Add-in with C#

To create a VBE add-in for the Visual Basic Editor you can use C#.
Create a [[COM Add-in]] that implements the IDTExtensibility2 interface.

Creating

Open Visual Studio 2022 as Administrator.
New Project, Visual C#, Windows Desktop, Class Library (.NET Framework).
Change the Name to "VBECOMAddin".
Change the Location to somewhere on your C drive.
Check the .NET Framework version is correct and press OK.
Rename the Class1.cs file to MyConnect.cs.
Add four references to this project, (Project > Add Reference).
(1) Assemblies, Framework: "System.Windows.Forms"
(2) COM, Type Libraries: "Microsoft Office 16.0 Object Library".
(3) COM: Type Libraries: "Microsoft Visual Basic for Applications Extensibility 5.3" (VBIDE)
(4) COM: Type Libraries: "Microsoft Add-in Designer" (Extensibility)
Remove the default class.
Add the following class underneath the interface in the MyConnect.cs file.

namespace VBECOMAddin 
{ 
    [System.Runtime.InteropServices.ProgId("VBECOMAddin.MyConnect")] 
    [System.Runtime.InteropServices.Guid("0B58C36E-DE4E-4C40-8ED2-960437BFB7B1")] 
    [System.Runtime.InteropServices.ClassInterface(System.Runtime.InteropServices.ClassInterfaceType.None)] 
    [System.Runtime.InteropServices.ComVisible(true)] 
    [System.Runtime.InteropServices.ComDefaultInterface(typeof(Extensibility.IDTExtensibility2))] 
    public class MyConnect : Extensibility.IDTExtensibility2 
    { 
        public void OnConnection( 
            object Application, 
            Extensibility.ext_ConnectMode ConnectMode, 
            object AddInInst, 
            ref System.Array custom) 
        { 
            System.Windows.Forms.MessageBox.Show("onConnection"); 
        } 
        public void OnDisconnection( 
            Extensibility.ext_DisconnectMode RemoveMode, 
            ref System.Array custom) 
        { } 
        public void OnAddInsUpdate(ref System.Array custom) 
        { } 
        public void OnStartupComplete(ref System.Array custom) 
        { } 
        public void OnBeginShutdown(ref System.Array custom) 
        { } 
    } 
} 

Make sure the Guid Id is replaced using (Tools > Create GUID).
Select Build > Build Solution).
At this point no registry entries are added to the registry.
Project Properties
Build, Platform Target x64
Open "AssemblyInfo.cs"

[assembly: ComVisible(true)] 

When the solution is built the RegAsm tool WILL NOT RUN automatically.
The RegAsm tool adds the necessary class and interface information to the registry.
Tools > Command Line > Developer Command Prompt

"C:\Windows\Microsoft.NET\Framework64\v4.0.30319\RegAsm.exe" /codebase /tlb VBEAddin.dll 

Registry Keys - COM Add-in

Microsoft Office COM add-ins are identified by reading from the registry.
You can view the registry by going to the Windows start menu, choosing Run, typing regedit and pressing OK.
There must be a key representing the COM Add-in under the Addins key:
64 Bit Office - HKEY_CURRENT_USER\Software\Microsoft\VBA\VBE\6.0\Addins64\

HKEY_CURRENT_USER\Software\Microsoft\VBA\VBE\6.0\Addins64\VBECOMAddin.MyConnect 
(String)(Default) - (value not set) 
(String) Description - VBACOMAddin 
(String) FriendlyName - VBACOMAddin 
(DWord) LoadBehavior - 3 

Description - A brief description of the add-in.
FriendlyName - A user friendly name that is displayed in the COM add-ins dialog box.
Load Behavior - The load behaviour of the add-in.

Visual Basic Editor

(Tools > Addin Manager)