1. AOP in .NET Business Rules + Plumbing = Enterprise Software
PostSharp
AOP in .NET
Business Rules + Plumbing = Enterprise Software
David Ross W: E:

2. Application Concerns Core Crosscutting Non-functional Requirements
Model needs to provide capability for many different layers
At layer boundaries
Logging
Validation
Security
Data marshalling
Functional Requirements
Domain/Object Model
Business Logic

3. Crosscutting Concerns – Plumbing
Logging/Monitoring/Alerting
Type safety (Beyond value types int, string)
Validation/Enforcement
Security
Authorisation/Authentication
Data Binding
INotifyPropertyChanged
DependencyProperties
Transactions
Concurrency - Locking

4. Aspects - Reduce noise in source
Move plumbing out of source, but keep behaviour the same
Advice
The duplicated plumbing code we are removing
Typically under 30 lines of code
Behaviour that is injected at a join point
Join points
Places within the code where the Aspect is inserted
Examples
Entry/Exit of a method or property
Class’s Type Definition
In AOP

5. Aspects Lingo Point cut Locates Join Points to apply advice
Filter driven – Automatic injection
Find all Methods, of type Setter, in all classes where Namespace equals “Application.Entities”
Attribute driven – Manual injection

6. Aspects Lingo
Weaving
Process of injecting functionality back into a component
Can be performed by
Text post processor – Magic comments can replaced by code
Proxy container – Uses decorator pattern/hooks to allow code to be inserted
Binary manipulation - Modifying assemblies - by replacing and injecting IL code

7. Compile-time MSIL Injection process
Copyright © by Gael Fraiteur
If Postsharp modifies a type’s signature...
No Intellisense for a referenced projects that are modified
Intellisense for referenced assemblies that are modified

8. PostSharp Weaving Compile time Weaving - LGPL
Automatically if a component references PostSharp.Laos.dll – Default Install configures MSBuild
Manual application using an MSBuild task
Run time Weaving – GPL/Commercial License
Manual via PostSharp.Core.dll
Statically modify the assembly just before it is loaded into the domain

9. Example class
public class Demo { public Demo() ActivityRecorder.Record("In constructor"); } public void Foo() ActivityRecorder.Record("In Foo"); public void StaticFoo() ActivityRecorder.Record("In StaticFoo");

10. [Test] public void Verify_Aspect_Called_On_Method_Invocation() {
var d = new Demo ();
d.Foo();
d.StaticFoo();
ActivityRecorder.AssertActivityOccured("In constructor");
ActivityRecorder.AssertActivityOccured("Calling Void Foo()");
ActivityRecorder.AssertActivityOccured("In Foo");
ActivityRecorder.AssertActivityOccured("Calling Void StaticFoo()");
ActivityRecorder.AssertActivityOccured("In StaticFoo");
ActivityRecorder.AssertNoMoreActivities(); }

11. Method Invocation - Aspect
[Serializable]
public class ExampleOnMethodInvocationAspect : OnMethodInvocationAspect {
public override void OnInvocation(MethodInvocationEventArgs context) {
ActivityRecorder.Record(string.Format("Calling {0}", context.Delegate.Method.Replace(“~”, “”));
// Do I want to continue??
context.Proceed(); }

12. Code after Injection
private void ~Foo() { ActivityRecorder.Record("In Foo"); } [DebuggerNonUserCode, CompilerGenerated] public void Foo(){ Delegate delegateInstance = new ~PostSharp~Laos~Implementation.~delegate~0(this.~Foo); MethodInvocationEventArgs eventArgs = new MethodInvocationEventArgs(delegateInstance, null); ~PostSharp~Laos~Implementation.SkillsMatter.PostSharp.Aspects. ExampleOnMethodInvocationAspect~1.OnInvocation(eventArgs);

13. Filter based Point Cut
[assembly: ExampleOnMethodInvocationAspect (AttributeTargetAssemblies = "SkillsMatter.PostSharp", AttributeTargetTypes = "SkillsMatter.PostSharp.OnMethodInvocation.*")] Very simple to apply changes to all business objects in the solution with a single filter...

14. On Boundary Invocation - Aspect
[Serializable] public class ExampleOnMethodBoundaryAspect : OnMethodBoundaryAspect{ public override void OnEntry(MethodExecutionEventArgs eventArgs) { ActivityRecorder.Record(string.Format("Before {0}", eventArgs.Method)); } public override void OnExit(MethodExecutionEventArgs eventArgs) { ActivityRecorder.Record(string.Format("After {0}", eventArgs.Method)); public override void OnException(MethodExecutionEventArgs eventArgs) {

15. [Test] public void Verify_Aspect_Called_On_Method_Boundary() {
var e = new Demo();
e.Foo();
e.StaticFoo();
ActivityRecorder.AssertActivityOccured("Before Void .ctor()");
ActivityRecorder.AssertActivityOccured("In constructor");
ActivityRecorder.AssertActivityOccured("After Void .ctor()");
ActivityRecorder.AssertActivityOccured("Before Void Foo()");
ActivityRecorder.AssertActivityOccured("In Foo");
ActivityRecorder.AssertActivityOccured("After Void Foo()");
ActivityRecorder.AssertActivityOccured("Before Void StaticFoo()");
ActivityRecorder.AssertActivityOccured("In StaticFoo");
ActivityRecorder.AssertActivityOccured("After Void StaticFoo()");
ActivityRecorder.AssertNoMoreActivities(); }

16. Manual Point Cut
public class FoodQuestionaire { [RegularExpressionValidator("^([A-PR-UWYZ0-9][A-HK-Y0-9][AEHMNPRTVXY0-9]?[ABEHMNPRVWXY0-9]? {1,2}[0-9][ABD-HJLN-UW-Z]{2}|GIR 0AA)$")] public string Postcode { get; set; } [RangeValidator(0, 5)] public int? LikesDiary { get; set; } public int? LikesBeef { get; set; } public int? LikesFish { get; set; } }

17. public override void OnEntry (MethodExecutionEventArgs eventArgs) {
if (!eventArgs.Method.Name.StartsWith(“set_"))
return;
int value = (int)args[0];
if (value < LowerBoundary || value > UpperBoundary)
throw new ValidationException
("Value must be between " + LowerBoundary + " and “ + UpperBoundary);
base.OnEntry(eventArgs); }

18. [Test] public void RangeTest() { FoodQuestionaire q = new FoodQuestionaire(); q.LikesBeef = 5; Assert.AreEqual(5, q.LikesBeef); Assert.Throws<ValidationException>(() => q.LikesBeef = 100); Assert.Throws<ValidationException>(() => q.LikesBeef = -3); q.Postcode = "E14 0AN"; Assert.AreEqual("E14 0AN", q.Postcode); Assert.Throws<ValidationException>(() => q.Postcode = "Hello World"); }

19. On Method Boundary can be used for...
How long did a method take to execute?
Resource Management – Create resource before a method is called and release afterwards
Transactions – Commit/Rollback as required
Concurrency – Mutual Exclusion around code

20. Other Aspects Implement Method Aspect Composition Aspect
Replace a method’s content with the advice in the aspect
Useful for modifying 3rd party components
Composition Aspect
Allows an interface/state to be injected into a component
Used to simulate multiple inheritance
Examples include
Adding .NET Win Form data binding to a POCO
Adding Entity Framework interfaces to a POCO

21. Breaking the Build All NHibernate methods must be virtual – Davy Brion
public class RequireVirtualMethodsAndProperties : OnMethodBoundaryAspect   {
public override bool CompileTimeValidate(MethodBase method)   {   
          if (!method.IsVirtual) {
                string methodName = method.DeclaringType.FullName + “.”
+ method.Name;
                var message = new Message(SeverityType.Fatal, “MustBeVirtual”,
                    string.Format(“{0} must be virtual”, methodName), GetType().Name);
                MessageSource.MessageSink.Write(message);
                return false;
            }
            return true;
        } }

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