An interface is a reference type object with no implementation. You can think of it as an abstract class with all the implementation stripped out and everything that is not public removed. Abstract classes are classes that can not be instantiated. No properties or methods are actually coded in an interface, they are only defined. So the interface doesn't actually do anything but only has a signature for interaction with other classes or interfaces.
Interface can be consider as a contract. All the members and methods defined in the interface should be implemented in derived class. Two interfaces can have same method. But when implement method should be implemented as interface name.method name.
For Each ctrl As Control In Me.Controls("pnlMainPanel").Controls
If ctrl.GetType Is GetType(System.Windows.Forms.Panel) Then
For Each subCtrl As Control In ctrl.Controls
If subCtrl.GetType Is GetType(System.Windows.Forms.TextBox) Then
If subCtrl.GetType Is ...