C#

namespace DoubleBufferPanelNameSpace
{
    class DoubleBufferPanel : Panel
    {
        public DoubleBufferPanel()
        {
            this.SetStyle(ControlStyles.AllPaintingInWmPaint |
                     ControlStyles.UserPaint |
                     ControlStyles.DoubleBuffer |
                     ControlStyles.ResizeRedraw,
                     true);
            this.UpdateStyles();
        }
    }
}

store drawable objects in an array. Each drawable object knows how to draw itself. 
On repaint, just iterate over the array of drawable objects. 
So for a line, create a Line class that implements IDrawable. 
Store the state of the line in the line object so that the line knows where it goes and 
what it looks like. The OS clips the drawing to the dirty drawing rect so that 
only the dirty portion of the screen is redrawn. 
If you call invalidate the entire screen will be redrawn.

class Line : IDrawable
{
    public Line(Point startingPoint1, Point startingPoint2)
    {
        p1 = startingPoint1;
        p2 = startingPoint2;
    }

    private Point p1;
    public Point P1
    {
        get
        {
            return p1;
        }
        set
        {
            p1 = value;
        }
    }

    private Point p2;
    public Point P2
    {
        get
        {
            return p2;
        }
        set
        {
            p2 = value;
        }
    }
}

bool first = false;
OnEnter()
{
  //change colours
  first = true;
}
.
OnLeave()
{
  //change colours back
}
.
KeyDown(KeyEventArgs e)
{
  if(key is tab OR key is enter) {
    e.Handled = true;
    MyForm.Focus(); //should give the form focus, OnLeave event should fire
   }
.
  if(first)
    //clear data
.
  first = false;
}

e.Handled = char.IsDigit(e.KeyChar )

if (!((e.KeyChar >= 'A' && (e.KeyChar <= 'Z') || ((e.KeyChar >= 'a') && (e.KeyChar <= 'z')))))
{

    e.Handled = true;


}

if (char.IsSeparator(e.KeyChar))
{
    e.Handled = false;
}