Using VBA in excel:
I have a function where I want to access class properties using variables for property names. I was able to use a variable with CallByName to MsgBox the property value back to me:
MsgBox CallByName(oThisInvoice, DataType, VbGet)
Where oThisInvoice is a class object and DataType is the variable that contains the property name I want to access. This statement works and is a rewrite of:
MsgBox oThisInvoice.InvoiceDate
However, using the same method doesn't work when assigning a value to the same property:
CallByName(oThisInvoice, DataType, VbGet) = 5
doesn't work, I get an "Run-time Error 424: Object Required" error. Using VbLet and VbSet didn't work either, throwing "Run-time error 446: Object doesn't support named arguments".
It is a rewrite of the following:
oThisInvoice.InvoiceDate = 5
which does work.
Does anyone know what I can use to assign a value to a class property in VBA when using a variable to reference the property name?
CallByName(oThisInvoice, DataType, VbGet) = 5
Of course that doesn't work to set a value. You are calling a Get procedure, as evidenced by the use of VbGet. Get doesn't set values, it gets them.
What you need to do instead is use CallByName with VbLet:
CallByName oThisInvoice, DataType, VbLet, 5
Related
Solution: Just put brackets around Value in the CallByName statement to force evaluation of it.
Ex. CallByName MobClass, TargetData, vbLet, (Value)
Credit goes to Rory from the other post, which I will probably be deleting since it is no longer relevant and a possible duplicate.
I've spent a long time trying to figure out what was wrong with how I was using CallByName. I finally realized that its fourth argument (Args) will throw a type mismatch if the input is not either EXACTLY the same type as the input argument of what its calling or its hard-coded in.
(I don't even understand how, or why, it does this since VarType(Variant/Integer) = VarType(Integer))
So I either need a way to make it accept variant inputs or convert variables from Variant/Integer to Integer (or create a new variable) without a giant select case.
Edit: So my question wasn't clear so I'll explain it in more detail. I have a bunch of classes that I want to cycle through and call the Let property on. My simplified setup is:
Dim AllClasses as Collection
Sub SetAll(TargetProperty as String, Value as Variant)
For each ClassX in AllClasses
CallByName ClassX, TargetProperty, vbLet, Value
Next ClassX
End Sub
The problem is Value when it is initialized as Variant. The only time I can get it to not throw a type mismatch exception is when I initialize Value as the exact same type that the property wants, but I can't do that since the data types in the class vary.
Edit 2: I'm going to ask another question about the whole problem since no one seems to know much about CallByName
Edit 3: Here's a summary of what we have so far:
CallByName's fourth argument (Args) throws a type mismatch when trying to call the Let property on a class.
This only happens when the value trying to be assigned is stored in a Variant data type. It works perfectly if the variable is initialized to the same type the Let property is expecting OR if the value is hard-coded into the argument.
The Let property works fine on its own. It accepts Variant data types just fine.
My question is: Is there a way to stop this exception? I'm creating another post about other possible solutions to my overall problem.
The Problem is that you pass the properties-arguments by reference not by value, but you can't pass a reference to a different datatype (variant -> long) as the types don't match and it can't be converted as this would change the data type in the caller too. By using brackets, you force the argument to be passed by value and it can be casted as typeLong.
You can avoid this by using ByValin theProperty Letterinstead ofByRef(what is implicit used if not set). You are aware that by referencing a variable, changes made in the property change the callers value too?
Example:
Class PassExample
'Save as class module PassExample
Public Property Let PropByVal(ByVal NewVal As Long)
NewVal = 1
End Property
Public Property Let PropByRef(ByRef NewVal As Long)
NewVal = 1
End Property
Module with test sub:
'save as standard module
Sub test()
Dim v As Variant
v = 0
Dim ExampleInstance As New PassExample
CallByName ExampleInstance, "PropByVal", VbLet, v 'this works
CallByName ExampleInstance, "PropByRef", VbLet, v 'type mismatch
CallByName ExampleInstance, "PropByRef", VbLet, (v) 'this works as ByRef is changed to byval
Debug.Print v ' shows 0, not 1 as it should be if referenced
CallByName ExampleInstance, "PropByRef", VbLet, CVar(v) ' works too as it passes a function-result that can't be referenced
End Sub
Thanks to Rory and chris neilsen for providing the solution!
I answered this question, apparently to OP's satisfaction, but still find their question puzzling. Their question involved an expression in which a workbook object was being concatenated with a string, triggering Run-time Error '438': Object doesn't support this property or method. You can reproduce this sort of error by simply typing
?"Hello, " & ThisWorkbook
In the Immediate Window.
My question is -- why does this raise that error, instead of a error 13 -- type mismatch? A reasonable guess is that VBA tries to find a default property for a workbook object and that a default property doesn't exist. But, if so, I would expect it to be the following error from Microsoft's list of Visual Basic 6.0 error codes: Automation object doesn't have a default value (Error 443).
It is mostly of academic interest, but if the result of concatenating an object without a default property with a string is always Error 438, and that is the only way of triggering Error 438 rather than possibly another error when concatenating a string with an object, then the following code might be of use:
Function HasDefault(O As Variant) As Boolean
Dim i As Long
If Not IsObject(O) Then Exit Function
On Error Resume Next
i = Len("Hello, " & O)
If Err.Number = 438 Then
HasDefault = False
Else
HasDefault = True
End If
End Function
I've tested this on a variety of objects, and for those I've tested it on it has returned False exactly when _Default doesn't show up as a (hidden) member of the object when viewed in the Object Browser. Nevertheless, I don't quite trust this function and am still puzzled by what is going on here.
VBA will try and convert the expressions on each side of the & operator to a data value. The language spec states that:
If the value type of the expression’s target variable is a class:
If the declared type of the target is Variant, runtime error 9
(Subscript out of range) is raised.
If the declared type of the target is not Variant, and the target has
a public default Property Get or function, the data value’s value is
the result of invoking this default member for that target with this
argument list. This consumes the argument list.
Otherwise, runtime error 438 (Object doesn’t support this property or
method) is raised.
As regards your function, I'd just use:
callbyname(O, "_Default", VbGet)
which will raise a 438 error as appropriate.
All I'm trying to do here is save a reference to the currently active window, but it doesn't seem to be working. It gives me a run time error on the last line.
Dim SourceWindow As Window, QACheckWindow As Window
SourceWindow = ActiveWindow
I'm not exactly sure why. Isn't ActiveWindow supposed to return the currently active window? If not, how can I make a reference to it?
EDIT: The above is right at the beginning of my function, so all there is before it is Sub FuncName()
In VB object variables require the Set keyword to be assigned. Object properties that are objects also need to be Set. Runtime error 91 "object variable not set" is raised when the assignment doesn't use that keyword.
This is inherited from legacy Let keyword to assign values, and Set keyword to assign references; the Let eventually was deprecated (although still needed for defining properties) and the Set remained, leaving the VB6/VBA value assignment syntax like [Let] variable = value, where "Let" is optional.
In the declaration and assignment:
Dim SourceWindow As Window, QACheckWindow As Window
'this is like saying "Let SourceWindow = ActiveWindow":
SourceWindow = ActiveWindow
SourceWindow is an object, assigned as if it were a value - this causes VBA to attempt let-coercion through a default member call. If the object wasn't initialized, the member call fails with error 91. If the object was initialized but doesn't have a default member, error 438 is raised.
So in this case error 91 is being raised because of an implicit member call; the .net equivalent would be a NullReferenceException:
Dim SourceWindow As Window, Dim WindowTitle As String
'"SourceWindow" reference isn't set, the object can't be accessed yet:
WindowTitle = SourceWindow.Caption
I'm going to go a bit overboard here, but the legacy Let statement should not be confused with the Let clause (in VB.net) which, in the LINQ query syntax (in VB.net), computes a value and assigns it to a new, query-scoped variable (example taken from MSDN):
From p In products
Let Discount = p.UnitPrice*0.1 '"Discount" is only available within the query!
Where Discount >= 50
Select p.ProductName, p.UnitPrice, Discount
VB.net assigns both values and references, without the need to specify a Let or a Set, because in .net this distinction is a much thinner line, given how everything ultimately derives from System.Object... including System.ValueType. That's why the Set keyword was also deprecated in VB.net, and also why the VB.net syntax for defining properties has dropped the Let in favor of Set - because parameterless default members are illegal in VB.NET, so this ambiguous let-coercion doesn't happen.
A property in an Excel/VBA class I'm writing returns a Range. I made it the default property for the class using the technique described at http://www.cpearson.com/excel/DefaultMember.aspx. I expected to use all the Range class's built-in properties and methods with objects of my class without specifying the property explicitly. It doesn't work. Here are a couple of much simpler classes to illustrate. (These listings are the exported source viewed with a text editor since VBA's editor hides the Attribute statements.)
' clsDefLong: This class just verifies that default properties work as I expected.
Public Property Get DefProp() As Long
Attribute DefProp.VB_UserMemId = 0
DefProp = 125
End Property
' clsDefRange: This class is identical except the default property returns a Range.
Public Property Get DefProp() As Range
Attribute DefProp.VB_UserMemId = 0
Set DefProp = ActiveCell
End Property
Here's a Sub in a normal module to instantiate and test the classes. The comments indicate what happens when I single step through it:
Sub DefTest()
Dim DefRange As New clsDefRange, DefLong As New clsDefLong
Debug.Print DefLong.DefProp '(1) Displays 125. Verifies the class behaves as intended.
Debug.Print DefLong '(2) Same as (1). Verifies VBA uses the DefProp property as the default.
Debug.Print DefRange.DefProp.Value '(3) Displays the ActiveCell content. Verifies that this class works as intended.
Debug.Print DefRange.DefProp '(4) Same as (3). Verifies VBA knows DefProp returns a Range without further prompting.
Debug.Print DefRange '(5) Aborts with the messge "Run-time error '13': Type mismatch"
End Sub
Why doesn't DefRange in statement (5) behave just like DefRange.DefProp in statement (4)?
If I change statement (5) to:
Debug.Print DefRange.Cells(1, 1)
The compiler selects ".Cells", says "Compile error: Method or data member not found" and stops so the problem is in the object model - not just something getting messed up at run-time. Am I doing something wrong? Or isn't it possible to have a default property that returns a Range? How about other built-in classes? User defined classes?
Debug.Print DefRange
This seems like you're asking it to chain default properties and it won't do it. You can only pull the default property from the object you provide. In this case, you're returning a range object and that can't be printed. VBA won't go to the next level to see if the default property returns an object and if that object type has a default property. I suppose if it did, you could create an infinite loop - two objects each the result of the default property of the other.
Debug.Print DefRange.Cells(1, 1)
No default property will insert itself into a dot-chain. I assume this is because if DefRange did have a Cells property of its own, which would it use? I can't think of any objects in Excel's model that behave this way. You can use this
Debug.Print DefRange(1,1)
This seems to be an example of chaining default properties, which I said it wouldn't do. I guess the (1,1) is enough to jump start the chain again. DefRange returns a range object, (1,1) returns a range object, and the Value (default) property is returned.
Interesting question. I wonder if the default property feature was built this way intentionally or it's just the way it worked out.
This question already has answers here:
What is a NullReferenceException, and how do I fix it?
(27 answers)
Closed 2 years ago.
I have been getting an error in VB .Net
object reference not set to an instance of object.
Can you tell me what are the causes of this error ?
The object has not been initialized before use.
At the top of your code file type:
Option Strict On
Option Explicit On
Let's deconstruct the error message.
"object reference" means a variable you used in your code which referenced an object. The object variable could have been declared by you the or it you might just be using a variable declared inside another object.
"instance of object" Means that the object is blank (or in VB speak, "Nothing"). When you are dealing with object variables, you have to create an instance of that object before referencing it.
"not set to an " means that you tried to access an object, but there was nothing inside of it for the computer to access.
If you create a variable like
Dim aPerson as PersonClass
All you have done was tell the compiler that aPerson will represent a person, but not what person.
You can create a blank copy of the object by using the "New" keyword. For example
Dim aPerson as New PersonClass
If you want to be able to test to see if the object is "nothing" by
If aPerson Is Nothing Then
aPerson = New PersonClass
End If
Hope that helps!
sef,
If the problem is with Database return results, I presume it is in this scenario:
dsData = getSQLData(conn,sql, blah,blah....)
dt = dsData.Tables(0) 'Perhaps the obj ref not set is occurring here
To fix that:
dsData = getSQLData(conn,sql, blah,blah....)
If dsData.Tables.Count = 0 Then Exit Sub
dt = dsData.Tables(0) 'Perhaps the obj ref not set is occurring here
edit: added code formatting tags ...
In general, under the .NET runtime, such a thing happens whenever a variable that's unassigned or assigned the value Nothing (in VB.Net, null in C#) is dereferenced.
Option Strict On and Option Explicit On will help detect instances where this may occur, but it's possible to get a null/Nothing from another function call:
Dim someString As String = someFunctionReturningString();
If ( someString Is Nothing ) Then
Sysm.Console.WriteLine(someString.Length); // will throw the NullReferenceException
End If
and the NullReferenceException is the source of the "object reference not set to an instance of an object".
And if you think it's occuring when no data is returned from a database query then maybe you should test the result before doing an operation on it?
Dim result As String = SqlCommand.ExecuteScalar() 'just for scope'
If result Is Nothing OrElse IsDBNull(result) Then
'no result!'
End If
You can put a logging mechanism in your application so you can isolate the cause of the error. An Exception object has the StackTrace property which is a string that describes the contents of the call stack, with the most recent method call appearing first. By looking at it, you'll have more details on what might be causing the exception.
When working with databases, you can get this error when you try to get a value form a field or row which doesn't exist. i.e. if you're using datasets and you use:
Dim objDt as DataTable = objDs.Tables("tablename")
you get the object "reference not set to an instance of object" if tablename doesn't exists in the Dataset. The same for rows or fields in the datasets.
Well, Error is explaining itself. Since You haven't provided any code sample, we can only say somewhere in your code, you are using a Null object for some task. I got same Error for below code sample.
Dim cmd As IDbCommand
cmd.Parameters.Clear()
As You can see I am going to Clear a Null Object. For that, I'm getting Error
"object reference not set to an instance of an object"
Check your code for such code in your code. Since you haven't given code example we can't highlight the code :)
In case you have a class property , and multiple constructors, you must initialize the property in all constructors.