title: Import .TLB file gives "cannot open source file x.tlh"
body: <p>I'm updating a VS2010 C++ project to VS2019.</p>
  
  <p>The project is importing several tlb files, and for each of these, VS2019 is giving an error that it "cannot open source file" for the .tlh file.</p>
  
  <p>One of these is from a DLL that I've also upgraded (written in C#, built and registered OK in VS2019), and 2 others are 3rd party tlb files.</p>
  
  <p>Here's an example of the import statement (in my header file)...</p>
  
  <pre><code>#import "../ExcelInterop/bin/Debug/ExcelInterop.tlb" raw_interfaces_only, raw_native_types, no_namespace, named_guids
  </code></pre>
  
  <p>The relative path is correct from the header file location.</p>
  
  <p>How do I fix this error, and get VS2019 to correctly process my .tlb files?</p>