What is IExpando and where is it used?-Collection of common programming errors

Yeah, it is a bit out-of-place today. It is the managed declaration of the COM interface type. Also present, note ComTypes.IExpando and InteropServices.UCOMIExpando. Core interfaces for scripting runtimes to implement property bags. In particular for Javascript, check this thread.

Microsoft had high hopes for JScript, it was a primary language supported along-side C#, VB.NET and Managed C++. That didn’t work out.

Forgotten, but not quite dead. The ExpandoObject class is back in .NET 4.0