PostSharp's Great Reset: Announcing Project "Caravela", a Roslyn-based aspect framework

by Gael Fraiteur on 25 Jan 2021

Today we’re excited to make a one-in-ten-years announcement: we’re releasing the first public preview of PostSharp “Caravela”, a Roslyn-based framework for code transformation and aspect-oriented programming.

We intend PostSharp “Caravela” to become the successor of the MSIL-based PostSharp Framework and PostSharp SDK.

PostSharp “Caravela” builds on 15 years of experience in code transformation and aspect-oriented programming, but has been designed from scratch for C# 9 and modern development pipelines. It has a radically different approach than PostSharp.

Today, we’re demonstrating two components of PostSharp “Caravela”:

  • Caravela.Framework is a high-level aspect framework comparable to PostSharp Framework or AspectJ. This component is in a very early preview and is not considered to be of any commercial use yet.
  • Caravela.Framework.Sdk is a low-level extensibility point to the Roslyn compiler, similar to source generators, but allowing for arbitrary code modifications (instead of just additions of partial classes). This component can be compared to PostSharp SDK or Fody, but using the clean Roslyn code model instead of the arcane MSIL one. This component is already very usable and useful today and is not expected to change much in the future.

In this blog post:

Caravela.Framework: a high-level aspect framework

Caravela.Framework is a code transformation and aspect-oriented programming based on templates written in pure C#.

These templates make it easy to write code that combines compile-time information (such as names and types of parameters of a method) and run-time information (such as parameter values) in a natural way, without having to learn another language or having to combine C# with some special templating language.

Instead of a thousand words, let’s look at this example:

Example: logging

▶ Try in your browser

Here is the aspect code. It represents the code transformation.

public class LogAttribute : OverrideMethodAspect
{
    public override object Template()
    {
        Console.WriteLine(target.Method.ToDisplayString() + " started.");

        try
        {
            dynamic result = proceed();

            Console.WriteLine(target.Method.ToDisplayString() + " succeeded.");
            return result;
        }
        catch (Exception e)
        {
            Console.WriteLine(target.Method.ToDisplayString() + " failed: " + e.Message);

            throw;
        }
    }
}

Let’s apply the [Log] aspect to the following method:

[Log]
static int Add(int a, int b)
{
    if ( a == 0 ) throw new ArgumentOutOfRangeException(nameof(a));
    return a + b;
}

The following method gets actually compiled instead of your source code:

[Log]
static int Add(int a, int b)
{
    Console.WriteLine("Program.Add(int, int) started.");
    try
    {
        int result;
        if (a == 0)
            throw new ArgumentOutOfRangeException(nameof(a));
        result = a + b;
        Console.WriteLine("Program.Add(int, int) succeeded.");
        return (int)result;
    }
    catch (Exception e)
    {
        Console.WriteLine("Program.Add(int, int) failed: " + e.Message);
        throw;
    }
}

With Caravela, you can see and debug the C# code being actually compiled. For details, see Debugging code with Caravela.

Caravela.Framework.Sdk: hack the compiler

At PostSharp we are not fans of hacking because it turns out to be a hassle to maintain in the long term (and our frameworks are designed to make your code more maintainable), but sometimes there may be good reasons to overcome the limitations of the language.

Caravela.Framework.Sdk offers direct access to Caravela’s underlying code-modifying capabilities through Roslyn-based APIs. Aspect weavers written with Caravela SDK can perform arbitrary transformations of the project and syntax trees being compiled.

Example: CancellationToken

▶ Try in your browser

The next example demonstrates an aspect that adds a CancellationToken to your methods declarations and method calls where it is missing.

Because the code of an SDK-based aspect weaver is naturally more complex and would not easily fit in a blog post, please go to GitHub if you want to see the source code of the aspect weaver.

Here is some example input code:

[AutoCancellationToken]
class C
{
    public static async Task MakeRequests()
    {
        var client = new HttpClient();
        await MakeRequest(client);
    }

    private static async Task MakeRequest(HttpClient client) 
        => await client.GetAsync("https://httpbin.org/delay/1");
}

What actually compiles is this. You can see that the aspect added CancellationToken parameters and arguments as needed.

[AutoCancellationToken]
class C
{
    public static async Task MakeRequests(
      System.Threading.CancellationToken cancellationToken = default)
    {
        var client = new HttpClient();
        await MakeRequest(client, cancellationToken);
    }
 
    private static async Task MakeRequest(HttpClient client,
        System.Threading.CancellationToken cancellationToken = default) 
        => await client.GetAsync("https://httpbin.org/delay/1", cancellationToken);
}

Benefits of PostSharp “Caravela” over PostSharp MSIL

PostSharp “Caravela” was designed from scratch. It is based on best lessons learned from PostSharp MSIL during the last 15 years, and addresses the main obstacles that are now hindering PostSharp MSIL.

You will enjoy the following benefits with Caravela compared to PostSharp:

  • Faster builds: Caravela runs directly inside the compiler process (it is a fork of Roslyn), does not require an external process, and is therefore much faster;

  • More powerful transformations: The templating technology used by Caravela allows for more control over code than what is possible with PostSharp MSIL.

  • Better multi-platform support: Caravela does not load the whole project being built in the compiler process, therefore it avoids the cross-compilation issues that have plagued PostSharp for many years;

  • Better design-time experience: You will see introduced members and interfaces in Intellisense because Caravela will do the work at design time and not at post-compilation time. No need for weird casts.

  • Better run-time performance: Because of code generation improvements, you can create aspects that execute much faster.

  • Better debugging experience: You can switch from source code view to transformed code view and debug exactly the code that is executed.

Benefits of PostSharp “Caravela” over Roslyn source generators

Unlike Roslyn source generators, PostSharp “Caravela”:

  • can replace or enhance hand-written code (Roslyn source generators are additive only: you can only add partial classes);
  • allows you to write aspects (or code transformations):
    • in your main project (instead of a separate project),
    • using the C# language, with Intellisense and code validation (instead of building a string);
  • is therefore a real and complete framework for aspect-oriented programming in C#, with the same level of functionality that exists in other languages (such as AspectJ for Java) – which has never been the intent of Roslyn source generators.

Most Anticipated Questions

How long will the MSIL-based PostSharp be maintained?

Our current release plan with the MSIL-based PostSharp is:

  • 6.9 (Q1 2021): addressing performance issues in PostSharp Tools for Visual Studio.
  • 6.10 LTS (Q4 2021): support for .NET 6.

PostSharp 6.10 LTS will be our last supported version of the MSIL-based stack and we intend to support it according to our support policies, that is, 1 year after Caravela reaches a first LTS version. We will work with our customers to ensure the smoothest possible transition.

Will PostSharp “Caravela” be compatible with PostSharp 6.*?

How compatible do we intend to be with PostSharp MSIL? How much code will you need to rewrite?

It has been 12 years since the last major breaking change in PostSharp. Do you remember the .NET landscape in 2008? Clearly, we cannot build a new platform by keeping compatibility with designs that were optimal 12 years ago. However, we understand that PostSharp is used by thousands and we want to find a compromise between modernity and backward compatibility.

We have already taken the following compromise:

  • your aspect code (typically less than a dozen of classes) will need to be totally rewritten,
  • your business code should not be affected.

What will happen with PostSharp Patterns?

We intend to port PostSharp Patterns to PostSharp “Caravela” in a way that maximizes backward compatibility, but we may also take the the opportunity to make a few long-due breaking changes.

How will PostSharp “Caravela” be licensed and priced?

We don’t know yet. The preview releases are being licensed under the terms of the Evaluation License of PostSharp.

Summary

PostSharp “Caravela” is the future of aspect-oriented programming and metaprogramming in .NET. It will take us a long time to get there, but the possibilities are amazing and the path much less rocky than 10 years ago.

For more information, please have a look at the home of PostSharp “Caravela” on GitHub.

If you have any feedback or question regarding Caravela, please open an issue, start a discussion, or contact us directly at hello@postsharp.net.

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