问题
I like to use early binding in my VBA projects, since I like the auto-complete of method names, etc. during development. I also like the confidence of knowing that the compiler will warn me if I've mis-spelled a method name.
However, to use early binding I need to add a reference to the relevant library (for example, the "Microsoft Scripting Runtime"). That's fine for "standard" libraries like that, but sometimes I want to use a library that may or may not be present on the user's machine.
Ideally, I'd like to display a useful message if the library is not present (such as "xyz is not installed on this computer, and so this feature cannot be used"). If I was using only late binding, then I could do this:
Dim o As Object
Set o = CreateObject("foo", "bar")
If o Is Nothing Then
MsgBox "nope"
End If
But, if I've added a reference to the library in order to use early binding, then if the library is not present I get a compile error when my VBA project is loaded. Thus, none of the code runs (including the code to detect the non-existence of the library).
Is there any way around this catch-22?
回答1:
Not really.
However, one way I've dealt with this in development is to have two separate declaration lines. I comment one or the other depending on whether I am doing dev work or releasing to production. You can leave everything else alone (including the CreateObject line) and then you just need to remember to switch the commented line and add/remove the reference itself.
For example:
Dim o As foo.bar 'Comment out for production'
'Dim o As Object ''Comment out for dev work'
Set o = CreateObject("foo", "bar")
If o Is Nothing Then
MsgBox "nope"
End If
回答2:
You could create a class module as a proxy for an object library --- wrap all the methods, properties, and constants needed by your other procedures.
All those procedures would use the proxy class the same way, so you wouldn't need to revise those procedures to switch between early and late binding. And Intellisense would show you everything you expose with the proxy class.
The class would be a single point of control to switch between early and late binding. You mentioned Excel as one example:
#Const DevStatus = "PROD"
#If DevStatus = "DEV" Then
Private objApp As Excel.Application
Private objBook As Excel.Workbook
Private objSheet As Excel.Worksheet
#Else 'assume PROD
Private objApp As Object
Private objBook As Object
Private objSheet As Object
#End If
If there is a possibility Excel may not be installed on any users' machines, you can check its availability during class initialize.
Dim blnExcelAvailable As Boolean
Private Sub Class_Initialize()
blnExcelAvailable = IsExcelAvailable()
End Sub
Private Function IsExcelAvailable() As Boolean
Dim blnReturn As Boolean
Dim objTest As Object
On Error GoTo ErrorHandler
Set objTest = CreateObject("Excel.Application")
blnReturn = True
ExitHere:
On Error GoTo 0
Set objTest = Nothing
IsExcelAvailable = blnReturn
Exit Function
ErrorHandler:
blnReturn = False
GoTo ExitHere
End Function
Then your procedures which use the proxy class could check a property to see whether Excel is available.
Public Property Get ExcelAvailable() As Boolean
ExcelAvailable = blnExcelAvailable
End Property
I think this approach is possible, and it satisfies your requirements AFAICT. However, I'm unsure whether it's reasonable. Back to the example of Excel, you could do something like this for a manageable subset of its object model. But if you need all or most of its methods, properties, and constants, the proxy class would be a huge undertaking.
Personally I wouldn't use this approach. It's less work for me to manage early/late binding as mwolfe02 and JP. described. However my impression is this is more burdensome in your situation, so perhaps you're willing to invest more effort than I am in something like this.
来源：https://stackoverflow.com/questions/8911845/can-i-use-late-binding-to-check-the-existence-of-a-library-before-using-it-via-e