C#

bool bPaintLine = false;
Pen p = new Pen(Color.Blue);

private void btnLine_Click(object sender, System.EventArgs e)
{
    // if clicked - set flag, cause form to repaint and hide button
    // don't get graphics obj here and paint as line won't repaint itself
    //   rather perform line paint in the paint event
    bPaintLine = true;
    this.Invalidate();
    btnLine.Visible = false;
}
private void Form1_Paint(object sender, System.Windows.Forms.PaintEventArgs e)
{
    // if btnLine clicked (bPaintLine = false) then drawline
    if(bPaintLine)
    {
        using(Graphics g = e.Graphics)
        {
            Point pStart = new Point(10,10);
            Point pEnd = new Point(500,500);
            g.DrawLine(p,pStart,pEnd);
        }

    }
}

public class Form1 : Form
{
  // ...
  protected override void OnPaint(PaintEventArgs e)
  {
    base.OnPaint(e);
    // The rest of your handler code goes here.
  }
}

Graphics g = e.Graphics;
try
{
  // Do stuff with g
}
finally
{
  g.Dispose();
}

for(int i = pt1.X; i <= 40; )
{
   ...
}

using System;
using System.ComponentModel;
using System.Drawing;
using System.Runtime.InteropServices;
using System.Windows.Forms;
 
public class Levels : Control
{
  int mixerId = -1;
  IntPtr hmx;
 
  const int CALLBACK_WINDOW = 0x00010000;
  const int MIXER_OBJECTF_MIXER = 0x0;
  const int MM_MIXM_CONTROL_CHANGE = 0x03d1;
 
  [DllImport("winmm.dll")]
  [return: MarshalAs(UnmanagedType.U4)]
  static extern int mixerOpen(out IntPtr phmx,
    [MarshalAs(UnmanagedType.U4)] int uMxId,
    IntPtr dwCallback, IntPtr dwInstance,
    [MarshalAs(UnmanagedType.U4)] int fdwOpen);
 
  [DllImport("winmm.dll")]
  [return: MarshalAs(UnmanagedType.U4)]
  static extern int mixerClose(IntPtr hmx);
 
  public Levels()
  {
    SetStyle(ControlStyles.AllPaintingInWmPaint |
      ControlStyles.UserPaint | ControlStyles.ResizeRedraw, true);
    SetStyle(ControlStyles.Selectable, false);
  }
 
  protected override void OnHandleCreated(EventArgs e)
  {
    base.OnHandleCreated(e);
 
    if (hmx != IntPtr.Zero)
      mixerClose(hmx);
 
    if (mixerId >= 0)
    {
      int result = mixerOpen(out hmx, mixerId, Handle, IntPtr.Zero,
        CALLBACK_WINDOW | MIXER_OBJECTF_MIXER);
      if (result != 0) throw new Win32Exception(result);
    }
  }
 
  protected override void WndProc(ref Message m)
  {
    base.WndProc(ref m);
 
    if (m.Msg == MM_MIXM_CONTROL_CHANGE)
    {
      Console.WriteLine("{0}: Received a message.", DateTime.Now);
    }
  }
 
  public int MixerId
  {
    get { return mixerId; }
    set { mixerId = value; }
  }
}
 
class Test : Form
{
  static void Main()
  {
    Application.Run(new Test());
  }
 
  Test()
  {
    Levels c = new Levels();
    Controls.Add(c);
    c.Location = new Point(8, 8);
    c.MixerId = 0;
  }
}