I have 20+ MDI forms with consistently named Public variables. When the child form closes a method on the MDI Parent is called passing Me as a generic form type. How can I access the public variables by name via the Form reference? I only need to read the variables. Of course the Variables() method does not exist...
Public Sub CleanupForm(ByVal frm As Form)
Dim sTable_Name As String = frm.Variables("TABLE_NAME") ' Public at form level
Dim cLock As clsRecLocks
cLock = frm.Variables("Rec_Lock")
cLock.DeleteThisLock()
'..
I've seen some posts on similar requests but most start out with "don't do it that way..." then go off in the weeds not answering the question. I concede it is poor design. I can't change all the calling forms in the short term so I need to use this approach.
VS2010, VB.Net, Win Forms, .Net 2.0
I was able to get to a simple variable using CallByName:
Try
Dim s As String = CallByName(frm, "TABLE_NAME", CallType.Get)
Stop
Catch ex As Exception
MsgBox(ex.Message)
End Try
On to the class object. Perhaps I can add a default Get for the class that returns the ID I need.
Default property won't work as the Locks object was not declared Public - at least for the CallByName() approach.
Can Reflection get to form level variables not declared Public? Seems like a security issue, but...
Can I get a "Parent" reference in the instantiated Locks class? i.e. A reference to the form that established the Locks object? I can change the clsRecLocks() class.
I found a property I could get to that told me the form was "read-only" and I can use that tidbit to delete the correct (or more correct - still not 100%) lock record. So the bug is 90% fixed. I think I need update all the forms with code that records the info I need to get to 100%.
Thanks to all!
Poor design but you can do this:
Public Sub CleanupForm(ByVal frm As Form)
Dim frmTest as object = frm
? = frmTest.TABLE_NAME
? = frmTest.Rec_Lock
End Sub
This will compile and if the variables exist, it will return them but if not, you get an error.
Converting to an interface after the fact is not that hard, you should do it now rather than later.
Related
I know this appears to be a rather common topic and should have been resolved from earlier posts. But what I am experiencing still does not seem to have a solution online:
I have a form called ExpenseEntry in which there is a sub procedure called Public Sub OpenVoucher.
I want to call this sub from another form for which I use the following code:
Dim ExpForm As New ExpenseEntry
ExpForm.Show()
ExpForm.OpenVoucher()
While this works well enough, the problem is everytime I click the button, a new window of ExpenseEntry is launched. As per how I have designed the application, repeat windows is not permissible and only one window should be available at a time.
I have tried various methods to restrict more than one form such as by using a variable to control the form but that gives rise to other issues.
If I use Application.OpenForms but still does not resolve the issue.
I have earlier queried in this regard in the following link:
Textbox not refreshing
I am using VB.NET 2019 which does not allow the launch of default instance of a form like Form.Show. I know this is bad practice but it was easier to manage with that till VB.NET 2017.
Now by creating a form variable and launching that creates an infinite loop where I cannot have just one instance of a form running on a single thread.
The really simple way to handle this is to use the default instance of the form type. In VB, since 2005, each form type has a default instance that you can access via the type name. Read here for more info. In your case, you can do this:
'Display the form if it is not already displayed.
ExpenseEntry.Show()
'Activate the form if it is already displayed.
ExpenseEntry.Activate()
'Do the deed.
ExpenseEntry.OpenVouncher()
That said, default instances are a bit dodgy. They do enable beginners to access forms from anywhere in their project under certain circumstances but they also have limitations that can cause issues. Most importantly though, they help to prevent you learning proper OOP by treating forms differently to other types. If you want to do this the way a proper developer would then simply declare a variable to refer to the current instance of the form:
Private expenseEntryDialogue As ExpenseEntry
When it's time to use the form, you simply check whether that variable refers to a usable instance and use it if it does, otherwise create a new one:
If expenseEntryDialogue Is Nothing OrElse expenseEntryDialogue.IsDisposed Then
expenseEntryDialogue = New ExpenseEntry
End If
expenseEntryDialogue.Show()
expenseEntryDialogue.Activate()
expenseEntryDialogue.OpenVoucher()
A third option would be to implement your own singleton, i.e. a type that can only ever have a single instance. You probably wouldn't do that in VB, given that the default instance is basically a thread-specific singleton and does more automatically but, if you wanted to, you could do this:
Public Class ExpenseEntry
Private Shared _instance As ExpenseEntry
'The one and only instance of the type.
Public Shared ReadOnly Property Instance As ExpenseEntry
Get
If _instance Is Nothing OrElse _instance.IsDisposed Then
_instance = New ExpenseEntry
End If
Return _instance
End Get
End Property
'The constructor is private to prevent external instantiation.
Private Sub New()
' This call is required by the designer.
InitializeComponent()
' Add any initialization after the InitializeComponent() call.
End Sub
End Class
and then this:
ExpenseEntry.Instance.Show()
ExpenseEntry.Instance.Activate()
ExpenseEntry.Instance.OpenVoucher()
I am updating some code from a vb6 application to VB.NET.
I have a problem that occurs when I try to open a form from the main form.
It calls this function:
Public Sub optDrawSafeFile_CheckedChanged(ByVal eventSender As System.Object, ByVal eventArgs As System.EventArgs) Handles optDrawSafeFile.CheckedChanged
If eventSender.Checked Then
'--------------------------------------------------------------------------------
' JRL 11-03-06
' change the enables
UpdateGUI((False))
cboProject.SelectedIndex = frmMain.cboProjects.SelectedIndex
SelectJob()
End If
End Sub
And when it goes to execute this line:
cboProject.SelectedIndex = frmMain.cboProjects.SelectedIndex
It blows up and says this:
frmMain is declared like this:
How can I fix this error?
TL;DR
It is described in more detail in this video.
Short answer: change frmMain to My.Forms.frmMain.
cboProject.SelectedIndex = My.Forms.frmMain.cboProjects.SelectedIndex
Long answer:
In VB6, referencing a form by its name allowed you to access it both as a class and an instance of that class. The instance that you access in this manner is called the default instance. This is not possible in VB.NET. However, VB.NET includes a dynamically generated class, My.Forms, that provides functionality similar to that of default instances.
See http://msdn.microsoft.com/en-us/library/ms379610%28v=vs.80%29.aspx#vbmy_topic3 for more information about My.Forms and the "My" namespace.
A better and more object-oriented way to handle this, however, would be to pass the instance of the main form to the constructor of the frmAddMethod form and store it in an instance field.
So, within the class definition in frmAddMethod.vb:
Sub New(ByVal mainForm As frmMain)
_mainForm = mainForm
End Sub
Private _mainForm as frmMain
And when you create the frmAddMethod instance from frmMain, pass in "Me" to the constructor:
Dim addMethodForm as new frmAddMethod(Me)
"Me" is the instance of the class from which a non-shared class method was called.
This will allow you to use the _mainForm class field to access the instance of the main form from non-shared methods of frmAddMethod.
*Edited to recommend My.Forms instead of DefInstance per Plutonix's comment.
Nothing is "blowing up", your program is not crashing. Using a type name, like frmMain, where an object reference is expected is something that the VB.NET compiler allows. Specifically for the Form class, an appcompat hack for VB6 code. It is the debugger that doesn't think much of it. It merely gives you a diagnostic on your watch expression since it doesn't have the same appcompat hack as the compiler does. So doesn't know what to display.
You can use My.Forms instead to get the active form object reference. So make your watch expression:
My.Forms.frmMain.cboProjects.SelectedIndex
Only do this when you are single-stepping the code, it will still go wrong if you use Debug + Break All to break into the program. Setting a watch expression on Me.cboProject is otherwise the obvious workaround in this specific case.
Below is a question that I will answer myself, however it caused a GREAT deal of frustration for me and I had a lot of trouble searching for it on the web, so I am posting here in hopes of saving some time & effort for others, and maybe for myself if I forget this in the future:
For VBA (in my case, MS Excel), the Public declaration is supposed to make the variable (or function) globally accessible by other functions or subroutines in that module, as well as in any other module.
Turns out this is not true, in the case of Forms, and I suspect also in Sheets, but I haven't verified the latter.
In short, the following will NOT create a public, accessible variable when created in a Form, and will therefore crash, saying that the bYesNo and dRate variables are undefined in mModule1:
(inside fMyForm)
Public bYesNo As Boolean`
Public dRate As Double
Private Sub SetVals()
bYesNo = Me.cbShouldIHaveADrink.value
dRate = CDec(Me.tbHowManyPerHour.value)
End Sub
(Presume the textbox & checkbox are defined in the form)
(inside mModule1)
Private Sub PrintVals()
Debug.Print CStr(bYesNo)
Debug.Print CStr(dRate)
End Sub
However, if you make the slight alteration below, it all will work fine:
(inside fMyForm)
Private Sub SetVals()
bYesNo = Me.cbShouldIHaveADrink.value
dRate = CDec(Me.tbHowManyPerHour.value)
End Sub
(Presume the textbox & checkbox are defined in the form)
(inside mModule1)
Public bYesNo As Boolean`
Public dRate As Double
Private Sub PrintVals()
Debug.Print CStr(bYesNo)
Debug.Print CStr(dRate)
End Sub
mModule1 will work perfectly fine and, assuming that the fMyForm is always called first, then by the time the PrintVals routine is run, the values from the textbox and checkbox in the form will properly be captured.
I honestly cannot possibly fathom what MS was thinking with this change, but the lack of consistency is a huge suck on efficiency, learning idiosyncracies like these, which are so poorly documented that a Google search in 2013 for something that has likely been around for a decade or more is so challenging to search.
First comment:
Userform and Sheet modules are Object modules: they don't behave the same way as a regular module. You can however refer to a variable in a userform in a similar way to how you'd refer to a class property. In your example referring to fMyForm.bYesNo would work fine. If you'd not declared bYesNo as Public it wouldn't be visible to code outside of the form, so when you make it Public it really is different from non-Public. – Tim Williams Apr 11 '13 at 21:39
is actually a correct answer...
As a quick add-on answer to the community answer, just for a heads-up:
When you instantiate your forms, you can use the form object itself, or you can create a new instance of the form object by using New and putting it in a variable. The latter method is cleaner IMO, since this makes the usage less singleton-ish.
However, when in your userform you Call Unload(Me), all public members will be wiped clean. So, if your code goes like this:
Dim oForm as frmWhatever
Set oForm = New frmWhatever
Call oForm.Show(vbModal)
If Not oForm.bCancelled Then ' <- poof - bCancelled is wiped clean at this point
The solution I use to prevent this, and it is a nice alternative solution for the OP as well, is to capture all IO with the form (i.e. all public members) into a separate class, and use an instance of that class to communicate with the form. So, e.g.
Dim oFormResult As CWhateverResult
Set oFormResult = New CWhateverResult
Dim oForm as frmWhatever
Set oForm = New frmWhatever
Call oForm.Initialize(oFormResult)
Call oForm.Show(vbModal)
If Not oFormResult.bCancelled Then ' <- safe
There are other limitations to Public within Excel VBA.
MSoft documentation in learn.microsoft.com states that public variables are global to the VBA project - it's not true.
Public variables are only global to the workbook within which they are declared, and then only across standard modules. Public variables declared within workbook code are not visible in standard modules, even though standard module sub's are - which are defined to be public.
Public variables declared in one workbook's standard modules are certainly not accessible from other workbooks in the same VBA project, contrary to the MSoft documentation.
I created two user controls with different user interfaces. Depending on a selection the user makes, one of these interfaces will be used in my class. Since I don't know until after the user makes a selection, I cannot declare the user control ahead of time so I created a public variable to later assign the correct user control to.
The error occurs when I try to access a control (textbox) on the user control. However, if I declare the user control without assigning it to the public variable, then I don't get an error. Also, if I were to assign the user control to the public variable as its being declared then I don't get an error either. I really do need to be able to pick between the two user controls though. I don't know what to do. Am I missing something? I appreciate any help.
Public Class VesselData
Public RCAVesselData
Public AOLVesselData
Public Sub New()
If Main.UserSelectedModule = "Arrival on Location" Then
OperatorView = New AOLVesselData 'User Control 1
ElseIf Main.UserSelectedModule = "Running Conventional Anchors" Then
OperatorView = New RCAVesselData 'User Control 2
End If
OperatorView.txtDistanceToFairlead.text = "A" 'THROWS MissingMemberException - Public member 'txtDistanceToFairlead' on type 'AOLVesselData' not found.
Dim Test as New AOLVesselData
Test.txtDistanceToFairlead.text = "A" 'DOES NOT THROW EXCEPTION
End Sub
The problem is that you are accessing a member that doesn't exist. From the code you have posted, of the AOLVesselData and RCAVesselData classes, it looks like the RCAVesselData class does not have a txtDistanceToFairlead member.
If you have a common set of methods/properties you expect both user controls to expose, refactor them into an Interface and have both user controls implement that Interface. That will make it easy to use them interchangeably.
Try using Shared instead of Public when you declare OperatorView. And, like tcarvin said, you might want to turn "Explicit" on (project compile options) or use "Option Explicit".
This is probably a simple one but I can't seem to figure it out.
I have a bunch of form items created by the form designer declared as (in frmAquRun.Designer.vb)
Public WithEvents btnAquRunEvent1 As VisibiltyButtonLib.VisibilityButton
Public WithEvents btnAquRunEvent2 As VisibiltyButtonLib.VisibilityButton
... etc
And I basically want to be able to supply a number to a function access each of these fields. So I wrote this function. (in frmAquRun.vb)
Const EVENT_BUTTON_PREFIX As String = "btnAquRunEvent"
Public Function getEventButton(ByVal id As Integer) As Windows.Forms.Button
Dim returnButton As Windows.Forms.Button = Nothing
Try
returnButton = DirectCast(Me.GetType().InvokeMember(eventButtonName, Reflection.BindingFlags.GetField Or Reflection.BindingFlags.Public Or Reflection.BindingFlags.Instance, Nothing, Me, Nothing), Windows.Forms.Button)
Catch ex As Exception
End Try
Return returnButton
End Function
But it always seems to be generating field not found exceptions.
The message in the exception is "Field 'ATSIS_ControlProgram.frmAquRun.btnAquRunEvent1' not found.".
The namespace and form name in the message are correct. Any idea what i'm doing wrong?
The problem is that for WithEvents fields, VB actually creates a property that does the necessary event handler attaching and detaching. The generated property has the name of the field. The actual backing field gets renamed to _ + original name.1)
So in order for your code to work just prefix the button name by _ or use the BindingFlag that corresponds to the property getter (instead of GetField).
Alternatively, you can do this a lot easier by using the Controls collection of the form:
returnButton = DirectCast(Me.Controls(eventButtonName), Windows.Forms.Button)
But beware that this only works if the button is top-level, i.e. not nested within a container control on the form.
1) This is an implementation detail of the VB compiler but it’s portable (especially to Mono’s vbnc compiler) since the handling for WithEvents fields is described in great detail in the VB language specifications.
The problem is that the event handlers aren't really fields. As compiled they're really properties that implement add_btnAquRunEventX, remove_btnAquRunEventX and fire_btnAquRunEventX methods. There are ways of using reflection to get around this, but that's probably not the best way to approach the problem. Instead you can simply create a List<> and populate it with the event handlers, then index into that list.
I'm a little rusty in VB syntax but it should look something like this:
Dim events = New List<EventHandler>()
events.Add( btnAquRunEvent1 )
events.Add( btnAquRunEvent2 )
....
events( 0 )( null, EventArgs.Empty )
Take a step back though and evaluate why you're invoking by index. There may be a simpler way of abstracting the whole thing that doesn't involve all this indirection.