetValue(Ellipse.FillProperty, back);
            fef.SetValue(Ellipse.StrokeProperty, new SolidColorBrush((Color)ColorConverter.ConvertFromString("#999999")));
            fef.SetValue(Ellipse.StrokeThicknessProperty, (double)2);

            return fef;
        }

        private void Thumb_DragDelta(object sender, DragDeltaEventArgs e)
        {
            var c = _adornedElement as FrameworkElement;
            var thumb = sender as FrameworkElement;
            double left, top, right, bottom, width, height;
            if (thumb.HorizontalAlignment == HorizontalAlignment.Left)
            {
                right = c.Margin.Right;
                left = c.Margin.Left + e.HorizontalChange;
                width = (double.IsNaN(c.Width) ? c.ActualWidth : c.Width) - e.HorizontalChange;
            }
            else
            {
                left = c.Margin.Left;
                right = c.Margin.Right - e.HorizontalChange;
                width = (double.IsNaN(c.Width) ? c.ActualWidth : c.Width) + e.HorizontalChange;
            }

            if (thumb.VerticalAlignment == VerticalAlignment.Top)
            {
                bottom = c.Margin.Bottom;
                top = c.Margin.Top + e.VerticalChange;
                height = (double.IsNaN(c.Height) ? c.ActualHeight : c.Height) - e.VerticalChange;
            }
            else
            {
                top = c.Margin.Top;
                bottom = c.Margin.Bottom - e.VerticalChange;
                height = (double.IsNaN(c.Height) ? c.ActualHeight : c.Height) + e.VerticalChange;
            }

            if (thumb.HorizontalAlignment != HorizontalAlignment.Center)
            {
                if (width > 0)
                {
                    c.Margin = new Thickness(left, c.Margin.Top, right, c.Margin.Bottom);
                    c.Width = width;
                }
            }
            if (thumb.VerticalAlignment != VerticalAlignment.Center)
            {
                if (height > 0)
                {
                    c.Margin = new Thickness(c.Margin.Left, top, c.Margin.Right, bottom);
                    c.Height = height;
                }
            }
        }
    }
}

用法

<Window>
<Grid>
        <Border x:Name="border" Width="200" Height="200" Background="Gray" />
    </Grid>
</Window>

public partial class WinResize : Window
    {
        public WinResize()
        {
            InitializeComponent();
            this.Loaded += (s, e) => {
               var layer  =  AdornerLayer.GetAdornerLayer(border);
                layer.Add(new GridAdorner(border));
            };
        }
    }