C#

Process.Start("mailto:le.nhan@freenet.de?subject=Error notification");

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; )
{
   ...
}