C# outlook: Unable to cast COM object of type 'System._ComObject' to Interface type 'Microsoft.Office.Interop.Outlook.MailItem.'

Question

I'm new to c#.

I have been trying on my own to fix this issue since 1 week until i've got no other alternative than to ask for help here.

My goal is to create a console app which would look for Unread Mails on the secondary mailbox[mailboxB@gmail.com]/second outlook account and reply those mails based on the Subject.

The code works fine on my Machine .

Unfortunately when the same exe is deployed on other machine , the following error gets generated:

unable to cast com object of type 'microsoft.office.interop.outlook.mailitem'.This operation failed because the QueryInterface call on the COM component for the interface with IID'{00063034-0000-0000-C000-000000000046}' failed due to the following error: No such interface supported(Exception from HRESULT: 0x80004002 (E_NOINTERFACE))

Please note i've tried the following to resolve the issue but the issue still prevails:

1. Restarted Outlook as administrator.

   Change the compatibility of the app under properties-> compatibility to run as administrator

2. Checked the outlook version which is 16.0 and the same ref is being used for in the code.

Please find my code below:

            Outlook.Application oApp = new Outlook.Application();
            Outlook.NameSpace oNS = (Outlook.NameSpace)oApp.GetNamespace("MAPI");
            
            MAPIFolder theInbox = oNS.Folders["mailboxB@gmail.com"].Folders["Inbox"];
            
            Outlook.Items unreadItems = theInbox.Items.Restrict("[Unread]=true");
            string t = "Test Mail";
            try
            {
                foreach (Outlook.MailItem it in unreadItems)
                {
                    if (it.Subject == t)
                    {
                        Outlook.MailItem replyMail = it.Reply();
                        it.HTMLBody = DateTime.Now.ToString("HH:mm:ss");
                        
                        
                        replyMail.Send();
                        it.Unread=false;
                    }
                }
            }
            catch (Exception e)
            {
                Console.WriteLine(e.ToString());
            }

Any sort of help would be highly appreciated.

Answer 1

You can have items other than MailItem in your Inbox folder, eg ReportItem or MeetingItem .

Change your code to

foreach (object obj in unreadItems)
{
   if (obj is Outlook.MailItem it)
   {
      ...