Interface Problems

Question

I tried to implement a new class that is implementing two interfaces in Excel VBA but I have compilation problems as the members of the interfaces do not seem to be properly implemented in the implementing class (they are not callable).

The Interfaces are the following:

ICrawlable:

Option Explicit

Public Function GetCrawler() As ICrawler

End Function

IEquatable:

Option Explicit

Public Function Equals(CompareObject As Variant) As Boolean

End Function

Whereas ICrawlable also holds a function returning the interface ICrawler:

Option Explicit

Public Property Get CurrentItem() As Variant

End Property

Public Sub MoveNext()

End Sub

Public Function GetNext() As Variant

End Function

Public Function ItemsLeft() As Boolean

End Function

I created a sample class InterfaceTester using those two first interfaces:

Option Explicit

Implements ICrawlable
Implements IEquatable

Private Function ICrawlable_GetCrawler() As Variant

End Function

Private Function IEquatable_Equals(CompareObject As Variant) As Boolean

End Function

When trying to use that class in a Module or wherever else I do not have Intellisense support. Furthermore, the code does not compile and I get a "Method or Data member not found" in this module code highlighting the .Equals:

Sub TestInterfacing()
    Dim TestInstance As InterfaceTester
    Set TestInstance = New InterfaceTester

    Dim VerificationInstance As InterfaceTester
    Set VerificationInstance = New InterfaceTester

    Dim Result As Boolean

    Result = TestInstance.Equals(VerificationInstance)
End Sub 

Is this a bug in VBA? Have I declared things in the interfaces that are not allowed (I have tried changing all return data types to Variant already and have tried disabling each of the functions of the interfaces)? Do I use reserved keywords (in the object explorer I do not see duplicates of my interface names)? Does it compile on your machines?

Answer 1

If you want to use methods from the interface, your variable must be declared as this interface type:

Sub TestInterfacing()
    Dim TestInstance As IEquatable 'InterfaceTester
    Set TestInstance = New InterfaceTester

    Dim VerificationInstance As InterfaceTester
    Set VerificationInstance = New InterfaceTester

    Dim Result As Boolean
    Result = TestInstance.Equals(VerificationInstance)

End Sub

However, in this case you cannot use methods from the second interface implemented by this class: ICrawlable.

---

The reason for this is that in VBA implementing method looks like that:

Private Function ICrawlable_GetCrawler() As ICrawler

while using the rules from other languages it should look like below:

Public Function GetCrawler() As ICrawler

VBA would not understand that this is implementation of interface's method GetCrawler.

---

In order to overcome this issue you should add another two public methods to InterfaceTester class - Equals and GetCrawler. Implementing methods should only direct to those methods:

InterfaceTester class:

Implements ICrawlable
Implements IEquatable


Public Function Equals(CompareObject As Variant) As Boolean
    'implementation
End Function

Public Function GetCrawler() As ICrawler
    'implementation
End Function

Private Function ICrawlable_GetCrawler() As ICrawler
    Set IEquatable_Equals = GetCrawler
End Function

Private Function IEquatable_Equals(CompareObject As Variant) As Boolean
    IEquatable_Equals = Equals(CompareObject)
End Function

Now you can declare your variable TestInstance as InterfaceTester class and use methods from both interfaces.