Outlook VSTO添加上下文菜单项在Outlook 2016中不起作用

Question

我正在尝试在Outlook中的上下文菜单项中添加按钮。我确实覆盖了CreateRibbonExtensibilityObject()中的ThisAddin.cs 并创建了如下的自定义 ContextMenuItem 。

ThisAddin.cs

protected override Office.IRibbonExtensibility CreateRibbonExtensibilityObject()
  {
     return new OutlookAddInExtensibility();
    }

OutlookAddInExtensibility 类可实现自定义功能区上下文菜单项。

[ComVisible(true)]
    internal class OutlookAddInExtensibility : IRibbonExtensibility
    {

        public string GetCustomUI(string RibbonID)
        {
            return
    @"<?xml version=""1.0"" encoding=""UTF-8""?>
<customUI xmlns=""http://schemas.microsoft.com/office/2009/07/customui"">
    <contextMenus>    
        <contextMenu idMso=""ContextMenuMailItem"">
            <button 
                id=""MyContextMenuMailItem""
                label=""ContextMenuMailItem""
                onAction=""RibbonMenuClick""
                getVisible=""GetVisible""/>
                />
        </contextMenu>  
    </contextMenus>
</customUI>
";
        }

        public bool GetVisible(Microsoft.Office.Core.IRibbonControl control)
        {
            return true;
        }

        public void RibbonMenuClick(IRibbonControl control)
        {
            var selection = control.Context as Microsoft.Office.Interop.Outlook.Selection;

            var mailItems = selection.OfType<Microsoft.Office.Interop.Outlook.MailItem>().ToList();

            System.Diagnostics.Trace.WriteLine($"RibbonMenuClick control: {control} type {control?.GetType().Name ?? "(null)"}");
        }
    }

它也删除了我现有的 Addin 图标。当我执行此操作时，右键单击 MailItem 时没有显示上下文菜单项。

第一次使用 Outlook插件时，在此方面苦苦挣扎，但却不明白我在做什么错？