In Excel VBA how to persist key variables over a 'state loss' (without writing to a cell or a file)? - vba

Question

Excel VBA is a flexible development environment. It is pesudo-compiled. However, sometimes during development a "state loss" can occur. A "state loss" is when all variables are torn down. Indeed, VBA has an option "Notify before state loss" option for triage. It is unsurprising that one cannot Edit and Continue code in all cases.
However, sometimes state losses happen whilst running in production because opening some other workbook may cause trauma to your application session (trust me, it happens!)
I know one can persist data to a worksheet cell or even a file but this is inappropriate for trying to retain an instance of a class, especially if that is the anchor for a whole object graph.
So in the case where one insists on a memory held variable how do you persist state over a state loss?

Answer 1

One way to keep the data persistent during the lifetime of Excel is to store them on the default .Net domain attached to the instance:
Sub Usage()
Dim dict As Object
Set dict = GetPersistentDictionary()
End Sub
Public Function GetPersistentDictionary() As Object
' References:
' mscorlib.dll
' Common Language Runtime Execution Engine
Const name = "weak-data"
Static dict As Object
If dict Is Nothing Then
Dim host As New mscoree.CorRuntimeHost
Dim domain As mscorlib.AppDomain
host.Start
host.GetDefaultDomain domain
If IsObject(domain.GetData(name)) Then
Set dict = domain.GetData(name)
Else
Set dict = CreateObject("Scripting.Dictionary")
domain.SetData name, dict
End If
End If
Set GetPersistentDictionary = dict
End Function

Answer 2

Attempting answer of my own question ...
The solution is to have a simple container, I choose Scripting.Dictionary, compiled into a DLL and make accessible to VBA using COM. In the old days one could have used VB6.
These days, one can also use C++ but here I present a C# solution (uses COM interop).
using System.Runtime.InteropServices;
namespace VBAStateLossProofStorageLib
{
// Code curated by S Meaden from Microsoft documentation
// 1. C# Shared Class library
// 2. In AssemblyInfo.cs set ComVisible(true)
// 3. In Project Properties->Build check 'Register for Interop'
// 4. Add Com reference to Microsoft Scripting Runtime
public interface IVBAStateLossProofStorage
{
Scripting.Dictionary getGlobalDictionary();
}
[ClassInterface(ClassInterfaceType.None)]
[ComDefaultInterface(typeof(IVBAStateLossProofStorage))]
public class VBAStateLossProofStorage : IVBAStateLossProofStorage
{
public Scripting.Dictionary getGlobalDictionary()
{ return CVBAStateLossProofStorage.m_dictionary; }
}
// https://msdn.microsoft.com/en-gb/library/79b3xss3.aspx
// "a static class remains in memory for the lifetime of the application domain in which your program resides. "
[ComVisible(false)]
static class CVBAStateLossProofStorage
{
public static Scripting.Dictionary m_dictionary;
static CVBAStateLossProofStorage()
{
m_dictionary = new Scripting.Dictionary();
}
}
}
And here is some client VBA code to demonstrate. Requires a Tools->Reference to the type library (.tlb file) created alongside the Dll.
Option Explicit
Public gdicLossy As New Scripting.Dictionary
Public gdicPermanent As Scripting.Dictionary
Sub RunFirst()
Set gdicLossy = New Scripting.Dictionary
gdicLossy.add "Greeting", "Hello world!"
Dim o As VBAStateLossProofStorageLib.VBAStateLossProofStorage
Set o = New VBAStateLossProofStorageLib.VBAStateLossProofStorage
Set gdicPermanent = o.getGlobalDictionary
gdicPermanent.RemoveAll '* clears it down
gdicPermanent.add "Greeting", "Bonjour!"
End '* THIS PROVOKES A STATE LOSS - ALL VARIABLES ARE TORN DOWN - EVENT HANDLERS DISAPPEAR
End Sub
Sub RunSecond()
Debug.Assert gdicLossy.Count = 0 '* sadly we have lost "Hello world!" forever
Dim o As VBAStateLossProofStorageLib.VBAStateLossProofStorage
Set o = New VBAStateLossProofStorageLib.VBAStateLossProofStorage
Set gdicPermanent = o.getGlobalDictionary
Debug.Assert gdicPermanent.Count = 1 '* Happily we have retained "Bonjour!" as it was safe in its compiled Dll
Debug.Assert gdicPermanent.Item("Greeting") = "Bonjour!"
End Sub