Building an ArcGIS Explorer add-in project in Visual Studio can result in a warning in the Visual Studio Error List window with the following message:
"Validation warning: The xmlns 'http://www.esri.com/schemas/ArcGISExplorer/1500/Addins' is missing from the ExplorerAddIns element. It is not required, but adding it will enable intellisense support in the addins.xml file."
An additional optional XML attribute has been added in ArcGIS Explorer 1500, which enables IntelliSense within Visual Studio for the AddIns.xml file. This attribute is not present in any add-in projects created by a previous version of the ArcGIS Explorer SDK for the Microsoft .NET Framework. This causes a warning if the add-in project is built on a machine on which ArcGIS Explorer 1500 or later is installed.
<?xml version="1.0" encoding="utf-8"?>