How to use LinearGradientBrush whith fill rectangle in c#.net

///////////////////     CODE    /////////////////////////

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 _125
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void panel1_Click(object sender, EventArgs e)
        {
            LinearGradientBrush lgb = new LinearGradientBrush(new Point(20, 70), new Point(70, 20), Color.Blue, Color.Violet);
            Graphics g = panel1.CreateGraphics();

            ColorBlend cb = new ColorBlend();


            cb.Colors = new Color[] {Color.White,Color.Black,Color.Blue,Color.GreenYellow };

            cb.Positions = new float[] { 0, .33F, .66F, 1F };

            g.FillRectangle(lgb, 20, 20, 100, 100);

        }
    }
}

///////////////////     OUTPUT   //////////////////////

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