using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Drawing.Drawing2D;

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

        bool dragable = false;
        int firstX;
        int firstY;

        private void Form1_MouseDown(object sender, MouseEventArgs e)
        {
            dragable = true;
            firstX = e.X;
            firstY = e.Y;
        }

        private void Form1_MouseMove(object sender, MouseEventArgs e)
        {
            Graphics g = this.CreateGraphics();
            Pen myPen = new Pen(Color.Black, 1);
           // GraphicsPath myPath=new GraphicsPath(

            if (dragable == true)
            {
                //g.Clear(Color.White);
                g.DrawLine(myPen, e.X-1, e.Y-1, e.X, e.Y);
                //g.DrawPath(myPen, GraphicsPath
                //g.DrawRectangle(myPen, e.X, e.Y, 1, 1);
            }
            
        }

        private void Form1_MouseUp(object sender, MouseEventArgs e)
        {
            dragable = false;
        }
    }
}

DrawLine(OldX, OldY, NewX, NewY);
OldX = NewX;
OldY = NewY;