I have a very simple Class definition.
Class Sheetwriter is defined as follows:
Option Explicit
'Works off of the ActiveCell
'Helps you write data into the cells
Private pCornerCell As String
Public Property Get CornerCell()
CornerCell = pCornerCell
End Property
Public Property Let CornerCell(Value As String)
pCornerCell = Value
Range(Value).Select
End Property
I get a compile error that I don't understand.
Definitions of property procedures for the same property are inconsistent, or the property procedure has an optional parameter.
What am I doing wrong?
Public Property Get CornerCell()
That's returning an implicit Variant, since no return type was specified.
Public Property Get CornerCell() As String
That's returning the String that the compiler is seeing in the Property Let member, and fixes your problem.
FWIW, that Range(Value).Select statement doesn't belong in there at all, and you don't want to work off the active cell and sprinkle Select and Activate statements everywhere.
See How to avoid using Select in Excel VBA macros for some tips about avoiding that.
Related
I have been trying to clean up my code a bit and make it more similar to the Excel object model, and I was wondering if it is possible to create a "loopable" container class in VBA, e.g. similar to how you can do:
Dim Sheet As Worksheet
For Each Sheet In ThisWorkbook.Worksheets
' ...
Next Sheet
I want this functionality for my own container.
Say I create my own class called Container which contains items of some class ItemType (this can simply be an empty class for this example):
' Class Container
' The container contains items of a class I will call ItemType
Private Type MContainer
Items As Collection ' Could also be implemented in terms of an array
End Type
Private This As MContainer
Public Property Get Item(ByVal Index As Long) As ItemType
Attribute Item.VB_UserMemId = 0 'Makes it so I can access elements like a Collection
Set Item = This.Items(Index)
End Property
Public Function Add() As ItemType
This.Items.Add
Set Add = This.Items(This.Items.Count)
End Function
Private Sub Class_Initialize()
Set This.Items = New Collection
End Sub
I then want to loop through the items in my container with the For Each..., but this doesn't work. See the following example for how I ideally want it to work:
Public Sub MyMethod()
Dim Stuff As New Container
Stuff.Add
Dim Element As ItemType
For Each Element In Stuff ' <- This will not work
' Do something
Next Element
End Sub
The final For loop is what I am looking at making work. Is this possible? Basically the issue is that I can't call For Each on my Container class similar to how you can with e.g. the Excel.Sheets class. Is this possible to achieve in VBA?
For Each iteration requires a special member attribute value to work, and a NewEnum property or function returning an IUnknown.
Every collection class that can be iterated with a For Each loop has a hidden [_NewEnum] member (the square brackets are required for accessing the hidden member, since the underscore prefix is illegal for an identifier in VBA.
Tweaking module and member attributes isn't possible to do directly in the VBE, so you need to remove/export the module, modify it in e.g. Notepad++, save the changes, then re-import it into your project.
Or, have Rubberduck (disclaimer: I contribute to this open-source project) do it for you, using annotations (aka "magic comments"):
'#Enumerator
'#Description("Gets an enumerator that iterates through the internal object collection.")
Public Property Get NewEnum() As IUnknown
Set NewEnum = this.Items.[_NewEnum]
End Function
'#DefaultMember
'#Description("Gets/sets the element at the specified index.")
Public Property Get Item(ByVal index As Long) As ItemType
Set Item = this.Items(index)
End Property
Then parse the project (Ctrl+`) and bring up the Inspection Results toolwindow (Ctrl+Shift+i) - there should be a number of "Missing Attribute" results under "Rubberduck Opportunities":
Click "Fix all occurrences in module" in the bottom pane, to synchronize the hidden attributes with the annotation comments.
If you have "Missing Annotation" results, Rubberduck has determined that a module/member has a non-default value for a given attribute, and is able to similarly add an annotation comment that surfaces/documents it with a comment.
The Code Explorer (Ctrl+R), the Rubberduck toolbar, and the VBE's own Object Browser (F2) will display the contents of the VB_Description attribute, so #Description annotations are particularly useful to have on any public procedure.
Object Browser:
Code Explorer:
Rubberduck toolbar:
Add this to your class
Public Function NewEnum() As IUnknown
Attribute NewEnum.VB_UserMemId = -4
Set NewEnum = Items .[_NewEnum]
End Function
An alternative approach to this issue is not to use a Collection but a Scripting.Dictionary. One of the advantages of a scripting dictionary is that it can return arrays of the keys and items of the dictionary. Iterating over an array in VBA is a trivial exercise.
These two subs do the same thing when inside a class.
Sub DemoMe( )
Me.AboutMe ' Calls AboutMe procedure.
End Sub
Sub DemoMe( )
AboutMe ' Does the same thing.
End Sub
What is the point? Does the Me keyword do anything? What is the preferred way of an object accessing its own members?
tldr; No, although there are situations where it can be useful.
From the VBA language specification (5.3.1.5):
Each procedure that is a method has an implicit ByVal parameter called
the current object that corresponds to the target object of an
invocation of the method. The current object acts as an anonymous
local variable with procedure extent and whose declared type is the
class name of the class module containing the method declaration. For
the duration of an activation of the method the data value of the
current object variable is target object of the procedure invocation
that created that activation. The current object is accessed using the
Me keyword within the <procedure-body> of the method but cannot be
assigned to or otherwise modified.
That's all it is, just a "free" local variable that refers to the specific instance that the method is being called on. This also happens to be the default context for the procedures during their invocation, so it can be omitted if the code is intended to operate on the current instance. Although as #HansPassant points out in the comment above, it also allows the editor to bind to the interface and provide IntelliSense.
That said, there are a couple instances where you would either want to or have to use it (this is by no means an exhaustive list):
Naming collisions:
If your class has a member that "hides" a built-in VBA function, it can be used to make the scope explicit:
Public Property Get Left() As Long
'...
End Property
Public Property Get Right() As Long
'...
End Property
Public Property Get Width() As Long
Width = Me.Right - Me.Left
End Property
Equity Checks:
Public Function Equals(other As Object) As Boolean
If other Is Me Then
Equals = True
Exit Function
End If
'...
End Function
Fluent Functions:
This can be a useful pattern for compositing objects - you perform an action, then return the instance of the class so they can be "chained". Excel's Range interface does this in a lot of cases:
Public Function Add(Value As Long) As Class1
'Do whatever.
Set Add = Me
End Function
Public Sub Foo()
Dim bar As New Class1
bar.Add(1).Add(1).Add 1
End Sub
Not any more than there are reasons to use this in Java, C#, or any other language: it's a reserved identifier that represents the current instance of the class - what you do with that is up to your imagination.
What is the preferred way of an object accessing its own members?
Indeed, an object doesn't need the Me keyword to access it own public interface. Same as this in other languages, I'd even call it redundant. However it can sometimes be a good idea to explicitly qualify member calls with Me, especially when the class has a VB_PredeclaredId attribute (e.g. any UserForm): referring to UserForm1 in the code-behind of UserForm1 yields a reference to the default instance of the class, whereas qualifying member calls with Me yields a reference to the current instance of that class.
Accessing Inherited Members
VBA user code can't do class inheritance, but a lot of VBA classes do have a base class. The members of UserForm when you're in the code-behind of UserForm1, and those of Worksheet when you're in the code-behind of Sheet1, aren't necessarily easy to find. But since the inherited members show up in IntelliSense/auto-complete, you can type Me. and browse a list of members inherited from the base class, members that you would otherwise need to know about in order to invoke.
A class creating an instance of itself inside itself? That I've never seen.
You're missing out! I do this all the time, to enable referring to the object instance held by a With block, inside a Factory Method - like this GridCoord class.
Public Function Create(ByVal xPosition As Long, ByVal yPosition As Long) As IGridCoord
With New GridCoord
.X = xPosition
.Y = yPosition
Set Create = .Self
End With
End Function
Public Property Get Self() As IGridCoord
Set Self = Me
End Property
Note that while the GridCoord class exposes a getter and a setter for both X and Y properties, the IGridCoord interface only exposes the getters. As a result, code written against the IGridCoord interface is effectively working with read-only properties.
Another use is to get the name of the class module, without needing to hard-code it. This is particularly useful when raising custom errors: just use TypeName(Me) for the Source of the error.
The Builder Pattern notoriously returns Me, which enables a "fluent API" design that makes it possible to write code that incrementally builds complex objects through chained member calls, where each member returns Me (except the final Build call, which returns the type of the class being built):
Dim thing As Something
Set builder = New ThingBuilder
Set thing = builder _
.WithFoo(42) _
.WithBar("test") _
.WithSomething _
.WithSomethingElse
.Build
#PBeezy : In addition to my comment :
Me, refers to the object it's coming from so AboutMe resides in the class. If you had another instance, say this is Class1, you'd have dim c as Class1, as soon as you create an instance of Class1 in Class1, you need to tell the compiler which class you are using, the holding class or the instance created in, where, me.class1.aboutme would be logically valid. You can also create, a class for each cell in a workbook, then you could refer to A1's class from B1's class. Also, if there is a public function/sub called AboutMe, this also helps.
Class (clsPerson)
Public c1 As clsPerson
Public strPersonName As String
Public Function NAME_THIS_PERSON(strName As String)
strPersonName = strName
End Function
Public Function ADD_NEW_CHILD(strChildName As String)
Set c1 = New clsPerson
c1.strPersonName = strChildName
End Function
Normal module
Sub test()
Dim c As New clsPerson
c.NAME_THIS_PERSON "Mother"
c.ADD_NEW_CHILD "Nathan"
Debug.Print c.strPersonName
Debug.Print c.c1.strPersonName
End Sub
Gives these results
Mother
Nathan
I'm having an issue creating this class and it's stating that the Set parameters are invalid and error on the below code. I've triple checked this and can't seem to find the issue! Any idea why this error is occurring?
CODE:
Option Explicit
Private pIsChargeable As Boolean
Public Property Set IsChargeable(value As Boolean)
pIsChargeable = value
End Property
Public Property Get IsChargeable() As Boolean
IsChargeable = pIsChargeable
End Property
ERROR:
Compile Error:
Definitions of property procedures for the same property are inconsistent, or property procedure has an optional parameter, a ParamArray, or an invalid Set final parameter
Property Set is for Object type variables. You're looking for Property Let.
It might be helpful to read Chip Pearson's explanation of classes as well.
Our object model contains a class called Unit and a collection of these called Units (which is stored in a Dictionary). These objects have unique Names and Keys (they originally came from a SQL db that enforced this) so I have added:
Public Units(N as String) As Unit ...
Public Units(K as Integer) As Unit...
which return a Unit object from the Units collection.
In Excel VBA, one can refer to most objects using similar methods; Worksheets(1) returns the first sheet, while Worksheets("Bob") returns the named sheet. But they have one additional method, Worksheets, which returns the entire collection. It's as if they have this method...
Public Worksheets() As List(Of Worksheet)
But you can't use List in interop (right?) so it's more like...
Public Worksheets() As ArrayList
So how would I do the same basic API in .net with interop? That is, have three methods...
Public Units(N as String) As Unit ...
Public Units(K as Integer) As Unit...
Public Units() As ArrayList...
As I understand it only the first method of a given name is exported (is this correct?). So how does Excel do it, and can I fake that in .net?
VBA's Worksheets is not a method. It is a class, Worksheets, that has a default property Item that accepts a parameter of type Variant. There is no overloading (COM does not support it), it's just that Variant can hold both a number or a string.
If you want a similar structure in VB.NET, you can have a collection class that implements a default property as VB.NET understands it, and this time you can overload it.
Public Class UnitsCollection
Default Public ReadOnly Property Item(ByVal i As Integer) As Unit
Get
Return ...
End Get
End Property
Default Public ReadOnly Property Item(ByVal i As String) As Unit
Get
Return ...
End Get
End Property
End Class
Private oiCustoms() As CCustomClass
Public Property Get Partners() As CCustomClass()
Set Partners() = oiCustoms()
End Property
Public Property Set Partners(values() As CCustomClass)
ReDim oiPartners(values.Count)
Set oiCustoms() = values()
End Property
When I try to run I get a Compile error:
Definitions of property procedures for the same property are inconsistent, or property procudure has n optional parameter, a ParamArray, or an invalid Set final parameter.
What's wrong here? I have done some looking, and it looks like I can't use an array as a property parameter. Is this correct? Any good workarounds. I will need to have this data for the object stored in an array for use elsewhere.
You are confusing arrays with objects.
Private oiCustoms() As CCustomClass
Public Property Get Partners() As CCustomClass()
Partners = oiCustoms
End Property
Public Property Let Partners(values() As CCustomClass)
ReDim oiPartners(LBound(values) To UBound(values))
oiCustoms = values
End Property