I have been tasked with modifying some legacy ActiveX DLLs written in Visual Basic 6. One of the things I need to do is to emulate the "ScriptingContext" object, (so that we can support other mechanisms for running the DLLs other than IIS without having to re-write large chunks of the code).
Something that has been causing me some grief is the "ASPTypeLibrary.Application" object which has two very different ways to access its stored values, eg:
.Application("KeyName")
or
.Application.Value("KeyName")
How can I create my own VB6 class which supports both of these access mechanisms? I can do one or the other but not both?
(a simple code example would be great thanks, I'm not a VB6 programmer)
I have found a way to do this, see the code snippet below taken from two classes, "clsContext" and "clsContextApp". The latter implements the ".Value" functionality and the former has the ".Application" property...
I have now discovered an even more difficult problem. The ScriptingContext's "ASPTypeLibrary.Request" object has three different ways to access its ".Request.QueryString" property:
.Request.QueryString("KeyName")
or
.Request.QueryString.Value("KeyName")
or
.Request.QueryString
The last method returns a string comprised of all the Key/Value pairs concatenated by "&" characters. I have no idea how to implement this?
' clsContext
Public ContextApp As clsContextApp
Public Property Get Application(Optional ByRef Key As Variant = Nothing) As Variant
If (Key Is Nothing) Then
Set Application = ContextApp
Else
If (Not ContextApp.p_Application.Exists(Key)) Then
Application = ""
Else
Application = ContextApp.p_Application.Item(Key)
End If
End If
End Property
Public Property Let Application(ByRef Key As Variant, ByVal Value As Variant)
If (VarType(Key) = vbString) Then
If (VarType(Value) = vbString) Then
If (Not ContextApp.p_Application.Exists(Key)) Then
ContextApp.p_Application.Add Key, Value
Else
ContextApp.p_Application.Item(Key) = Value
End If
End If
End If
End Property
' clContextApp
Public p_Application As Scripting.Dictionary
Public Property Get Value(Key As String) As String
If (Not p_Application.Exists(Key)) Then
Value = ""
Else
Value = p_Application.Item(Key)
End If
End Property
Public Property Let Value(Key As String, Value As String)
If (Not p_Application.Exists(Key)) Then
p_Application.Add Key, Value
Else
p_Application.Item(Key) = Value
End If
End Property
Well I've managed to answer the additional question regarding ScriptingContext's "ASPTypeLibrary.Request" object which has three different ways to access its ".Request.QueryString" property.
I've included a code snippet below that is based on the code from my previous answer for the "ASPTypeLibrary.Application" object. If I add a new Property to the "clsContextApp" class and make it the default property for that class, then it will be called when the ".Application" property is called without any qualification eg:
MyString = Context.Application
Setting a particular property as the default property in VB6 is a little obscure, but I followed the directions I found here.
' clsContextApp Default Property
Property Get Values(Optional ByVal Index As Integer = -1) As String ' This is the Default Value for clsContextApp
Attribute Values.VB_UserMemId = 0
Dim KeyName As String, Value As String
Values = ""
If (Index < 0) Then
For Index = 0 To p_Application.Count - 1
KeyName = p_Application.Keys(Index)
Value = p_Application.Item(KeyName)
If (Index > 1) Then
Values = Values + "&"
End If
Values = Values + KeyName + "=" + Value
Next Index
Else
If (Index < p_Application.Count) Then
KeyName = p_Application.Keys(Index)
Value = p_Application.Item(KeyName)
Values = KeyName + "=" + Value
End If
End If
End Property
Adding a reference to Microsoft Active Server Pages Object Library, and to COM+ Services Type Library, and then using the object browser reveals some basic things you seem to be missing.
GetObjectContext is a global method in COMSVCSLib with no arguments used to retrieve the current ObjectContext as its return value.
ObjectContext is a Class. It has a read-only default property, named Item that takes a String argument and is of type Variant.
Passing "Application" as an argument to Item returns the current instance of the Application Class.
ScriptingContext is a Class. It is obsolete.
Application is another Class. It has a default property named Value that takes a String argument and is of type Variant.
Value is a property of the Application Class and provides access to a read-write key/value pair store where keys are always Strings. Since it is of type Variant you can store objects as well as simple values and arrays of various types.
None of this looks difficult to replicate in VB6. The key/value store could be a Collection or Scripting.Dictionary.
Related
I have a class, implementing two interfaces, one interface for normal use, and one interface that only reveals a function to create the class with initial parameters. The class has a field that is a Structure with two variables within, I am using the Get property of the class to get the structure, and then using dot notation to access the field within the structure, and then I am trying to set it to a number, but it never works. If I try to access the structure with the private variable within the class it works, but I want to be consistent and only use the properties to modify it within the Create function.
Public Function Create(WorksheetName As String, Optional CurrentRow As Long = 4) As ISheetInfo
With New clsSheetInfo
Set .WS = ThisWorkbook.Worksheets(WorksheetName)
Set .Cols = CreateColumnDictionary(.WS, 3)
Let .Rows.Current = CurrentRow
Let .Rows.Final = .WS.Cells(.WS.Rows.Count, 1).End(xlUp).Row
Set Create = .Self
End With
End Function
When I step through, it first goes to this property (RowData is the UDT with two fields, .Current and .Final:
Property Get Rows() As RowData
Rows = pRows
End Property
But then after the assignment, .Rows.Current is still 0, I'm not sure why.
I want to get an Object within a Collection as a String without calling a function for it.
e.G.:
In java i can
System.out.print(objVarXY)
and the compiler will automatically call the objVarXY.toString() function (if implemented)
in VBA something like this
Debug.Print parameterListe.LList.Item(1)
will cause an error.
Debug.Print parameterListe.LList.Item(1).toString
will work, if i implemented a toString Subfunction.
But what if i dont know what kind of object will be inside my LList collection?
Debug.Print will implicitly attempt coerce the given value expression into a String for output.
When you Debug.Print an object, VBA attempts to Let-coerce the object into a value - if the object doesn't have a default member that ultimately yields a value that can be implicitly converted to a String, then you get run-time error 438 "object doesn't support this property or method" if the class doesn't have a default member.
If the object is user code (i.e. your own class module), and if it makes sense to do so, you could add a default member yourself, and have the class responsible for knowing how to represent itself as a String (note that the VB attribute is hidden in the VBE code panes, and must be edited outside the VBE - unless you're using Rubberduck, in which case you can simply add a #DefaultMember annotation and synchronize annotations/attributes):
'#DefaultMember
Public Function ToString() As String
Attribute ToString.VB_UserMemId = 0
'...
End Function
But all this does is give the class the ability to be essentially treated as a String, through implicit member calls. I'd call that something like an abuse a language feature (it's through this mechanism that Debug.Print Excel.Application outputs the application's Name, or Debug.Print adoConnection outputs the connection's ConnectionString property), since as you noted, you might as well just invoke that ToString method explicitly.
If the object doesn't know how to represent itself as a String, then something, somewhere will have to. In Java (IIRC) and .NET this would essentially be the default ToString implementation:
Debug.Print TypeName(objVarXY)
...which is rather useless, but that's essentially what ToString does by default.
Whether you're writing Java, C#, or VBA, there needs to be code responsible for knowing how to represent objVarXY as a String.
Sadly VBA doesn't do fancypants pattern matching, so we can't Select Case TypeOf obj like in C# (it only recently got that ability - don't know about Java), and since Select Case TypeName(obj) wouldn't be type-safe, I'd go with If...ElseIf:
Public Function Stringify(ByVal obj As Object) As String
If TypeOf obj Is Something Then
Dim objSomething As Something
Set objSomething = obj ' cast to Something interface
Stringify = objSomething.SomeProperty
ElseIf TypeOf obj Is SomethingElse Then
Dim objSomethingElse As SomethingElse
Set objSomethingElse = obj ' cast to SomethingElse interface
Stringify = objSomethingElse.AnotherProperty & "[" & objSomethingElse.Foo & "]"
'ElseIf TypeOf obj Is ... Then
' ...
Else
' we don't know what the type is; return the type name.
Stringify = TypeName(obj)
End If
End Function
Obviously if the collection always involves classes that you own, the better solution is to have each object know how to represent itself as a String value.
But, having user classes expose a ToString method on their default interface isn't ideal: since we don't know what type we're getting from the collection, all we have is Object and a late-bound call - and no compile-time guarantee that the class implements a ToString method, and no compiler warning if we try to invoke, say, ToStrnig.
The solution is to not put ToString on the classes' default interface, and formalize the behavior with some IString class module, which might look like this:
Option Explicit
Public Function ToString() As String
End Funtion
Yup, that's the whole class.
Now the user classes that need to be representable as strings, can do this:
Option Explicit
Implements IString
Private Function IString_ToString() As String
' todo: implement the method!
End Function
And now we can have early-bound assurance that the objects have a ToString method:
Dim o As Object
For Each o In MyCollection
If TypeOf o Is IString Then
Dim s As IString
Set s = o 'cast to IString interface
Debug.Print s.ToString
Else
Debug.Print TypeName(o)
End If
Next
At the end of the day there's no magic, regardless of what language you're using.
Something like that does not exist in VBA there are only the Type conversion functions like CStr() to convert eg. an Integer into a String.
If you eg need to convert a Collection into an Array you will need to use a function for that.
But what if I don't know what kind of object will be inside my LList collection
Then you will need to determine which object it is (you would probably expect eg 5 different possible objects) and do something like a Select Case for each different object type to convert this to a String.
I recently came accross `Attribute Values.VB_UserMemId = 0'. I like lists so I thought I'd build a bespoke collection type object.
The minimal code for the class that can reproduce the error is:
Class Lst
Option Explicit
Public c As New Collection
'this is the default property
Public Property Get item(Optional index)
'Attribute Values.VB_UserMemId = 0
If IsMissing(index) Then
Set item = Me
'DoEvents
Else
item = c(index)
End If
End Property
Public Property Let item(Optional index, itm)
If IsMissing(index) Then 'assume itm is list
If IsObject(itm) Then Set c = itm.c Else c.add itm
Else
c.add itm, , index
c.Remove index + 1
End If
End Property
Essentially, lst(i) returns the ith element of the private collection, Lst(i)=6 sets the ith element. (errorhandling and index checking code stripped for clarity).
I noticed that objects that return themselves from the default property can be returned from a function in a variant (e.g LstFunc=L below), without the need for a set removing complexity from my students eyes...(you cant do that with a collection object)
Unfortunately, I encountered two challenges...the minimum code for these is:
The Problem
Function LstFunc() As Variant
Dim L As New Lst
L = 4 'replaces L.item=3
LstFunc = L 'this is not normally allowed, but desirable (for me!)
End Function
Sub try()
Dim L As New Lst
L = LstFunc 'replaces L.item=LstFunc-->L.c: [4]
L = 3 'L.c: [4,3]
If L = 6 Then DoEvents
End Sub
Here is what happens
1) when the expression L = 6 is evaluated excel hangs. Some times ESC gets you it back in, but my experience is that excel stops responding and needs a restart.
To evaluate the expression the L.item function is called initially, returning a Lst, for which item is called, etc.etc. resulting in unwanted, and undetected infinite repetition (not quite recursion). Uncommenting the DoEvents statement in the get item property allows you to stop without a crash
2) after uncommenting the DoEvents, I run in debugger mode step by step. If i now hover (by accident..) over the variable L, the debugger crashes, and I get the green triangle of death, which I fear will be very confusing for the students:
Note this behaviour is recoverable if the DoEvents statement in the class is commented out again. A veritable catch 22...
Bit of an intricate one this, but any sugesstions as to how I can trap the unwanted repetition in (1) at low computational cost and without losing the ability to pass the object like a variant would be greatfully received.
PS this is a code snipped that provides an unsafe workaround discussed in a comment below:
Public Property Get item(Optional index)
'Attribute Values.VB_UserMemId = 0
static i
If IsMissing(index) Then
Set item = Me
i=i+1:if i>1000 then item="":exit property
'DoEvents
Else
item = c(index)
i=0
End If
End Property
The recursion can't be avoided.
From section 5.6.2.2 of the VBA language specification:
If the expression’s value type is a specific class:
If the source object has a public default Property Get or a public default function, and this default member’s parameter list is
compatible with an argument list containing 0 parameters, the simple
data value’s value is the result of evaluating this default member as
a simple data value.
Note that with your sample class, this line of code meets all of those conditions:
If L = 6 Then DoEvents
The type of the expression L = 6 is Boolean, with an Lst on the left hand side and an Integer on the right hand side. That means the type of the comparison is Integer, so the run-time checks to see if there is a default Property Get, which you provide here:
Public Property Get item(Optional index)
'Attribute Values.VB_UserMemId = 0
The parameter list is compatible with an argument list containing 0 parameters, because the index is optional. So, it evaluates to L.item() = 6. The only test you do inside the property is If IsMissing(index), which is guaranteed to be true if it's called as the default member - remember, it can't require a parameter to be passed. As you found out, this leads you to...
5.6.2.3 Default Member Recursion Limits
Evaluation of an object whose default Property Get or default function
returns another object can lead to a recursive evaluation process if
the returned object has a further default member. Recursion through
this chain of default members may be implicit if evaluating to a
simple data value and each default member has an empty parameter list,
or explicit if index expressions are specified that specifically
parameterize each default member.
How this is handled is implementation specific. Office VBA implementations, however, do not cap the recursion depth and will simply crash the host when it runs out of stack space.
That said, the rest of your question is simply an x-y problem, although my suggestion is to scrap this. Using default members hides the intent of your code and robust, maintainable code should be readable.
Every time i try to attribute any type of string to this i get Object reference not set to an instance of an object. I have tried every combination of possible way to handle the string, convert it to a string again and all the fuzz. It's very frustrating and i guess it's some kind of base principle of the structure/class usage and the string array or whatnot (which is also very dumb)
Private Class movie
Public name As String
Public actors As String
Public year As Integer
Public country As String
Public votes As Integer
End Class
Private movies() As movie
If File.Exists(OpenFileDialog1.FileName) Then
lblPath.Text = OpenFileDialog1.FileName
Dim iFile As New StreamReader(lblPath.Text)
While Not iFile.EndOfStream
current = iFile.ReadLine
movies(i).name = "sasasasa"
lbMovies.Items.Add(movies(i).name)
i = i + 1
End While
End If
these are the code parts where i use it
You are creating an empty array of movie objects, as was pointed out previously. Consequently movies(i) is Nothing. When you try to access a member (movies(i).name) the appropriate exception is generated. Note that your code does not even reach the assignment operator = but fails prior to that. In other words, this has nothing to do with strings altogether; you will get the same error if you write movies(i).votes = 42 instead. To fix your code you will first have to create a movie object, populate it and append it to your array.
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