Allowing third party code to access my code

I'm trying to Invoke third party code in my application, I have been able to do this with if the third party assembly is com registered with createobject the first sub will return a user control the second will run a function called Execute like so:

Code:

---

Public Sub CreateUserControl(ByVal ProjectName As String, ByVal ClassName As String)
        'Create a com visible user control
        Dim objUserControl As Object
        objUserControl = CreateObject(ProjectName & "." & ClassName)
        Dim user As New UserControl
        user = objUserControl
    End Sub

    Public Sub InvokeUserDefinedCode(ByVal ProjectName As String, ByVal ClassName As String)
        'Run user defined code - com visible
        Dim objUserDefinedInstance As Object
        objUserDefinedInstance = CreateObject(ProjectName & "." & ClassName)
        objUserDefinedInstance.Execute()
    End Sub

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So, the above will allow me to run third party code in my application, provided the assembly is com registered.
My question is, I would like some of the objects in my application (such as variables and thier values) to be visible for the third party code to access
is this called an API? if anyone could point me in the right direction links or examples or anything that will be most helpful

Thanks in advance
enex