One of the most interesting features of PostSharp that sets it apart from other AOP tools is its ability to apply aspects at compile-time. As I've explored in previous blog posts, this gives you the ability to do compile time checking and initialization, insteaad of costly and error prone runtime validation. For instance, one could use CompileTimeValidate to enforce that a given aspect can only be used on MVC Controller methods.
[Serializable]
public class ConstrainedAspect : OnMethodBoundaryAspect
{
public override bool CompileTimeValidate(System.Reflection.MethodBase method)
{
if(!method.DeclaringType.IsSubclassOf(typeof(Controller)))
{
Message.Write(MessageLocation.Of(method),
SeverityType.Error,
"987",
"Aspect can only be used on Controllers. " +
"You applied it on type {0}",
method.DeclaringType.Name);
return false;
}
return true;
}
public override void OnEntry(MethodExecutionArgs args)
{
var controller = (Controller) args.Instance;
controller.ViewData["aspect"] =
"Constrained Aspect was here at " + DateTime.Now;
}
}
This nifty feature has led some Postsharp users to create "aspects" that only contain compile time validation. An architect could put some code in here that helps to validate and enforce the architectural design: an "architectural unit test", if you will. With PostSharp 2.1, these "constraints" become a first-class feature.
There are two types of constraints available in PostSharp 2.1: scalar constraints and referential constraints. The separation is partially a semantic one, as both types of constraints are just ways of enforcing rules at compile time that the C# compiler itself doesn't give you. It's also a technical separation, as referential constraints are checked on all assemblies that reference the code element. (Note that you'll need to turn on architectural validation in the "PostSharp" tab of your project properties, and that this feature is available only in the professional version).
Scalar Constraints
A scalar constraint is a simple constraint that is meant to affect a single piece of code in isolation. This is the most like using a CompileTimeValidation method in an aspect, except without the aspect part. For instance, if you are a user of NHibernate, you know that your entity classes must have virtual properties. However, if you're like me, you might add a new property and forget to make it virtual. Then, you compile your project, run it, go through a test case, and get a runtime error. Wasted time! Here's a scalar constraint that you can apply to your entities to make sure you don't forget.
[Serializable]
[MulticastAttributeUsage(MulticastTargets.Class)]
public class NHEntityAttribute : MulticastAttribute, IScalarConstraint
{
public void ValidateCode(object target)
{
var targetType = (Type)target;
var properties = targetType.GetProperties(
BindingFlags.Public | BindingFlags.Instance);
var virtualProperties = properties.Where(p => !p.GetGetMethod().IsVirtual);
foreach (var propertyInfo in virtualProperties)
{
Message.Write(MessageLocation.Of(targetType),
SeverityType.Error,
"998",
"Property {0} in Entity class {1} is not virtual",
propertyInfo.Name, targetType.FullName);
}
}
public bool ValidateConstraint(object target)
{
return true;
}
}
Note that ValidateConstraint exists to validate the application of the constraint itself (a validation of a validation!). In my example above, I'm performing no validation at all and just returning true, but certainly you could check to make sure this validation is not applied to a static class, for instance. If ValidateConstraint method returns false, then the constraint is considered not valid, and will not be applied.
If you have all your entities in a single namespace, it's very easy to apply this constraint to all your entities (even ones that you haven't written yet) by multicasting that attribute. (For more info on multicasting, check out Dustin's excellent blog posts on multicasting: part 1 and part 2).
[assembly: NHEntity(AttributeTargetTypes = "YourNamespace.Models.Entities")]
When you forget the 'virtual' after you add a new property, you'll see a compilation error.
You could use constraints like this for similar situations, like WCF DataMembers in a DataContract or OperatingContracts in a ServiceContract. You can avoid a lot of frustration and wasted time.
Referential Constraints
Referential constraints are meant to enforce architectural design across assemblies, references, and relationships. This feature can be very useful, especially if you are writing an API. PostSharp actually ships with 3 out-of-the-box constraints for common scenarios: ComponentInternal, InternalImplements, and Internal.
ComponentInternal raises a compiler error if the code its applied to is used in a namespaces besides the one it resides in. For instance:
// NamespaceA
namespace PostsharpArchitecturalConstraints.API.NamespaceA
{
[ComponentInternal(Severity = SeverityType.Error)]
internal class ApiA
{
public string GetFriendsName()
{
return "Mr. Friendly";
}
}
}
// NamespaceB
using PostsharpArchitecturalConstraints.API.NamespaceA;
namespace PostsharpArchitecturalConstraints.API.NamespaceB
{
public class ApiB
{
public string GetFriendsName()
{
var a = new ApiA();
return a.GetFriendsName();
}
}
}
If you want to specify some exceptions (a specific namespace that you want the internal class to be used in), you can do that in the ComponentInternal's constructor, but by default, it only allows code within its own namespace (and child namespaces) to call it.
InternalImplements is for use on interfaces, and limits implementations of the interface to its own assembly. This means the interface can stay public, for instance, but nothing outside the assembly can implement it.
// in PostsharpArchitecturalConstraints.API assembly
namespace PostsharpArchitecturalConstraints.API.Interface
{
[InternalImplement(Severity = SeverityType.Error)]
public interface IPublicInterface
{
void DoOperation();
string GetValue();
}
}
// in PostsharpArchitecturalConstraints assembly
using PostsharpArchitecturalConstraints.API.Interface;
namespace PostsharpArchitecturalConstraints.Models.Services
{
public class MyPublicInterfaceImpl : IPublicInterface
{
private string _value;
public void DoOperation()
{
_value = "operation complete";
}
public string GetValue()
{
return _value;
}
}
}
Why would you want to do this? If you are designing an API, you may want the user to be able to reference an interface, and use your provided implementations of that interface, but you also may want to change your interface somewhere down the line. But, if you make changes to your interface, you could potentially break any implementations that the user has already made. By using InternalImplement, the user retains some flexibility in how they consume your API without the potential of their code breaking when they upgrade to your new version.
And finally, Internal. If you put Internal on a public item, it will remain public, but cannot be used by another assembly.
// in one assembly
[Internal]
public class PublicAndInternal
{
public string GetValue()
{
return "this can only be called in its own assembly";
}
}
// in another assembly
public class TryingToUseInternal
{
public string Execute()
{
var publicInternal = new PublicAndInternal();
return publicInternal.GetValue();
}
}
Of course, the door is wide open for you to write your own referential constraints. Here's one I wrote called Unsealable. It makes any classes inherited from the selected class unable to be sealed.
[MulticastAttributeUsage(MulticastTargets.Class)]
public class Unsealable : ReferentialConstraint
{
public override void ValidateCode(object target,
System.Runtime.InteropServices._Assembly assembly)
{
var targetType = (Type) target;
var sealedSubClasses = ReflectionSearch.GetDerivedTypes(targetType)
.Where(t => t.DerivedType.IsSealed)
.Select(t => t.DerivedType)
.ToList();
sealedSubClasses.ForEach(sealedSubClass => Message.Write(
MessageLocation.Of(sealedSubClass),
SeverityType.Error,
"997",
"Error on {0}: subclasses of {1} cannot be sealed.",
sealedSubClass.FullName, targetType.FullName));
}
}
This example also makes use of the handy ReflectionSearch utility that comes with PostSharp, which makes certain reflection tasks (like finding derived types in the above example) cleaner, easier, and Linq-ready. Here's an example of applying that constraint:
[Unsealable]
public class MyUnsealableClass
{
protected string _value;
public MyUnsealableClass()
{
_value = "I'm unsealable!";
}
public string GetValue()
{
return _value;
}
}
public sealed class TryingToSeal : MyUnsealableClass
{
public TryingToSeal()
{
_value = "I'm sealed!";
}
}
Constraints can be very valuable for larger teams, or for building an API that will be consumed by a larger audience (even an internal one). Don't go too crazy, though. Only build constraints that will help you, your team, and your API consumers save time and make less mistakes. By enforcing constraints on how code is to be used, you can protect yourself and your users from costly breaking changes down the road. It may be a politically sensitive issue, so good communication is still important, but it's very much like defensive programming, just at the architectual design level.