title: Excel reference to .NET DLL breaks with new version
body: <p>I have a .NET DLL which is installed to the GAC, as it is used by several applications.</p>
  
  <p>One of these is an Excel workbook which references the DLL using .NET's COM interop facility.  The COM tlb file is installed to an application directory.</p>
  
  <p>This all works fine except when an updated version of the DLL is installed.  On some PCs the COM reference within the workbook breaks (it's shown as "Missing").  Running the relevant macro crashes Excel.  This can be fixed by un-ticking the missing reference within the VBA editor and re-adding the reference to the DLL, but I'd rather my users didn't have to do this each time they open it.</p>
  
  <p>All the relevant objects and methods have unchanged signatures and are decorated with unchanging GUIDs.</p>
  
  <p>Any ideas why the reference breaks?</p>
  
  <p>EDIT:  After some investigation deep in the Registry, it looks like the COM registration isn't happening correctly on install.  This didn't show up as an install failure, however.  Still no closer to finding the underlying cause, though.</p>