I had a look at the text of the first exception, and from that it was obvious:


Unable to cast COM object of type 'System.__ComObject' to interface type 'IWhatever'. This operation failed because the QueryInterface call on the COM component for the interface with IID '{CAD939D0-5E5D-11D7-AA0B-0002B33FE9DX}' failed due to the following error: Bad variable type. (Exception from HRESULT: 0x80020008 (DISP_E_BADVARTYPE)).


The message contains curly brackets around the IID, and they’re being intrepreted as insertion points by a call to String.Format!

Workaround

So that explained why it only happens in debugger runs; as this post helpfully points out the WPF databinding is enabled by default in that case. To workaround it, you can disable logging programmatically or using the App.config file to only display Critical output. So if your app is named foo.exe, create a file foo.exe.config that contains:

<configuration>

  <system.diagnostics>

    <sources>

      <source name="System.Windows.Data" switchName="SourceSwitch"/>

    </sources>

    <switches>

      <add name="SourceSwitch" value="Critical" />

    </switches>

  </system.diagnostics>

</configuration>

Fix…?

Hopefully there’ll be a fix for this at some point soon, as disabling the logging is a pretty large hammer to crack this small nut.

This can be a problem if you’ve got any tools set-up that try and do something with the built binary. Normally you can assume that referenced assemblies will also be in that directory, so you’d be able to load and execute your built file. If you’re pointing to a copy in the intermediate directory, that’s not the case.

It looks like it’s just an artifact of the way they’ve integrated the F# compiler (fsc.exe) with msbuild. The F# team are aware of this bug, so hopefully it’ll be fixed in the next drop.