Issues with classes again

Kevin Davis

Hello again,

I have created a class, and two forms, and the "app" works like it should. However, when I try to compile to a .dll it throws a Error "Reference to a non-shared member requires an object reference". This is exactly what Vulpes helped me with with an earlier project. However, I just can't seem to get a firm grasp on using textboxes and other form methods and properties in another form. I have attached the file for your reference. Anf If someone could recommend a great way to learn(book, dvd, etc..) about how to use classes with multiple forms and their properties, I'd appreciate it. I am bound and determined to learn how a class can be used on or with multiple forms.

Thanks,

Kevin

3 Replies

Know the answer? Post it — somebody with the same question will find it here.

Vulpes15y ago
Hmm, it looks like you also have a form in the exe testing the dll which has a Name of "Form1" (even though its class is called frmMain) and this is causing a conflict with the one in the dll itself.

Whilst it is possible to work around this by iterating through the Application.OpenForms collection and testing the type of each form until you find one which is of type ContactTest.Form1, it's not worth the hassle if the other approach works OK.

So, we've learnt from experience that the Application.OpenForms approach is best avoided for Windows Forms class libraries (it's OK for exes where you shouldn't have naming conflicts) which is not something you're likely to find in a book :)
Kevin Davis15y ago
I entered in the code that you stated I could add into my AddContact sub, and it compiled into a .dll file perfectly. However, when I added that .dll file to a sample project to test it's functionallity, this is what happened:

However, the first way to do this that you mentioned, by using the Initilaize component methos, worked just fine. Thank you very much for your help and pointing me in the right direction.

Kevin
Vulpes15y ago
What it boils down to is that you need a reference to a Form if you want to do something to it from another Form or Class.

There are many ways to obtain such references but, if you only intend to create a single instance of a form, then an easy way is to create a shared field in your Form class which makes this reference available to all other Forms or Classes within your application.

You can do this here as follows:

' In the Form1 class add the highlighted lines

Public Class Form1

Friend Shared Instance As Form1

Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
Dim form As New frmAddContact
Form.ShowDialog()
End Sub

Public Sub New()

' This call is required by the designer.
InitializeComponent()
Instance = Me

' Add any initialization after the InitializeComponent() call.

End Sub
End Class

' in your contact class amend the line which is giving the error as follows

Public Sub AddContact()

Dim LVI As ListViewItem
LVI = Form1.Instance.ListView1.Items.Add(Name)
With LVI
.SubItems.Add(Email)
.SubItems.Add(Phone)
.SubItems.Add(Carrier)
End With

End Sub

Although I have a couple of 'heavy' books on Windows Forms, they don't discuss what to do when you have more than one form. In fact, even a 1500 page book can only skim the surface of what is a huge subject.

The best thing to do here is to learn by experience and you'll see this, or similar, questions asked many times on .NET forums with a range of possible things that you can do about it depending on the circumstances.

Another thing you could have to done here would have been to make no changes at all to Form1 but change the code in the AddContact() method as follows:

Public Sub AddContact()

Dim LVI As ListViewItem
Dim f1 As Form1 = CType(Application.OpenForms("Form1"), Form1)
LVI = f1.ListView1.Items.Add(Name)
With LVI
.SubItems.Add(Email)
.SubItems.Add(Phone)
.SubItems.Add(Carrier)
End With

End Sub

The Application.OpenForms collection contains references to all open forms and you can get a particular one using its name (normally the same as the class name). You need then to cast it to its actual type as the property returns a generic Form reference.