How do I detect click on a line in Windows Forms

Question

I have a winforms application

Here is my code

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using S         


        

Answer 1

I implemented a simple Line class to check if a dot fall on the line.
You can capture a mouse position out of Form_Click event

Here's the snippet

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

namespace WindowsFormsApplication
{
    public partial class Form1 : Form
    {
        Line myLine;
        int x1 = 10;
        int x2 = 40;
        int y1 = 0;
        int y2 = 30;
        public Form1()
        {
            InitializeComponent();
            myLine = new Line() { Start = new Point(x1, y1), Stop = new Point(x2, y2), Epsilon = 10 };
        }

        private void Form1_Paint(object sender, PaintEventArgs e)
        {
            Pen pen = new Pen(Color.FromArgb(255, 0, 0, 0));
            e.Graphics.DrawLine(pen, x1, y1, x2, y2);
            pen.Dispose();
        }

        private void Form1_Click(object sender, EventArgs e)
        {
            MouseEventArgs me = (MouseEventArgs)e;
            bool contain = myLine.contain(new Point(me.X,me.Y));
        }
    }

    public class Line
    {
        public Point Start { get; set; }
        public Point Stop { get; set; }
        public float Epsilon { get; set; }

        public bool contain(Point p)
        {
            // y = mx + c
            float m = (Stop.Y - Start.Y) / (Stop.X - Start.X);
            float c = Stop.Y - (m * Stop.X);
            return p.X >= Math.Min(Start.X, Stop.X)
                && p.X <= Math.Max(Start.X, Stop.X)
                && p.Y >= Math.Min(Start.Y, Stop.Y)
                && p.Y <= Math.Max(Start.Y, Stop.Y)
                && Math.Abs(Math.Abs(p.Y) - Math.Abs((m * p.X) + c)) < epsilon; //with relax rules
                //&& (p.Y == (m*p.X)+c); // strict version
        }
    }

UPDATE
careful of a case where X1 == X2. It will throw exception.

Answer 2

Using GraphicsPath.IsOutlineVisible method you can determine whether the specified point is under the outline of the path when drawn with the specified Pen. You can set width of the pen.

So you can create a GraphicsPath and then add a line using GraphicsPath.AddLine to the path and check if the path contains the point.

Example:

The below method, checks if the p is on the line with end points p1 and p2 using the specified width.

You can use wider width to increase the tolerance or if the line is wider than 1:

//using System.Drawing;
//using System.Drawing.Drawing2D;
bool IsOnLine(Point p1, Point p2, Point p, int width = 1)
{
    using (var path = new GraphicsPath())
    {
        using (var pen = new Pen(Brushes.Black, width))
        {
            path.AddLine(p1, p2);
            return path.IsOutlineVisible(p, pen);
        }
    }
}