Aspect-Oriented Programming on .NET Framework

Understand what AOP is and how it is implemented on .NET. This article is PostSharp-centric, but not limited to PostSharp. We always try to refer to other existing AOP tools when relevant.

What is Aspect-Oriented Programming (AOP)?

Aspect-Oriented Programming (AOP), also named Aspect-Oriented Software Development (AOSD), is an approach to software development that goes further in the direction of separation of concerns. Separation of concerns is one of the most important rules in software development. It states that the same concern should be solved in a single unit of code. This is also called modularization . In procedural programming, the unit of code is the procedure (or function, or method). In object-oriented programming , the unit of code is the class.

Some concerns cannot be implemented successfully using a pure procedural or object-oriented programming. An example of this is code security. If you want to secure objects and methods, you have to modify the code of each method. That's why security is called a crosscutting concern, because it crosscuts the unit of modularization of the programming paradigm, in this case the class.

An aspect is a concern that crosscuts many classes and/or methods.

So AOP is a technique that allows to address issues that crosscut classes. AOP is frequently used to implement caching, tracing, security or failure injections.


Say we have a large set of business objects and some of their methods need to be secured. The current user is required to be in some roles in order to be given the permission to execute the method. Using the AOP programming model, you could, instead of modifying each method, develop an aspect and 'apply' it on methods of interest. In PostSharp Framework, the aspect could look like this:

public sealed class RequiresRoleAttribute : OnMethodBoundaryAspect
    string[] roles;
    public string[] Roles
        get { return this.roles; }
        set { this.roles = value; }

    public override void OnEntry(MethodExecutionEventArgs e)

This defines a custom attribute that can be applied to any method. You can use wildcards to apply the aspect to a set of objects and methods:

[assembly: RequiresRole(Roles = new string[] { "Delete" }, TargetMethods = "Delete*")]

There are of course a lot of AOP techniques to achieve the same result. The illustration above is only one of them.

Major AOP techniques in .NET

There are basically two approaches to change the behavior of a program in .NET (i.e. to weave a .NET program):

  • Compile-Time Weaving : the program is modified during the build process on the development machine, before deployment.
  • Runtime Weaving : the program is modified during its execution, after deployment.

Expressing the semantics

We know what we want: apply additional behaviors to existing code. Now we need to know how to express it. We have to express the following facts:

  • What behavior we want to add (advices),
  • Where we want to add it (specification of join points using pointcuts).

The following approaches are available when the .NET Framework is targeted:

  • Extending the Language : Create a new .NET language with new constructs.
  • Companion Files : Use external files, for instance XML files.
  • Custom Attributes : Use 'normal' .NET custom attributes to annotate the code.
  • Domain-Specific Languages: Use of a graphical designer to express the semantics, as well as IDE integration to simplify build and debugging experiences
  • Programmatic Tipping : All previous approaches were declarative. Another solution is to have normal .NET imperative code to add advices to join points.

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