I have a BackgroundWorker that is used to carry a time consuming process while a form is shown. The form and the BackgroundWorker are in separate classes, and when the BackgroundWorker has finished what it has to do, I need to carry out some basic actions on the form.
However, the below does not work and produces the warning Reference to non-shared member requires an object reference.
Private Sub bw_RunWorkerCompleted(ByVal sender As Object, ByVal e As RunWorkerCompletedEventArgs)
mainForm.btnCancel.Enabled = False
mainForm.btnFinish.Enabled = True
End Sub
I researched the warning and it suggested that I had to ensure the object mainForm was declared, which for this scenario seems odd from the get go. Regardless, I changed my code to this, and the warning disappeared, but as suspected, it doesn't work. It seems that a new instance of the form would be referenced, which is not what I require.
Private Sub bw_RunWorkerCompleted(ByVal sender As Object, ByVal e As RunWorkerCompletedEventArgs)
Dim objForm As New mainForm
objForm.btnCancel.Enabled = False
objForm.btnFinish.Enabled = True
End Sub
Can someone please tell me how I can interact with a form from a BackgroundWorker? Thanks.
The problem is not that you need to declare a new mainForm object. The problem is that you need a reference to the right mainForm object. Since it is possible to create any number of mainForm objects, you need a reference to the particular mainForm object that you want to modify. Remember, mainForm is the class (a type of object). It is not, itself, an object.
The simplest way fix this would be to give a reference to the mainForm object to the class that is performing the work, like this:
Public Class MyBusiness
Public Property TheMainForm As mainForm
' ...
Private Sub bw_RunWorkerCompleted(ByVal sender As Object, ByVal e As RunWorkerCompletedEventArgs)
TheMainForm.btnCancel.Enabled = False
TheMainForm.btnFinish.Enabled = True
End Sub
End Class
Then, before starting the work, you need to make sure you set the TheMainForm property. For instance, something like this:
Dim business As New MyBusiness
business.TheMainForm = Me
business.DoWork()
Related
I understand its possible to open the "default instance" of a form in vb.net by calling NameOfTheForm.show()
How do I pass an object to the default form instance so i can work on the object and use it to populate the forms Text boxes?
I've tried to add a parameter to the frmNames New method but I'm not sure how then to open the form. The old style instantiate an object works:
Dim DetailsForm As New frmOrder(oOrder)
DetailsForm.Show()
But I'm used to using the :
frmOrder.show
syntax.
Should I use the top method or should I use the bottom method and have a public property on the form to accept the Object?
Have a missed a better method of doing this?
Thanks
Your calling method should use your first option
'Careful about declaring this in a sub, because when that sub ends, the form might get closed.
'It might be best to declare this as an instance var (aka form-level var)
Private DetailsForm As frmOrder
'this could go in an event handler, or anywhere
DetailsForm = New frmOrder(oOrder)
DetailsForm.Show()
You will need to add a constructor to your DetailsForm:
Private _oOrder as OrderType
Public Sub New(oOrder As OrderType)
'Best to save it to a private instance var and process it during Form_Load
_oOrder = oOrder
End Sub
Then when your Form_Load() runs, it can use your private instance var to fill your TextBoxes, like you want.
A second, but less eloquent approach would be to add a public property to the form and after you call .Show(), you can assign a value DetailsForm.OrderObject = oOrder, and then process the object that was passed-in.
The constructor approach is better because it can be "compiler checked"
If you really feel you must use default instance.
In Form1
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Form2.myObject = New Coffee("Breakfast Blend", 7)
Form2.Show()
End Sub
In Form2
Public myObject As Coffee
Private Sub Form2_Load(sender As Object, e As EventArgs) Handles MyBase.Load
TextBox1.Text = myObject.Name
TextBox2.Text = myObject.ID.ToString
End Sub
I want to fire an event in another class.
And my problem is I don't know how to make it.
I'm trying to use Inherits statement to my Form and add my class name to it, and it works as I hope:
Public Class Frm_Main_Copy
Inherits ToolStripMenuApp
'I have a ToolStripMenu that has declared before on my class and it sounds like this:
'Public Shared WithEvents Cat000x86_64App As ToolStripMenuItem
...
Private Sub IsClicked(ByVal sender As Object, ByVal e As EventArgs) Handles Cat000x86_64App.Click
End Sub
End Class
but the designer form messed up(returns a fatal error) and I should delete the Inherits statement, and the others.
Tried to act as form designer script(Trying to put this code to my class):
Friend WithEvents BlahBlah As RadioButton 'For example
It didn't worked,
Declaring a variable for my class and It didn't worked too
Searched on the Internet and it seems likely more complicated than I thought...
Anyone can help? Any help is appreciated.
A form is not a form unless it inherits, either directly or indirectly, the Form class. You cannot inherit any type that is not itself a form and expect your type to be a form. With that code, if ToolStripMenuApp is not a form then Frm_Main_Copy is not a form either, hence no form designer.
If what you're actually saying is that you have an instance of that ToolStripMenuApp that contains a TooStripMenuItem whose Click event you want to handle in Frm_Main_Copy then the first step is to not declare Cat000x86_64App as Shared. Frm_Main_Copy needs to declare a method capable of handling that event:
Private Sub IsClicked(ByVal sender As Object, ByVal e As EventArgs)
'...
End Sub
Note that there is no Handles clause because there's no WithEvents variable in that class whose event you are handling.
Next, Frm_Main_Copy must have access to the appropriate instance of ToolStripMenuApp. It's impossible for us to say how best to do that based on the information provided but it might be as simple as this:
Dim tsma As New ToolStripMenuApp
You then register your method as a handler for the appropriate event:
AddHandler tsma.Cat000x86_64App.Click, AddressOf IsClicked
If you use AddHandler, make sure to use RemoveHandler when you're done with either object. I suggest that you do some reading based on this information.
I have two classes.
Public Class MainForm
Private Project As clsProject
Private Sub btnDo_Click
...
Backgroundworker.RunWorkerAsync()
End Sub
Private Sub BackgroundWorker1_DoWork(ByVal sender As System.Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
Project = New clsProject
End Sub
and two methods inside MainForm
Public Shared Sub setLabelTxt(ByVal text As String, ByVal lbl As Label)
If lbl.InvokeRequired Then
lbl.Invoke(New setLabelTxtInvoker(AddressOf setLabelTxt), text, lbl)
Else
lbl.Text = text
End If
End Sub
Public Delegate Sub setLabelTxtInvoker(ByVal text As String, ByVal lbl As Label)
end class
I want to update the labels of MainForm from the clsProject constructor.
MainForm.setLabelTxt("Getting prsadasdasdasdasdry..", MainForm.lblProgress)
but it does not update them.
What am I doing wrong?
The problem is that you are using the global MainForm instance to access the label in a background thread here:
Public Class clsProject
Public Sub New()
' When accessing MainForm.Label1 on the next line, it causes an exception
MainForm.setLabelTxt("HERE!", MainForm.Label1)
End Sub
End Class
It's OK to call MainForm.setLabelTxt, since that is a shared method, so it's not going through the global instance to call it. But, when you access the Label1 property, that's utilizing VB.NET's trickery to access the global instance of the form. Using the form through that auto-global-instance variable (which always shares the same name as the type) is apparently not allowed in non-UI threads. When you do so, it throws an InvalidOperationException, with the following error message:
An error occurred creating the form. See Exception.InnerException for details. The error is: ActiveX control '8856f961-340a-11d0-a96b-00c04fd705a2' cannot be instantiated because the current thread is not in a single-threaded apartment.
I'm guessing that the reason you are not seeing the error is because you are catching the exception somewhere and you are simply ignoring it. If you stop using that global instance variable, the error goes away and it works. For instance, if you change the constructor to this:
Public Class clsProject
Public Sub New(f As MainForm)
' The next line works because it doesn't use the global MainForm instance variable
MainForm.setLabelTxt("HERE!", f.Label1)
End Sub
End Class
Then, in your MainForm, you would have to call it like this:
Private Sub BackgroundWorker1_DoWork(ByVal sender As System.Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
Project = New clsProject(Me) ' Must pass Me
End Sub
Using the global instance from the background thread is not allowed, but when we use the same label from the background thread, without going through that global variable it works.
So it's clear that you cannot use the global MainForm variable from a background thread, but what may not be clear is that it's a bad idea to use it ever. First, it's confusing because it shares the same name as the MainForm type. More importantly, though, it is a global variable, and global state of any kind is almost always bad practice, if it can be avoided.
While the above example does solve the problem, it's still a pretty poor way of doing it. A better option would be to pass the setLabelTxt method to the clsProject object or even better have the clsProject simply raise an event when the label needs to be changed. Then, the MainForm can simply listen for those events and handle them when they happen. Ultimately, that clsProject class is probably some sort of business class which shouldn't be doing any kind of UI work anyway.
You cannot execute any action on GUI-elements from the BackgroundWorker directly. One way to "overcome" that is by forcing the given actions to be performed from the main thread via Me.Invoke; but this is not the ideal proceeding. Additionally, your code mixes up main form and external class (+ shared/non-shared objects) what makes the whole structure not too solid.
A for-sure working solution is relying on the specific BGW methods for dealing with GUI elements; for example: ProgressChanged Event. Sample code:
Public Class MainForm
Private Project As clsProject
Public Shared bgw As System.ComponentModel.BackgroundWorker
Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
bgw = BackgroundWorker1 'Required as far as you want to called it from a Shared method
BackgroundWorker1.WorkerReportsProgress = True
BackgroundWorker1.RunWorkerAsync()
End Sub
Private Sub BackgroundWorker1_DoWork(sender As System.Object, e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
Project = New clsProject
End Sub
Public Shared Sub setLabelTxt(ByVal text As String)
bgw.ReportProgress(0, text) 'You can write any int as first argument as far as will not be used anyway
End Sub
Private Sub BackgroundWorker1_ProgressChanged(sender As Object, e As System.ComponentModel.ProgressChangedEventArgs) Handles BackgroundWorker1.ProgressChanged
Me.Label1.Text = e.UserState 'You can access the given GUI-element directly
Me.Label1.Update()
End Sub
End Class
Public Class clsProject
Public Sub New()
MainForm.setLabelTxt("Getting prsadasdasdasdasdry..")
End Sub
End Class
Try:
Me.Invoke(...)
instead of lbl.Invoke(.... I had to do this. This is my implementation:
Delegate Sub SetTextDelegate(ByVal args As String)
Private Sub SetTextBoxInfo(ByVal txt As String)
If txtInfo.InvokeRequired Then
Dim md As New SetTextDelegate(AddressOf SetTextBoxInfo)
Me.Invoke(md, txt)
Else
txtInfo.Text = txt
End If
End Sub
And this worked for me.
I need some help getting my head wrapped about instances and classes. In the code below I have a main_form, and I am also loading a user_control into the main_form. I have a property that resides inside main_form that I will be setting data called obj. My question is when I am doing work inside of the user_control, how can I reach in and set the obj property of main_form. I tried main_form.obj but I keep getting the error "object reference not set to an instance of an object". So, I'm not sure how to do it. Here's the dumbed down code
Public Class FormControlClass
Private _obj As New objCollection
Public Property obj As objCollection
Get
Return _obj
End Get
Set(ByVal value As objCollection)
_obj = value
End Set
End Property
'Load User Control Into Form from here.
me.controls.add('UserControl')
End Class
Public Class UserControlClass
'Access the obj property in the form control class from here.
FormControlClass.obj = 1
End Class
Even if you could do what you are trying to do, it would be a bad idea, because you would be coupling your user control to this particular form, making it useless outside of this form.
Instead you need to have your user control generate an event that the form can subscribe to and handle itself. In other words, you want to make the user control create a message that can be delivered to the form, like this:
Public Class UserControlClass
' Define event that will be raised by user control to anyone interested in handling the event
Public Event UC_Button1Click()
' Mechanism to allow event to be raised by user control
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
RaiseEvent UC_Button1Click()
End Sub
End Class
Now in your form class, you need to add a handler for the event raised by the user control, like this:
AddHandler userControl1.UC_Button1Click, AddressOf Button1_Click
Finally, you would create the method that is referenced in the AddressOf syntax, like this:
Public Sub Button1_Click(ByVal sender As Object, ByVal args As EventArgs)
' Do something here
End Sub
I have a mdicontainer form that summons forms. My problem is when the a user clicks again the menu for that form, it also make another instance of it.
What I did is declare a public class with a public variable on it ex: Boolean isFormOneOpen = false. Then every time formOne opens, it checks first the global variable I declared a while ago if it's false, if it is, instantiate an object of a formOne and then show it. Otherwise, do nothing. Very static, imagine if I have many forms, I have to declare a variable for each form to check if it's already open. Can you provide me a solution for this? Maybe a method that accepts a Form? Or any more clever way to do this.
You don't need a variable, you could iterate the MdiChildren collection to see if the form is already opened. For example:
Private Sub btnViewChild_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnViewChild.Click
For Each child In Me.MdiChildren
If TypeOf child Is Form2 Then
child.WindowState = FormWindowState.Normal
child.Focus()
Exit sub
End If
Next
Dim frm As New Form2
frm.MdiParent = Me
frm.Show()
End Sub
The VB.NET-centric solution:
Private Sub btnViewChild_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnViewChild.Click
Form2.MdiParent = Me
Form2.WindowState = FormWindowState.Normal
Form2.Show
End Sub
Rather than a boolean, declare a variable of the form's type. Then just make sure the variable isn't Nothing and call it's .Open() method. This has the nice side effect of also bringing your existing form instance to the front if it's already open.
Even better, in VB.Net 2.0 and later all forms have a default instance with the same name as their type, so you can just say FormName.Open() and be done with it. However, I haven't tried this in an mdi situation before.