Is it possible to add attributes at runtime or to change the value of an attribute at runtime?
Attributes are static metadata. Assemblies, modules, types, members, parameters, and return values aren't first-class objects in C# (e.g., the
System.Type class is merely a reflected representation of a type). You can get an instance of an attribute for a type and change the properties if they're writable but that won't affect the attribute as it is applied to the type.
This really depends on what exactly you're trying to accomplish.
The System.ComponentModel.TypeDescriptor stuff can be used to add attributes to types, properties and object instances, and it has the limitation that you have to use it to retrieve those properties as well. If you're writing the code that consumes those attributes, and you can live within those limitations, then I'd definitely suggest it.
As far as I know, the PropertyGrid control and the visual studio design surface are the only things in the BCL that consume the TypeDescriptor stuff. In fact, that's how they do about half the things they really need to do.