I'm using SharpShell.dll v2.7.2 and I carefully followed the instructions here:

https://www.codeproject.com/Articles/512956/NET-Shell-Extensions-Shell-Context-Menus

I did change some very minor things such as class name and function name, but I left most functionality intact. It compiles fine and regasm was successful. But the new shell extension menu item doesn't appear in Windows10 or Windows11. Here is my final code:

using System;
using System.Text;
using System.Runtime.InteropServices;
using System.IO;
using System.Windows.Forms;
using SharpShell.Attributes;
using SharpShell.SharpContextMenu;


namespace AcmeFileSender
{
   [ComVisible(true)]
   [COMServerAssociation(AssociationType.ClassOfExtension, ".*")]
   public class UploadFileExtension : SharpContextMenu
   {
      protected override bool CanShowMenu()
      {
         //  We always show the menu
         return true;
      }

      protected override ContextMenuStrip CreateMenu()
      {
         //  Create the menu strip
         var menu = new ContextMenuStrip();

         //  Create an 'Upload file' item
         var itemCountLines = new ToolStripMenuItem
         {
            Text = "Upload file"
         };

         //  When we click, we'll count the lines
         itemCountLines.Click += (sender, args) => UploadFiles();

         //  Add the item to the context menu.
         menu.Items.Add(itemCountLines);

         //  Return the menu
         return menu;
      }

      private void UploadFiles()
      {
         //  Builder for the output
         var builder = new StringBuilder();

         //  Go through each file.
         foreach(var filePath in SelectedItemPaths)
         {
            //  Count the lines
            builder.AppendLine(string.Format("{0} - {1} Lines", Path.GetFileName(filePath), File.ReadAllLines(filePath).Length));
         }

         //  Show the output
         MessageBox.Show(builder.ToString());
      }
   }
}

A few notes about the code above:

1. I used .* instead of .txt, because the eventual purpose of this shell extension will be to upload files to my server with minimal mouse clicks, and I want it to apply to ALL files, not just .txt files.
2. I removed the image reference beneath Text = "Upload file" because it wouldn't compile. The image icon didn't seem important so I removed it.

As you can see, most of the code is the same as the sample. In Win11, I'm using the classic/legacy context menu and it still doesn't work.

I have tried clearing all registry entries related to this shell extension twice, and re-ran regasm twice (always succeeds), with no luck in see the new shell menu item. Is a reboot required?

Here is my DLL loaded in Server Manager:

I see 2 potential problems:

1. 64-bit servers shows not installed. Why? Is this a problem?
2. Both 32-bit and 64-bit servers show as not registered. Definitely seems like a problem (and maybe THE problem), but I ran regasm against my DLL and received the message "Types registered successfully".

What am I doing wrong? How can I get Upload file to appear in the context menu in Win10/Win11 when I right-click any file inside windows explorer?