Interfaces

An Interface is a way to define a set of behaviors that a class will perform. The definition of an interface is a list of method signatures (name, parameters, and return type). A class having all of the methods is said to "implement" that interface.

In VBA, using interfaces lets the compiler check that a module implements all of its methods. A variable or parameter can be defined in terms of an interface instead of a specific class.

Simple Interface - Flyable

The interface Flyable is a class module with the following code:

Public Sub Fly()
    ' No code.
End Sub

Public Function GetAltitude() As Long
    ' No code.
End Function

A class module, Airplane, uses the Implements keyword to tell the compiler to raise an error unless it has two methods: a Flyable_Fly() sub and a Flyable_GetAltitude() function that returns a Long.

Implements Flyable

Public Sub Flyable_Fly()
    Debug.Print "Flying With Jet Engines!"
End Sub

Public Function Flyable_GetAltitude() As Long
    Flyable_GetAltitude = 10000
End Function

A second class module, Duck, also implements Flyable:

Implements Flyable

Public Sub Flyable_Fly()
    Debug.Print "Flying With Wings!"
End Sub

Public Function Flyable_GetAltitude() As Long
    Flyable_GetAltitude = 30
End Function

We can write a routine that accepts any Flyable value, knowing that it will respond to a command of Fly or GetAltitude:

Public Sub FlyAndCheckAltitude(F As Flyable)
    F.Fly
    Debug.Print F.GetAltitude
End Sub

Because the interface is defined, the IntelliSense popup window will show Fly and GetAltitude for F.

When we run the following code:

Dim MyDuck As New Duck
Dim MyAirplane As New Airplane

FlyAndCheckAltitude MyDuck
FlyAndCheckAltitude MyAirplane 

The output is:

Flying With Wings!
30
Flying With Jet Engines!
10000

Note that even though the subroutine is named Flyable_Fly in both Airplane and Duck, it can be called as Fly when the variable or parameter is defined as Flyable. If the variable is defined specifically as a Duck, it would have to be called as Flyable_Fly.

Multiple Interfaces in One Class - Flyable and Swimable

Using the Flyable example as a starting point, we can add a second interface, Swimmable, with the following code:

Sub Swim()
    ' No code
End Sub

The Duck object can Implement both flying and swimming:

Implements Flyable
Implements Swimmable

Public Sub Flyable_Fly()
    Debug.Print "Flying With Wings!"
End Sub

Public Function Flyable_GetAltitude() As Long
    Flyable_GetAltitude = 30
End Function

Public Sub Swimmable_Swim()
    Debug.Print "Floating on the water"
End Sub

A Fish class can implement Swimmable, too:

Implements Swimmable

Public Sub Swimmable_Swim()
    Debug.Print "Swimming under the water"
End Sub

Now, we can see that the Duck object can be passed to a Sub as a Flyable on one hand, and a Swimmable on the other:

Sub InterfaceTest()

Dim MyDuck As New Duck
Dim MyAirplane As New Airplane
Dim MyFish As New Fish

Debug.Print "Fly Check..."

FlyAndCheckAltitude MyDuck
FlyAndCheckAltitude MyAirplane

Debug.Print "Swim Check..."

TrySwimming MyDuck
TrySwimming MyFish

End Sub

Public Sub FlyAndCheckAltitude(F As Flyable)
    F.Fly
    Debug.Print F.GetAltitude
End Sub

Public Sub TrySwimming(S As Swimmable)
    S.Swim
End Sub

The output of this code is:

Fly Check... Flying With Wings! 30 Flying With Jet Engines! 10000 Swim Check... Floating on the water Swimming under the water