"The object does not support this method" - Error when trying to update the subject of an email (MailItem.Subject) - outlook-addin

Question

I'm maintaining a C# Outlook add-in. It runs, since many years, in Outlook 2003. Today, a user reported an error I never saw before. For two mails (both from the same sender), she's getting this error message when she tries to process them:
The object does not support this method.
at Microsoft.Office.Interop.Outlook._MailItem.set_Subject(String Subject)
The exception is raised when the add-in tries to update the subject:
private static void ForceReconnectToExchangeServer(Outlook.MailItem mi)
{
mi.Subject = mi.Subject + ""; // <-- HERE
mi.Save();
}
I can see a second error message (a tooltip at the end of the line in Visual Studio):
errorCS0433: The type 'MailItem' exists in both
'Microsoft.Office.Tools.Outlook.Implementation, Version=10.0.0.0,
Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' and
'Microsoft.Office.Tools.Outlook, Version=10.0.0.0, Culture=neutral
(... can't see the rest of the line ....)
Any idea why this is happening? Many thanks!
Note 1: This add-in uses a recent version of Redemption.
Note 2: I'm getting the same error when I forward the message to myself (an process it), even if I delete everything in the message. If I change the mail format to Plain Text however, no error.
Note 3: this problem maybe related to this one, but nobody answered it.

Answer 1

Do you have both references in your project? Make sure there are no conflicts.
Also keep in mind that updating Subject will cause conversation specific properties to be be reset. Resetting the message class (IPM.Note.Dummy, then back to the original one) might be a better idea.

Answer 2

The Microsoft.Office.Tools.Outlook namespace contains a set of classes and interfaces that enable you to create form regions that customize Microsoft Office Outlook forms.
I'd suggest reviewing the code base and remove unused references from the project. Also make sure that the Outlook namespace points to the Microsoft.Office.Interop.Outlook one, for example:
private static void ForceReconnectToExchangeServer(Microsoft.Office.Interop.Outlook.MailItem mi)
{
mi.Subject = mi.Subject + ""; // <-- HERE
mi.Save();
}