Outlook Add-In VSTO - Hook into click of existing button

Question

I'm currently trying to develop an Outlook Add-In that extends an already existing function. To get it work, I need to hook into the On-Click-Event of an existing button (I have the IdMso of it), and execute additional code. Is that possible, and if so, how? I'm using C# as programming language.

Answer 1

Yes, it is possible to repurpose built-in controls. The commands element in the ribbon XML can do the trick, for example:

<customUI xmlns="http://schemas.microsoft.com/office/2006/01/customui" 
   onLoad="OnLoad" > 
   <commands> 
     <command idMso="FileSave" onAction="mySave" /> 
   </commands> 
   <ribbon startFromScratch="false"> 
     <tabs> 
       <tab id="tab1" label="Repurpose Command Demo" > 
         <group id="group1" label="Demo Group"> 
           <toggleButton id="togglebutton1"  
             imageMso="AcceptInvitation"  
             size="large"  
             label="Alter Built-ins"  
             onAction="changeRepurpose" /> 
         </group> 
       </tab> 
     </tabs> 
   </ribbon> 
</customUI>

This sample creates a reference to the save file functionality in Microsoft Office by using the command element. You can determine that this component includes a reference functionality built into Microsoft Office through its use of the idMso attribute.

For ribbon buttons the callback looks like that:

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)

And for the toggleButton control:

C#: void OnAction(IRibbonControl control, bool pressed, ref bool cancelDefault)

VBA: Sub OnAction(control As IRibbonControl, pressed As Boolean, byRef cancelDefault)

C++: HRESULT OnAction([in] IRibbonControl *pControl, [in] VARIANT_BOOL *pvarfPressed, [in,out] VARIANT _BOOL *fCancelDefault)

Visual Basic: Sub OnAction(control As IRibbonControl, pressed As Boolean, byRef CancelDefault)

See Temporarily Repurpose Commands on the Office Fluent Ribbon for more information.

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By default, if a VSTO Add-in attempts to manipulate the Microsoft Office user interface (UI) and fails, no error message is displayed. However, you can configure Microsoft Office applications to display messages for errors that relate to the UI. You can use these messages to help determine why a custom ribbon does not appear, or why a ribbon appears but no controls appear. See How to: Show Add-in user interface errors for more information.