Attachments.Add COMException HRESULT: 0x80004004 (E_ABORT)

Question

I overrode the Outlook attach file button by my ribbon command:
<commands>
<command idMso="AttachFile" onAction="onAttachFile" />
</commands>
In the ribbon class I implemented the prompt to use my own attach file logic or default. I didn't found how to call default dialog using. So I added my own dialog:
public void onAttachFile(Office.IRibbonControl control, bool pressed)
var item = control.Context as Outlook.Inspector;
var mailItem = item.CurrentItem as Outlook.MailItem;
MessageBoxResult messageBoxResult = MessageBox.Show("Do you wish to use cusom logic to send files?", "Prompt", MessageBoxButton.YesNo);
if (messageBoxResult == MessageBoxResult.No)
Microsoft.Win32.OpenFileDialog dlg = new Microsoft.Win32.OpenFileDialog();
dlg.ValidateNames = false;
dlg.CheckFileExists = false;
dlg.CheckPathExists = true;
dlg.Filter = "All files|*.*";
dlg.Multiselect = true;
Nullable<bool> result = dlg.ShowDialog();
if (result == true)
foreach (var fileName in dlg.FileNames)
mailItem.Attachments.Add(fileName,Outlook.OlAttachmentType.olByValue, 1, fileName);
Execuring this code I get for MailItem.Attachments.Add() line COMException HRESULT: 0x80004004 (E_ABORT).
Thanks in advance for any ideas!

Answer 1

Hello,
The repurposed callback should look like the following one:
C#: void OnAction(IRibbonControl control, ref bool CancelDefault)
VBA: Sub OnAction(control As IRibbonControl, byRef CancelDefault)
C++: HRESULT OnAction([in] IRibbonControl *pControl, [in,out] VARIANT _BOOL *fCancelDefault)
Visual Basic: Sub OnAction(control As IRibbonControl, byRef CancelDefault)
The CancelDefault parameter allows to suppress the default action, you just need to set it to true. If you need to keep the default action running then you'd need to keep set to false.
Also I'd suggest avoiding any message boxes in the repurposed callbacks. If you do so, you need to implement all the required functionality in your code without relying to the built-in actions. Any dialog windows shown in the repurposed callbacks prevent
default actions from running.  
You can read more about that in the Temporarily Repurpose Commands on the Office Fluent Ribbon article in MSDN.