How can I get a list of classes that have been instantiated

I'm using vb.net and I'm creating instances of classes at runtime using reflection:

Dim my_ass As Assembly = Assembly.GetExecutingAssembly
Dim my_class As New Object
my_class = my_ass.CreateInstance(my_class_name, True, BindingFlags.OptionalParamBinding, Nothing, New Object() {param_list}, Nothing, Nothing)

But I don't want to create multiple instances of the same class so I'm trying to check to see if I have already created one. I don't know where the class would exist except in the root namespace so I need a way to iterate through all of the created classes and find them like:

for each c as class in Main
 if c.name/type = my_class_name then
  c.run_my_function()
 else
  ' create a new instance
  my_class = my_ass.CreateInstance(my_class_name, True, BindingFlags.OptionalParamBinding, Nothing, New Object() {param_list}, Nothing, Nothing)
 end if
next

Of course my example is fictional but that is the gist of what I need.

Take this code:

Dim my_ass As Assembly = Assembly.GetExecutingAssembly
Dim my_class As New Object
my_class = my_ass.CreateInstance(my_class_name, True, 
BindingFlags.OptionalParamBinding, Nothing, New Object() {param_list}, 
Nothing, Nothing)

And change it like this:

Static classCache As New Dictionary(Of String, Object)
Dim my_class As Object 
If Not classCache.TryGetVale(my_class_name, my_class) Then
    Dim my_ass As Assembly = Assembly.GetExecutingAssembly()
    my_class = my_ass.CreateInstance(my_class_name, True, 
        BindingFlags.OptionalParamBinding, Nothing, New Object() {param_list}, 
        Nothing, Nothing)
    classCache(my_class_name) = my_class
End If

Static is a special feature that is (as far as I know) unique to VB.Net. It's different from the static feature in most other languages in that it lets you declare the variable so it's scoped to a method and also ensures thread-safety for access.

Finally, FWIW this pattern from the question always scares me a bit:

Dim my_class As New Object
my_class = ...

It's often a sign someone might not really know what's going on, because it tells the computer to allocate memory for a new object and then immediately throws it away without ever using it. That code can ALWAYS be safely simplified to improve efficiency like this:

Dim my_class As Object = ...

Or like this if you have some weird coding standard that won't let you declare and assign on the same line:

Dim my_class As Object
my_class = ...