[RESOLVED] [2005] Showing Forms Dynamically

[Hack]

I have a listbox that contains a bunch of form names (all forms exist within my project)

When I user clicks on a form name, I need to load it. This is a rewrite of one of my VB6 projects. In Vb6, I did

Private Sub List1_Click()
Dim fForm As Form
Set fForm = Forms.Add(List1.List(List1.ListIndex))
fForm.Show
End Sub

Which worked just fine, however, in .NET, as I'm sure you know, the keyword Forms doesn't exist. Moreover Windows.Forms does not have an Add method, so I don't know how to translate this properly.

How would I do that?

[Atheist]

At the moment I cant think of anything better than this:

Dim formName As String = "Form1"
        Dim assm As System.Reflection.Assembly = System.Reflection.Assembly.GetExecutingAssembly()
        Dim frm As Form = TryCast(assm.CreateInstance(String.Format("{0}.{1}", assm.GetName.Name, formName)), Form)
        If Not frm Is Nothing Then
            frm.Show()
        End If

[nmadd]

I'm sure Atheist's code is much better, but I don't even know what it's doing.

Here's the simpleton way I've done it in the past (this, of course, could become painful with many forms):

Private Sub ListBox1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
    Select Case Me.ListBox1.SelectedItem.ToString()
        Case "Form2"
            Dim f2 As New Form2()
            f2.Show()
        Case "Form3"
            Dim f3 As New Form3()
            f3.Show()
    End Select
End Sub

[Dnereb]

I'm afraid you need to use a dependency injector to search your app assembly for classes that implement a specific Interface (The same as System.Windows.Form uses would be nice)
And the have the correct ClassName (a String)

and create an object of this typeoff interface

To do so quickly you need to Find a dependency injector written in c# or VB.Net

Good luck.

BTW Atheist is pretty close with his solution

[Tom Sawyer]

You can use the Activator.CreateInstance to do it:

Dim t As Type = Type.GetType("Namespace.FormName")

Dim frm As Form = CType(Activator.CreateInstance(t), Form)
frm.Show()

[Hack]

As a test I created a new project with two forms (Form1 Form2), and tried this

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim t As Type = Type.GetType("Namespace.FormName")
        Dim frm As Form = CType(Activator.CreateInstance(t), Form2)
        frm.Show()
    End Sub

With Option Strick On (which I always have), I got no errors, but when I ran it an clicked the button, I got

Value cannot be null - points to Form2
Parameter name: type

Troubleshooting tips:
Use the "new" keyword to create an object instance

Check to determine if the object is null before calling the method

Get general help for this exception (I just LOVE this one! )

I tried New is a variety of different places and in each I was immediately told that it wasn't valid the context in which I was using it.

[Atheist]

The string passed into Type.GetType must be in the format:
AssemblyName.ClassName
So change "Namespace" to whatever your assemblyname is.

[Tom Sawyer]

Yes, by Namespace, I meant AssemblyName.

So if your project is called Project1 and your listbox contains Form1, Form2 and Form3 and you click on Form2, your code would have to be:

Dim t As Type = Type.GetType("Project1.Form2")
Dim frm As Form = CType(Activator.CreateInstance(t), Form)
frm.Show()

Note that the second parameter for the CType is Form and not Form2. Having Form2 in there is what may be throwing your exception.

[Nerd44]

Hi!

In former Times I did a little other approach:

I filled al List(Of Type) or an array in a way like this:

dim Src=new Type() { Gettype(Form1),Gettype(Form2),Gettype(Form3) }

Then I told my Listbox:

Listbox1.DisplayMember="Name"
Listbox1.DataSource=Src

On Listbox1_SelectionChanged() I used the Activator to create Instances of the selected Type in the Listbox1.

[Hack]

Dim t As Type = Type.GetType("Project1.Form2")
Dim frm As Form = CType(Activator.CreateInstance(t), Form)
frm.Show()

I tried this and get the exact same error message as presented in Post 6. This time it is pointing to the word Form in the Dim frm As Form line.

[Tom Sawyer]

Does the New method of your Form2 have any parameters? This wouldn't work if you need to add in parameters during the instantiation.

[Hack]

Nope....in my entire test project, this is every single line of code I have

Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim t As Type = Type.GetType("Project1.Form2")
        Dim frm As Form = CType(Activator.CreateInstance(t), Form)
        frm.Show()
    End Sub
End Class

[Atheist]

Are you entirely sure that your assembly is called Project1?
What happens if you try the code I posted?

[Hack]

Atheist: Just looking at your code tells me that it will work.

However, I would prefer something that didn't hardcode the form names as forms in this project may come and go. As it is, I have loaded all the form names in a DB table that I just read into my listbox. So, if I add a new form, or delete an existing form, the only thing that I want to have to do is go into my DB table and make the appropriate adjustment.

[Hack]

Are you entirely sure that your assembly is called Project1?

From years and years of looking a VB6 and before projects, I assumed it would be called Project1, and didn't even bother to check.

Actually, new, yet to be named .NET projects are called WindowsApplication1

This works just fine

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim t As Type = Type.GetType("WindowsApplication1.Form2")
        Dim frm As Form = CType(Activator.CreateInstance(t), Form)
        frm.Show()
End Sub

Another VB6 holdover that bites me in the butt yet again.

Thanks guys.

[stanav]

From years and years of looking a VB6 and before projects, I assumed it would be called Project1, and didn't even bother to check.

Actually, new, yet to be named .NET projects are called WindowsApplication1

This works just fine

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim t As Type = Type.GetType("WindowsApplication1.Form2")
        Dim frm As Form = CType(Activator.CreateInstance(t), Form)
        frm.Show()
End Sub

Another VB6 holdover that bites me in the butt yet again.

Thanks guys.

I know you've marked the thread as resolved, but there's something I think worth to mention:

1. Activator.CreateInstance is very slow to compare with creating an instance of a class directly by its contructor.
2. The actual object you create in that code is of type Form2, but it is refered to just as Form. So you cannot access to any members of Form2 should you need to.
3. By combining #1 & #2, I think it's still better to use Select Case, and instantiate the appropriate form class depending on the case. The code can be longer and doesn't look as "advanced", but it'll run much better.

[Hack]

3. By combining #1 & #2, I think it's still better to use Select Case, and instantiate the appropriate form class depending on the case. The code can be longer and doesn't look as "advanced", but it'll run much better.

But wouldn't that mean hard coding the forms names?

[stanav]

But wouldn't that mean hard coding the forms names?

Are you not hard coding the form name here?

Dim t As Type = Type.GetType("WindowsApplication1.Form2")

[Hack]

That was just a test. When I moved the code over to production it became

        Dim ExistingForm As Type = Type.GetType("PaymentTracker." & lstFormNames.Text)
        Dim frm As Form = CType(Activator.CreateInstance(ExistingForm), Form)
        frm.Show()

[Tom Sawyer]

There's also the My.Forms and My.Application.OpenForms collections that may be useful. I've never used either, so I'm not sure whether or not they'd be able to apply to what you're doing.

[Hack]

Thanks Tom, maybe I'll play around with those.

[Dnereb]

Something I would check for as well is a specific interface.
this can be important if your application grows and in a diffrent namespace is a class with the same name as one of your forms...

That would be a nasty bug to find
So Define a Interface DynamicForm ande Let these Forms implement it.
Im your Dependency injector check for that Interface.

A snippet that does this but with a "foreign" assembly:

 ''' <summary>
    ''' returns an array of classes that implements interfaces of type Interface
    ''' Null will be return if there was no class found in this assembly that implements the specified interface
    ''' </summary>
    Public Function InterfacedClassNames(Of ObjectInterface)() As String()
        Dim classNames() As String = Nothing
        For Each _class As Type In ClassTypes
            Dim currentInterface As Object = _class.GetInterface(GetType(ObjectInterface).Name)
            If (Not (currentInterface) Is Nothing) Then
                If (classNames Is Nothing) Then
                    _classNamesList = New List(Of String)
                    _classNamesList.Add(_class.FullName)
                    ReDim classNames(0)
                    classNames(0) = _class.FullName
                Else
                    ReDim Preserve classNames(classNames.Length)
                    classNames(classNames.Length - 1) = _class.FullName
                    _classNamesList.Add(_class.FullName)
                End If
            End If
        Next
        Return classNames
    End Function

    Public Function GetObjectInstance(Of ObjectInterface)()

        Dim objectInstance As ObjectInterface
        Dim foundClassNames() As String = InterfacedClassNames(Of ObjectInterface)()
        If (Not (foundClassNames Is Nothing)) Then
            If ((ClassName Is Nothing) _
                        OrElse (ClassName = String.Empty)) Then
                If (foundClassNames.Length = 1) Then
                    ' if there is only one class found in the assembly and there is no preconfigured classname 
                    objectInstance = CType(MyAssembly.CreateInstance(InterfacedClassNames(Of ObjectInterface)(0)), ObjectInterface)
                Else
                    Throw New Exception(("there are more classes implementing interface " _
                                    + (GetType(ObjectInterface).Name + " found in the assembly." & vbCrLf & " You should specify which class to use by using the classname attribute in t" & _
                                    "he configuration file." & vbCrLf)))
                End If
            ElseIf _classNamesList.Contains(ClassName) Then
                objectInstance = CType(MyAssembly.CreateInstance(ClassName), ObjectInterface)
            Else
                Throw New Exception(("Configured class name " _
                                + (ClassName + " not found in the assembly.")))
            End If
        Else
            Throw New Exception(("now class implementing " _
                            + (GetType(ObjectInterface).Name + (" found in assembly " + Dll))))
        End If
        Return objectInstance
    End Function