This example demonstrates how to build aspects that automatically encrypt or decrypt parameter values. The only encryption algorithm implemented in this example is string reversal. This is not real encryption, of course! In practice, you would probably use a format-preserving encryption algorithm from a third party.

The FilterAttribute class is the abstract implementation of the aspect. It is not bound to any encryption implementation algorithm. To implement a specific algorithm, you need to derive a new class from FilterAttribute class. In this example, we just show a ReserseAttribute aspect.

In some situations, for instance in ASP.NET MVC controllers, you may need to decrypt or encrypt properties of a method parameter instead of the parameter itself (encrypting an object is impossible; you can only either serialize its members with a format-preserving algorithm, or the serialization of the object using binary encryption). For this scenario, we have the ApplyFiltersAttribute, also derived from FilterAttribute. This custom attribute causes the parameter value to be deeply decrypted or encrypted. You can apply any filter attribute (either [Reverse] or [ApplyFilters] in this example) to fields and properties of the parameter class.

What is being demonstrated?

The FilterAttribute class is more complex than it appears because it must support two cases: encryption of parameter values and deep encryption of object trees. Under the cover, the job is done by two separate aspects: FilterMethodArgumentsAspect and FilterTypePropertiesAspect. However, we don't want to ask developers to choose between the aspects. All we want to show to developers are front-end attributes derived from the FilterAttribute class. Therefore, we use FilterAttribute as an umbrella, hiding the implementation complexity. The FilterAttribute class whether it has to provide a FilterMethodArgumentsAspect or FilterTypePropertiesAspect aspect. This is done by implementing the IAspectProvider interface.

The FilterMethodArgumentsAspect class is quite simple and is based on a MethodInterceptionAspect. It is worth noting that we want to have a single instance of the FilterMethodArgumentsAspect class per method, even if several parameters are encrypted. The FilterAttribute has to provide the de-duplication. This is done by using the IAspectRepositoryService service from PostSharp, which returns which aspects have already been added to a declaration.

The FilterTypePropertiesAspect class is more complex. It introduces the IFilterable interface into the target type, an interface which has a single method named ApplyFilters. To have access to fields and properties at runtime without reflection, the aspect relies on an IAdviceProvider which provides a set of ImportLocationAdviceInstance advices: one for each encrypted property. At runtime, the bindings field of the aspect class is populated with a List<ILocationBinding>, which gives access to all properties.