Ok, bit of a weird combination. If you’re developing for Office 2010, you’re using VS2010, right? Uh huh…

Seriously, if you’re like a lot of folks, you might not be upgrading to VS2010 soon, so Microsoft can shake the bugs out in an SP1. But still, you have an Office VSTO addin in VS2008, that you’d like to be able to run in Office 2010, while debugging in the VS IDE.

If you’ve tried this without having Office 2007 installed ALSO along with the Office 2007 Primary Interops, you’ve most likely gotten a nasty message in the errors window telling you you “can’t compile this application because Office 2007 is not installed” or some such.

Well, fear not. With a little tweak to your addin’s project file, you should be good as gravy.

Here’s the untouched section from towards the bottom of a VSTO VBProj file. Notice the nasty line that starts with “VSTO_COMPATIBLEPRODUCTS”.

  <!-- This section defines VSTO properties that describe the host-changeable project properties. -->
  <ProjectExtensions>
    <VisualStudio>
      <FlavorProperties GUID="{BAA0C2D2-18E2-41B9-852F-F413020CAA33}">
        <ProjectProperties HostName="Word" HostPackage="{D2B20FF5-A6E5-47E1-90E8-463C6860CB05}" OfficeVersion="12.0" VstxVersion="3.0" ApplicationType="Word" Language="vb" TemplatesPath="" DebugInfoExeName="#Software\Microsoft\Office\12.0\Word\InstallRoot\Path#WINWORD.EXE" DebugInfoCommandLine="/w" AddItemTemplatesGuid="{2606E7C9-5071-4B63-9A83-C66A32B1669F}" />
        <Host Name="Word" IconIndex="0">
          <HostItem Name="MyAddin" Code="Connect.vb" CanonicalName="AddIn" CanActivate="false" IconIndex="1" Blueprint="Connect.Designer.xml" GeneratedCode="Connect.Designer.vb" />
        </Host>
        <ProjectClient>
          <VSTO_CompatibleProducts ErrorProduct="This project requires Microsoft Office Word 2007, but this application is not installed." ErrorPIA="This project references the primary interop assembly for Microsoft Office Word 2007, but this primary interop assembly is not installed.">
            <Product Code="{XX12XXXX-XXXX-XXXX-X000-X000000FF1CE}" Feature="WORDFiles" PIAFeature="WORD_PIA" />
          </VSTO_CompatibleProducts>
        </ProjectClient>
      </FlavorProperties>
    </VisualStudio>
  </ProjectExtensions>

The trick, it turns out, is to just comment that element out completely.

You’ll end up with this…

  <!-- This section defines VSTO properties that describe the host-changeable project properties. -->
  <ProjectExtensions>
    <VisualStudio>
      <FlavorProperties GUID="{BAA0C2D2-18E2-41B9-852F-F413020CAA33}">
        <ProjectProperties HostName="Word" HostPackage="{D2B20FF5-A6E5-47E1-90E8-463C6860CB05}" OfficeVersion="12.0" VstxVersion="3.0" ApplicationType="Word" Language="vb" TemplatesPath="" DebugInfoExeName="#Software\Microsoft\Office\12.0\Word\InstallRoot\Path#WINWORD.EXE" DebugInfoCommandLine="/w" AddItemTemplatesGuid="{2606E7C9-5071-4B63-9A83-C66A32B1669F}" />
        <Host Name="Word" IconIndex="0">
          <HostItem Name="MyAddin" Code="Connect.vb" CanonicalName="AddIn" CanActivate="false" IconIndex="1" Blueprint="Connect.Designer.xml" GeneratedCode="Connect.Designer.vb" />
        </Host>
        <ProjectClient>
          <!-- BEGIN COMMENTED OUT SECTION -->
          <!-- <VSTO_CompatibleProducts ErrorProduct="This project requires Microsoft Office Word 2007, but this application is not installed." ErrorPIA="This project references the primary interop assembly for Microsoft Office Word 2007, but this primary interop assembly is not installed.">
            <Product Code="{XX12XXXX-XXXX-XXXX-X000-X000000FF1CE}" Feature="WORDFiles" PIAFeature="WORD_PIA" />
          </VSTO_CompatibleProducts>
          END OF COMMENTING -->
        </ProjectClient>
      </FlavorProperties>
    </VisualStudio>
  </ProjectExtensions>

Happy VSTOing!