Generic All Controls Method
The problem is that Concat would want an IEnumerable<T> as well - not an IEnumerable<Control>. This should work though:
public static IEnumerable<T> GetAll<T>(this Control control) where T : class
{
var controls = control.Controls.Cast<Control>();
return controls.SelectMany(ctrl => GetAll<T>(ctrl))
.Concat(controls.OfType<T>()));
}