The document discusses using aspect-oriented programming (AOP) to add cross-cutting concerns like logging, validation, and retries to code. It provides examples of using AOP frameworks like PostSharp to add attributes that inject logging, validation, and retry code around method executions. While AOP can help avoid scattering such code throughout an application, it also introduces complexity and requires special tools or language features to be effective.

1. Yuriy Guts
Software Architect @ ELEKS

2. Clients
Application Services
Infrastructure Services
Desktop
Browser
Desktop
Application
Native
Mobile App
. . .
API
Business
Components
Reporting. . .ORM
RDBMS Cache
Search
Index
. . .Filesystem

3. Clients
Application Services
Infrastructure Services
Desktop
Browser
Desktop
Application
Native
Mobile App
. . .
API
Business
Components
Search. . .ORM
RDBMS Cache
Search
Index
. . .Filesystem
Cross-Cutting Concerns
Identity & Access Mgmt.
Audit
Logging
. . .
Import & Export

4. public class ReputationService : IReputationService
...
private readonly IUserDataService _userDataService;
public ReputationService(IUserDataService userDataService)
{
_userDataService = userDataService;
}
public int AddReputationForQuestion(Question question)
{
int pointsToAdd = question.UpVotes * 5 - question.DownVotes * 2;
int newReputation = _userDataService.AddReputationForUser(question.Author, pointsToAdd);
return newReputation;
}
public int AddReputationForAnswer(Answer answer)
{
int pointsToAdd = answer.UpVotes * 10 - answer.DownVotes * 3;
int newReputation = _userDataService.AddReputationForUser(answer.Author, pointsToAdd);
return newReputation;
}
...

5. public int AddReputationForQuestion(Question question)
{
int pointsToAdd = question.UpVotes * 5 - question.DownVotes * 2;
int newReputation = _userDataService.AddReputationForUser(question.Author, pointsToAdd);
return newReputation;
}

6. public int AddReputationForQuestion(Question question)
{
Console.WriteLine("{0:HH:mm:ss.fff}: AddReputationForQuestion started.", DateTime.Now);
int pointsToAdd = question.UpVotes * 5 - question.DownVotes * 2;
int newReputation = _userDataService.AddReputationForUser(question.Author, pointsToAdd);
Console.WriteLine("{0:HH:mm:ss.fff}: AddReputationForQuestion completed.", DateTime.Now);
return newReputation;
}

7. public int AddReputationForQuestion(Question question)
{
Console.WriteLine("{0:HH:mm:ss.fff}: AddReputationForQuestion started.", DateTime.Now);
if (question == null)
{
throw new ArgumentNullException("question");
}
if (question.Author == null)
{
throw new ArgumentNullException("question", "Author cannot be null.");
}
if (question.UpVotes < 0 || question.DownVotes < 0)
{
throw new ArgumentException("A question cannot have a negative number of votes");
}
int pointsToAdd = question.UpVotes * 5 - question.DownVotes * 2;
int newReputation = _userDataService.AddReputationForUser(question.Author, pointsToAdd);
Console.WriteLine("{0:HH:mm:ss.fff}: AddReputationForQuestion completed.", DateTime.Now);
return newReputation;
}

8. public int AddReputationForQuestion(Question question)
{
Console.WriteLine("{0:HH:mm:ss.fff}: AddReputationForQuestion started.", DateTime.Now);
if (question == null) throw new ArgumentNullException("question");
if (question.Author == null) throw new ArgumentNullException("question", "Author cannot be null.");
if (question.UpVotes < 0 || question.DownVotes < 0) throw new ArgumentException("A question cannot have a negative number of votes");
var attemptsLeft = 3;
try
{
while (attemptsLeft > 0)
{
try
{
int pointsToAdd = question.UpVotes * 5 - question.DownVotes * 2;
int newReputation = _userDataService.AddReputationForUser(question.Author, pointsToAdd);
Console.WriteLine("{0:HH:mm:ss.fff}: AddReputationForQuestion completed.", DateTime.Now);
return newReputation;
}
catch
{
attemptsLeft--;
if (attemptsLeft == 0)
{
throw;
}
}
}
}
catch (Exception ex)
{
Console.WriteLine("{0:HH:mm:ss.fff}: AddReputationForQuestion failed with exception: {1}.", DateTime.Now, ex);
}
throw new Exception();
}

9. public int AddReputationForQuestion(Question question)
{
QuestionValidator.ValidateQuestion(question);
return BoundaryLogging<int>.Run("AddReputationForQuestion", () =>
{
return MultipleAttemptExecutor<int>.Run(3, () =>
{
int pointsToAdd = question.UpVotes * 5 - question.DownVotes * 2;
int newReputation = _userDataService.AddReputationForUser(question.Author, pointsToAdd);
return newReputation;
});
});
}

10. [DomainValidation]
[BoundaryLogging]
[MultipleAttemptExecution(3)]
public int AddReputationForQuestion(Question question)
{
int pointsToAdd = question.UpVotes * 5 - question.DownVotes * 2;
int newReputation = _userDataService.AddReputationForUser(question.Author, pointsToAdd);
return newReputation;
}

12. Language Compiler
AOP Post-Compiler
Source Code
IL Code
IL Code with
generated aspects

13. IoC ContainerCalling Code
Dynamic Proxy
Original class
1
2
34
5

14. return this;
return F(this);

15. • PostSharp (IL weaving)
• Castle.DynamicProxy (call interception)
(just to name a few)

16. [Serializable]
public class BoundaryLogging : OnMethodBoundaryAspect
{
public override void OnEntry(MethodExecutionArgs args)
{
Console.WriteLine("{0:HH:mm:ss.fff}: {1} started.", DateTime.Now, args.Method.Name);
}
public override void OnExit(MethodExecutionArgs args)
{
Console.WriteLine("{0:HH:mm:ss.fff}: {1} completed.", DateTime.Now, args.Method.Name);
}
public override void OnException(MethodExecutionArgs args)
{
Console.WriteLine("{0:HH:mm:ss.fff}: {1} failed with exception: {2}.",
DateTime.Now, args.Method.Name, args.Exception);
}
}

17. [Serializable]
public class MultipleAttemptExecution : MethodInterceptionAspect
{
private readonly int _maxAttempts;
public MultipleAttemptExecution(int maxAttempts)
{
_maxAttempts = maxAttempts;
}
public override void OnInvoke(MethodInterceptionArgs args)
{
var attemptsLeft = _maxAttempts;
while (attemptsLeft > 0)
{
try
{
args.Proceed();
return;
}
catch
{
attemptsLeft--;
if (attemptsLeft == 0) throw;
}
}
throw new Exception("Maximum attempts reached.");
}
}

18. [DomainValidation]
[BoundaryLogging]
[MultipleAttemptExecution(3)]
public int AddReputationForQuestion(Question question)
{
int pointsToAdd = question.UpVotes * 5 - question.DownVotes * 2;
int newReputation = _userDataService.AddReputationForUser(question.Author, pointsToAdd);
return newReputation;
}

20. • [Authorize] attribute in ASP.NET MVC
• HttpModules in ASP.NET lifecycle
• Anything else?

21. • Logging & Audit
• Transactions
• Security
• Caching
• Exception Management
• Generating and validating code

22. 1. Introduces “magic”, can reduce understanding of the big picture (*).
2. Requires special tools or code decorations.
3. Even mature frameworks have bugs.
(*) Greg Young:
www.infoq.com/presentations/8-lines-code-refactoring

23. yuriy.guts @ eleks.com
https://github.com/YuriyGuts/refactoring-with-aop