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

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 var

Answer 1

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

Answer 2

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