If you have been doing much work in VSTO and especially around Excel and embeddings, you may have been bit by this bug. Does this error look familiar:

“The program used to create this object is Excel. That program is either not installed on your computer or it is not responding. To edit this object, install Excel or ensure that any dialog boxes in Excel are closed.”

This error can be caused by the following:

• A .NET3.5 Add-in or a .NET 4.0 Add-in
• You have attached to the WorkbookOpen and/or WorkbookActivate events.
• You are trying to edit/open/double-click on an embedded Excel instance inside a Word or PowerPoint document.

There have been several reports of this problem on the Microsoft MSDN site:

• Excel 2007 VSTO add-in causes error during edit of embedded Excel

There are also a number of KB articles that document the problem and attribute it to specific programs:

• You receive an error message in a program that interacts with …

The problem is specifically documented here:

• Excel OLE Embedding Errors if you have Managed Add-In Sinking Application Events in Excel

Simple Solution

The basic simple answer is to place a Marshal.ComReleaseObject(Wb) at the end (or better, in the Finally block) of your event handlers. This will properly allow Word and excel to handle the OLE communication by not having VSTO hang on to an instance handle of the workbook, therefore causing the error.

And this is not carte blanche to start placing ComReleaseObject() all over your code. I have found VERY VERY few limited cases where using ComReleaseObject() in an add-in necessary. And this is one of them.

Orphan Issue

It is not a panacea, either. While it resolves the issues of OLE initialization and allows you to edit your Excel embedding in Word (as one example), it does not prevent one other scenario that I like to call “Orphaned Excel.” In this scenario, you edit your embedded Excel instance in another Window (usually via a right-click / Ole Object / Open). If you leave Excel open, return to Word and close the document, Excel should close. But in the VSTO COM/OLE scenario it may not – it will remain open with the embedded workbook still editable. However, it is orphaned an no longer associated with its container. Any edits will be lost.

That is where a solution I created for Excel/Word OLE interaction comes in here:

• Excel Orphan Preventer

This add-in is very well commented and explains the following:

• When a workbook is opened, it looks to see if it is embedded.
• If it is, it connects to the running instance of Word, and gets a reference to the parent document.
• A timer in the add-in will then continually check the status of the document
• If the parent document is still opened, nothing happens. However, if Word is existed or the parent document is closed, the child embedding is forced closed.

IMPORTANT NOTE

However, and this is important note. For everything you do, you are in the sandbox with other kids. Your add-in is loading in the same AppDomain as everybody else. If there is another VSTO 3.5/4.0 Add-in loaded in Excel and that add-in is not doing any of the above – well, your still going to have problems.

That is what makes this issue so vexing. You can play by all the rules, but you cannot prevent other kids from throwing sand. This is why I see some customers going to the extreme to manage out (disable) all other COM add-ins when they load their solution. But this does not work for HKEY_LOCAL_MACHINE Loaded add-ins.

There are few options and most involve a lot of code. I say, TEST. If your are in an Enterprise environment, test all your VSTO add-ins together, find code owners and get everyone on the same page. If it is a vendor add-in causing the problem, point them here.