line.cs

//https://www.c-sharpcorner.com/article/gdi-tutorial-for-beginners/
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;
using System.Drawing.Drawing2D;

namespace _2018041501_CS_TrigoTest
{
    public partial class Form1 : Form
    {
        Graphics g;
        int i = 50;

        public Form1()
        {
            InitializeComponent();
        }

        protected override void OnPaint(PaintEventArgs pe)
        {
            g = pe.Graphics;

            Point pt1 = new Point(150, 150);
            Point pt2 = new Point(300, 150);
            Pen pn = new Pen(Color.Red);

            Rectangle rect = new Rectangle(i, i, 100, 100);
            LinearGradientBrush lBrush = new LinearGradientBrush(rect, Color.Red, Color.Yellow, LinearGradientMode.BackwardDiagonal);
            g.FillRectangle(lBrush, rect);
            g.DrawLine(pn,pt1, pt2);
           
        }

      
    }
}