鼠标向上时如何画线？答案

【问题标题】：How to draw line when mouse up?鼠标向上时如何画线？
【发布时间】：2015-06-04 18:54:41
【问题描述】：

我试图在 LMB 向下时设置起点，并在 LMB 向上时从起点到当前鼠标位置画一条线，就像 MSPaint 所做的那样。

我的问题是当 LMB 启动时，我似乎无法让线条出现在图片框上。有人可以请教我吗？

编辑：对不起，伙计们，我意识到问题出在其他地方，但我在此过程中学到了很多东西，感谢所有输入。

public partial class FormPaint : Form
{    
    Point? startPoint = Point.Empty;
    Point? endPoint = Point.Empty;
    bool isMouseDown = new Boolean();

    private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
    {
        if (Control.MouseButtons == MouseButtons.Left)
        {
            startPoint = e.Location;
            isMouseDown = true;
        }
    }

    private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
    {

         brush = new SolidBrush(color);
         using (Graphics g = Graphics.FromImage(pictureBox1.Image))
         {
              g.DrawLine(new Pen(brush), startPoint.Value, endPoint.Value);
              pictureBox1.Invalidate();

         }

        isMouseDown = false;

    }
    private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
    {
      endPoint = e.Location;
    }
     private void pictureBox1_Paint(object sender, PaintEventArgs e)
    {
         using (brush = new SolidBrush(color))
         {
               e.Graphics.DrawLine(new Pen(brush, 5), startPoint.Value, endPoint.Value);
         }
    }
}

【问题讨论】：

只是好奇......为什么你的Points 可以为空？另外，bool isMouseDown = false; 可能更清楚。
在pictureBox1_MouseMove中你不检查鼠标按钮是否被按下，在任何情况下都给endPoint赋值。
MouseMove 期间你也需要失效。
@EdS。我正在浏览一些论坛以查找类似的问题并看到使用它的帖子，所以我认为它可能会以某种方式做一些事情
了解程序中的每一行代码是个好主意。这是一篇似乎相关的文章：en.wikipedia.org/wiki/Cargo_cult_programming

标签： c# winforms drawing

【解决方案1】：

当您调用Invalidate 时，它会强制图片框重新绘制。问题在于它会丢弃您之前绘制的所有内容。然后调用图片框上的Paint。

我建议将绘图数据保存到一个列表中，并使用该保存的数据在图片框的Paint 事件中执行绘图。

另请阅读How do I draw a circle and line in the picturebox?

【讨论】：

【解决方案2】：

带有线条预览的完整示例，尽情享受吧。

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WinForm
{
    public partial class frmMain : Form
    {
        /// <summary>
        /// form constructor
        /// </summary>
        public frmMain()
        {
            InitializeComponent();
        }

        private PictureBox imgCanvas;
        private bool isMouseDown;
        private Point startPoint;
        private Point currentPoint;

        /// <summary>
        /// form load
        /// </summary>
        protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);

            imgCanvas = new PictureBox
            {
                Location = new Point(8, 8),
                Size = new Size(this.ClientSize.Width - 16, this.ClientSize.Height - 16),
                Anchor = AnchorStyles.Left | AnchorStyles.Top | AnchorStyles.Right | AnchorStyles.Bottom,
                BorderStyle = BorderStyle.Fixed3D,
            };
            imgCanvas.MouseDown += imgCanvas_MouseDown;
            imgCanvas.MouseUp += imgCanvas_MouseUp;
            imgCanvas.MouseMove += imgCanvas_MouseMove;
            imgCanvas.Paint += imgCanvas_Paint;
            this.Controls.Add(imgCanvas);
        }

        void imgCanvas_Paint(object sender, PaintEventArgs e)
        {
            if (isMouseDown)
            {
                e.Graphics.DrawLine(Pens.Red, startPoint, currentPoint);
            }
        }

        void imgCanvas_MouseMove(object sender, MouseEventArgs e)
        {
            if (isMouseDown)
            {
                currentPoint = e.Location;
                (sender as PictureBox).Refresh();
            }
        }

        void imgCanvas_MouseDown(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                isMouseDown = true;
                startPoint = e.Location;
            }
        }

        void imgCanvas_MouseUp(object sender, MouseEventArgs e)
        {
            if (isMouseDown)
            {
                if (e.Button == MouseButtons.Left)
                {
                    isMouseDown = false;

                    PictureBox pb = sender as PictureBox;

                    // create image
                    if (pb.Image == null)
                    {
                        pb.Image = new Bitmap(pb.ClientSize.Width, pb.ClientSize.Height);
                    }

                    // draw
                    using (Graphics g = Graphics.FromImage(pb.Image))
                    {
                        g.DrawLine(Pens.Green, startPoint, e.Location);
                        pb.Refresh();
                    }
                }
            }
        }
    }
}

结果：

【讨论】：