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vb.net how to instantiate a class based on a variable name at runtime

Posted on 2011-02-25
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Last Modified: 2012-05-11
I'm sure this is easy, but my search is coming up short.   I want to be able to instantiate an object (in this case a Prompt Screen), but the Class name is variable based on what the user is doing.  

This is the statement:
    myPromptScreen = New [VariableClassName]

Thanks in advance.
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Question by:jackiemeck
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6 Comments
 
LVL 86

Expert Comment

by:Mike Tomlinson
ID: 34984591
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LVL 9

Expert Comment

by:rawinnlnx9
ID: 34984603
Look at the 5th option down. Regardless though one of these will work for you. It's the right way:

http://msdn.microsoft.com/en-us/library/system.activator.createinstance.aspx
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LVL 41

Expert Comment

by:graye
ID: 34989711
Here is a small chunk of code that I've used to allow a program to dynamically load a "plug-in" at run time.  It uses the techniques as described above

Imports System.IO
Imports System.Reflection

Public Class Form1

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim DLL_File As String

        ' fill the combobox with the list of available plug-in DLLs
        For Each DLL_File In Directory.GetFiles(Application.StartupPath & "\" & My.Settings.PluginDirectory, "Plug*.dll")
            ComboBox1.Items.Add(DLL_File)
        Next

        ' make a default item
        If ComboBox1.Items.Count > 0 Then
            ComboBox1.Text = ComboBox1.Items(0).ToString
        End If
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim Assmbly As Reflection.Assembly
        Dim PlugInType As Type
        Dim pi As PluginBase

        ' load the DLL file using the "signature" of the base class
        Assmbly = Reflection.Assembly.LoadFrom(ComboBox1.Text)

        PlugInType = Assmbly.GetType(Assmbly.GetName.Name & "." & Assmbly.GetName.Name, True, True)
        pi = CType(Activator.CreateInstance(PlugInType), PluginBase)

        ' execute a method on the plug in
        pi.DoSomething()
    End Sub
End Class

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Author Comment

by:jackiemeck
ID: 35826224
I have searched numerous resources and have found a lack of detailed examples along the lines of what I'm trying to do.  I know I need "CreateInstance", but I don't know how to format it.  I'll restate by issue with hopefully a little better detail:

I have 15 Forms that each include the same methods.  I don't know which of the Forms the user will decide to activate until runtime.  Basically, there is a loop of 1 through 15, and if the Form is selected, we need to run the identical method inside it.

' The assembly is called "MyProgram", but I'm not sure where that goes.

FormName(1)   = "FormForReportA"
FormName(2)   = "FormForReportB"
FormName(...)   = (etc)
FormName(15)   = "FormForReportZ"

for x = 1 to 15
    myPromptScreen(x) = CreateInstance("NotSureWhatGoesHere", FormName(x))
next x

For x = 1 to 15
    if UserSelected(x) = true then
        myPromptScreen(x).InitializeData()
        myPromptScreen(x).CalculateData()
    end if
next x

Thanks in advance...
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LVL 86

Accepted Solution

by:
Mike Tomlinson earned 500 total points
ID: 35826833
Try something like this:

    Dim asm As System.Reflection.Assembly = System.Reflection.Assembly.GetExecutingAssembly
    ...
    for x = 1 to 15
        myPromptScreen(x) = asm.CreateInstance(asm.GetName.Name & "." & FormName(x))
    next x
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Author Closing Comment

by:jackiemeck
ID: 35833514
That's perfect!  It's exactly what I needed.  Thanks so much.
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