Opening Word-document (.doc) using C# causes COMEx

I'm trying to open a really old (binary) Word-file using C#/.NET and Microsoft.Office.Interop.Word. Whenever I try to open this file using the snippet below, it fails with a COMException telling me that "You are attempting to open a file type that is blocked by your File Block settings in the Trust Center."

This also used to happen when opening the same file in MS Word, but after adjusting the settings found in the Trust Center this now works just fine. However, I still cannot open the file using C#. Does anyone know if VS2010 caches these settings, or use its own settings somewhere? I have also tried to create a new project (after fixing the settings in Word) with the same snippet to see if that helped, but it did not.

Word.Application app;

string file = "<filename>";

app = new Word.Application();

try
{
    app.Documents.Open(file);
}
catch (COMException e)
{
    string s = e.Message;
}

This is implemented with an alternate data stream, supported by the NTFS file system. You access such a stream with the filename:stream syntax. The stream name that stores the info for the file blocking feature is Zone.Identifier:$DATA. It is written by whatever program copied the file, usually a browser.

You can see them with the DIR /R option. The SysInternals' Streams utility permits listing and deleting them. A silly way to get rid of it is copying it to a file system that doesn't support alternate data streams, like a .zip archive or a flash drive and copying it back.

You can look at the content of the stream or edit it with a command like notepad filename:zone.identifier, you'll see this:

[ZoneTransfer]
ZoneId=3

The ZoneId value identifies the origin of the file. Values are -1=unspecified, 0=local machine, 1=intranet, 2=trusted, 3=internet, 4=untrusted. What notepad can do is however not possible in .NET, it explicitly forbids using the : character in file names. The intention is for a human to deal with this, explicitly overriding the file blocking feature. Right-click the file in Explorer, Properties and click the Unblock button. That deletes the stream. Note the Powershell cmdlets mentioned in the first link.