C#

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;
}

int keycode = e.KeyChar;
if (!((e.KeyChar >= 'A' && e.KeyChar <= 'Z') || (e.KeyChar >= 'a' && e.KeyChar <= 'z') || keycode == 8 || keycode == 32)) {
     e.Handled = true;
}

if (!((e.KeyChar >= 'A' && (e.KeyChar <= 'Z') || ((e.KeyChar >= 'a') && (e.KeyChar <= 'z')))))
     {
           e.Handled = true;
     }
     if ((char.IsWhiteSpace(e.KeyChar)) || (e.KeyChar == (char)ConsoleKey.Backspace))
     {
         e.Handled = false;
     }