如何在WinForm中实现图形边框点击选中功能

我有一个C# WinForm项目，已经实现了矩形和椭圆的绘制功能，代码如下：

public partial class Form1 : Form
{

    List<Rectangle> _rectangles = new();
    List<Rectangle> _ellipses = new();
    Rectangle _rectInProgress;

    bool DrawingRectangle = false;
    bool DrawingEllipse = false;

    public Form1()
    {
        InitializeComponent();
        DoubleBuffered = true;
    }

    private void Form1_Load(object sender, EventArgs e)
    {

    }

    private void Form1_MouseDown(object sender, MouseEventArgs e)
    {
        if (DrawingRectangle || DrawingEllipse)
        {
            _rectInProgress = new Rectangle(e.Location, new Size());
        }
    }

    private void Form1_MouseMove(object sender, MouseEventArgs e)
    {
        if (MouseButtons == MouseButtons.Left && (DrawingRectangle || DrawingEllipse))
        {
            _rectInProgress.Width = e.Location.X - _rectInProgress.X;
            _rectInProgress.Height = e.Location.Y - _rectInProgress.Y;
            Invalidate();
        }
    }

    private void Form1_MouseUp(object sender, MouseEventArgs e)
    {
        if (DrawingRectangle)
        {
            _rectangles.Add(_rectInProgress);
            Invalidate();
        }
        else if (DrawingEllipse)
        {
            _ellipses.Add(_rectInProgress);
            Invalidate();
        }
    }

    private void Form1_Paint(object sender, PaintEventArgs e)
    {

        if (_rectangles.Any())
        {
            e.Graphics.DrawRectangles(new Pen(Color.Blue, 3), _rectangles.ToArray());
        }
        if (_ellipses.Any())
        {
            foreach (var ell in _ellipses)
            {
                e.Graphics.DrawEllipse(new Pen(Color.Blue, 3), ell);
            }
        }
        if (MouseButtons == MouseButtons.Left)
        {
            if (DrawingEllipse)
            {
                e.Graphics.DrawEllipse(new Pen(Color.Red, 3), _rectInProgress);
            }
            else if (DrawingRectangle)
            {
                e.Graphics.DrawRectangle(new Pen(Color.Red, 3), _rectInProgress);
            }
        }
    }

    private void button1_Click(object sender, EventArgs e)
    {
        DrawingRectangle = !DrawingRectangle;
        DrawingEllipse = false;
    }

    private void button2_Click(object sender, EventArgs e)
    {
        DrawingEllipse = !DrawingEllipse;
        DrawingRectangle = false;
    }
}

现在我需要实现图形选中功能，要求是检测光标是否点击在图形边框上（而非内部），之后才能进行调整大小操作。作为新手我毫无头绪，网上大多是检测点击内部的示例，恳请提供实现思路。

1. 定义选中状态存储字段 #

首先在Form1类中添加记录选中状态的字段：

// 选中状态相关
private int _selectedRectIndex = -1; // 选中矩形的索引，-1表示未选中
private int _selectedEllipseIndex = -1; // 选中椭圆的索引
private bool _isResizing = false; // 是否处于调整大小状态

2. 核心：边框点击检测方法 #

直接检测点是否在“线”上精度太低，我们可以用GraphicsPath.Widen把边框线扩展成一个有宽度的区域，再判断鼠标点是否在这个区域内。添加两个辅助方法：

// 检测点是否在矩形边框上（匹配绘制时的笔宽）
private bool IsPointOnRectangleBorder(Point p, Rectangle rect, int penWidth)
{
    using (var path = new GraphicsPath())
    {
        path.AddRectangle(rect);
        using (var pen = new Pen(Color.Black, penWidth))
        {
            var hitRegion = new Region(path.Widen(pen));
            return hitRegion.IsVisible(p);
        }
    }
}

// 检测点是否在椭圆边框上（匹配绘制时的笔宽）
private bool IsPointOnEllipseBorder(Point p, Rectangle rect, int penWidth)
{
    using (var path = new GraphicsPath())
    {
        path.AddEllipse(rect);
        using (var pen = new Pen(Color.Black, penWidth))
        {
            var hitRegion = new Region(path.Widen(pen));
            return hitRegion.IsVisible(p);
        }
    }
}

3. 修改MouseDown事件处理选中逻辑 #

在鼠标按下时，优先判断是否处于绘制状态，若不是则遍历图形检测边框点击：

private void Form1_MouseDown(object sender, MouseEventArgs e)
{
    if (DrawingRectangle || DrawingEllipse)
    {
        _rectInProgress = new Rectangle(e.Location, new Size());
        // 绘制时取消之前的选中
        _selectedRectIndex = -1;
        _selectedEllipseIndex = -1;
        Invalidate();
        return;
    }

    // 从后往前遍历，优先选中上层图形
    for (int i = _rectangles.Count - 1; i >= 0; i--)
    {
        if (IsPointOnRectangleBorder(e.Location, _rectangles[i], 3))
        {
            _selectedRectIndex = i;
            _selectedEllipseIndex = -1;
            _isResizing = true;
            Invalidate();
            return;
        }
    }

    for (int i = _ellipses.Count - 1; i >= 0; i--)
    {
        if (IsPointOnEllipseBorder(e.Location, _ellipses[i], 3))
        {
            _selectedEllipseIndex = i;
            _selectedRectIndex = -1;
            _isResizing = true;
            Invalidate();
            return;
        }
    }

    // 未点击到任何边框，取消选中
    _selectedRectIndex = -1;
    _selectedEllipseIndex = -1;
    Invalidate();
}

4. 修改MouseMove事件实现调整大小 #

选中状态下按住左键拖动时，调整图形尺寸：

private void Form1_MouseMove(object sender, MouseEventArgs e)
{
    if (DrawingRectangle || DrawingEllipse)
    {
        _rectInProgress.Width = e.Location.X - _rectInProgress.X;
        _rectInProgress.Height = e.Location.Y - _rectInProgress.Y;
        Invalidate();
        return;
    }

    if (_isResizing && MouseButtons == MouseButtons.Left)
    {
        // 调整选中的矩形
        if (_selectedRectIndex != -1)
        {
            var rect = _rectangles[_selectedRectIndex];
            // 此处为基础版：拖动右下角调整，可扩展为多控制点（四个角/边）
            rect.Width = e.Location.X - rect.X;
            rect.Height = e.Location.Y - rect.Y;
            _rectangles[_selectedRectIndex] = rect;
            Invalidate();
        }
        // 调整选中的椭圆
        else if (_selectedEllipseIndex != -1)
        {
            var ellipseRect = _ellipses[_selectedEllipseIndex];
            ellipseRect.Width = e.Location.X - ellipseRect.X;
            ellipseRect.Height = e.Location.Y - ellipseRect.Y;
            _ellipses[_selectedEllipseIndex] = ellipseRect;
            Invalidate();
        }
    }
    else
    {
        // 鼠标悬停边框时切换光标
        bool isOnBorder = false;
        foreach (var rect in _rectangles)
        {
            if (IsPointOnRectangleBorder(e.Location, rect, 3))
            {
                isOnBorder = true;
                break;
            }
        }
        if (!isOnBorder)
        {
            foreach (var ell in _ellipses)
            {
                if (IsPointOnEllipseBorder(e.Location, ell, 3))
                {
                    isOnBorder = true;
                    break;
                }
            }
        }
        Cursor = isOnBorder ? Cursors.SizeNWSE : Cursors.Default;
    }
}

5. 修改MouseUp事件结束调整状态 #

private void Form1_MouseUp(object sender, MouseEventArgs e)
{
    if (DrawingRectangle)
    {
        _rectangles.Add(_rectInProgress);
        Invalidate();
    }
    else if (DrawingEllipse)
    {
        _ellipses.Add(_rectInProgress);
        Invalidate();
    }
    _isResizing = false;
}

6. 修改Paint事件绘制选中样式 #

给选中的图形设置不同的边框样式，方便区分：

private void Form1_Paint(object sender, PaintEventArgs e)
{
    var normalPen = new Pen(Color.Blue, 3);
    var selectedPen = new Pen(Color.Red, 4); // 选中边框更粗更显眼

    if (_rectangles.Any())
    {
        for (int i = 0; i < _rectangles.Count; i++)
        {
            var pen = i == _selectedRectIndex ? selectedPen : normalPen;
            e.Graphics.DrawRectangle(pen, _rectangles[i]);
        }
    }
    if (_ellipses.Any())
    {
        for (int i = 0; i < _ellipses.Count; i++)
        {
            var pen = i == _selectedEllipseIndex ? selectedPen : normalPen;
            e.Graphics.DrawEllipse(pen, _ellipses[i]);
        }
    }
    if (MouseButtons == MouseButtons.Left)
    {
        if (DrawingEllipse)
        {
            e.Graphics.DrawEllipse(new Pen(Color.Red, 3), _rectInProgress);
        }
        else if (DrawingRectangle)
        {
            e.Graphics.DrawRectangle(new Pen(Color.Red, 3), _rectInProgress);
        }
    }

    normalPen.Dispose();
    selectedPen.Dispose();
}

7. 补充：切换绘制模式时取消选中 #

修改按钮点击事件，切换模式时清空选中状态：

private void button1_Click(object sender, EventArgs e)
{
    DrawingRectangle = !DrawingRectangle;
    DrawingEllipse = false;
    _selectedRectIndex = -1;
    _selectedEllipseIndex = -1;
    Invalidate();
}

private void button2_Click(object sender, EventArgs e)
{
    DrawingEllipse = !DrawingEllipse;
    DrawingRectangle = false;
    _selectedRectIndex = -1;
    _selectedEllipseIndex = -1;
    Invalidate();
}

内容的提问来源于stack exchange，提问作者Jeremy