I want to define some properties on a class using the [Indexable()] attribute in order to use the class with the i4o library (http://www.codeplex.com/i4o) but when I try and compile the code Visual Studio says it cannot find the Namespace for Indexable.
Is this part of the CLR and which namespace/library do I need to get this to compile?
Its not a part of the standard framework. You're going to need to identify what assembly contains this IndexableAttribute and reference it in your project.
Where is it located? I don't know. Who told you you needed to do this? Is it i4o? If so, its probably within one of the binaries that comes with the i4o project.
[Indexable()] isn't an attribute included in the .NET Framework. It was removed from i4o in favor of other approaches. See this blog post for details on how to use it.