I am developing a SolidWorks Add-in for SolidWorks 2026 x64 using:

• C#
• .NET Framework 4.8
• Visual Studio 2022
• Class Library project
• Platform: Release | x64

The add-in appears correctly in Tools → Add-ins, but when I try to activate it, the checkbox immediately unchecks itself and the add-in does not load.

What I verified

1. COM registration

   • The CLSID exists in:

   HKLM\\Software\\Classes\\CLSID\\{GUID}

   • InprocServer32 = mscoree.dll
   • RuntimeVersion = v4.0.30319
   • CodeBase points to the correct DLL (Release x64).

2. SolidWorks Addins registry

   HKLM\\SOFTWARE\\SolidWorks\\Addins\\{GUID}

   Exists with:

   • Title
   • Description
   • Default value = 1
3. COM instantiation works manually

From PowerShell (non-admin):

New-Object -ComObject "Automa.SwAddin.SwAddin"

This successfully creates the COM object.

4. Project settings

• Build configuration: Release
• Platform: x64
• Register for COM interop enabled.

Issue

When SolidWorks attempts to load the add-in:

• The checkbox immediately unchecks
• Constructor is not executed
• ConnectToSW() is not called
• No visible error message appears

Question

What could cause SolidWorks to fail loading a .NET COM Add-in that:

• is correctly registered
• can be instantiated manually via COM
• appears in the Add-ins list
• but immediately disables itself when activated?