Undocumented Office Ribbon Callback functionality

I just stumbled on an undocumented feature of the getImage callback for the Office 2010 Ribbon (well, backstage actually)

The “official” callback signature is

Public System.Drawing.Bitmap GetImage(Office.IRibbonControl control) {}

Or sometimes

Public stdole.IPictureDisp GetImage(Office.IRibbonControl control) {}

I don’t know which is “best”, but if you return a Bitmap it sure is easier to work with.

Now back in Office 2007 there are a couple controls with a GetImageMso callback that allows you to provide an Office Control ID for a built-in image. This is lacking from any 2010 backstage controls.

Whilst doing some unpleasant native debugging to try to “figure out” what office expects in these callback delegates, I tried creating a GetImage callback that returned a string.

<?xml version="1.0" encoding="utf-8"?>
<customUI xmlns="http://schemas.microsoft.com/office/2009/07/customui" onLoad="Ribbon_Load">
  <backstage>
    <tab id="MyTabShare" label="Testing">
      <firstColumn>
        <group id="MyGroup">
          <topItems>
            <imageControl id="imageControl1" getImage="GetImage"/>
          </topItems>
        </group>
      </firstColumn>
    </tab>
  </backstage>
</customUI>
        public string GetImage(Office.IRibbonControl control)
        {
            return "hello world";
        }

I was really hoping for an error about a bad callback signature, possible including an exception in MSO.DLL that I could dig into…

Instead, I got this:

Really… So when GetImage returns a string, Office interprets it as an ImageMso? I had to try:

        public string GetImage(Office.IRibbonControl control)
        {
            return "WindowKeepOnTop";
        }

The result: SUCCESS!

So, at least for the Office 2010 backstage ribbon, the getImage callback has an alternate signature:

string GetImage(IRibbonControl control)

Where string is an Office Image Id (or the value you would place statically in imageMso).

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