Roslyn-based Code Generation

[][NuPkg] [][AttrNuPkg] [][EngineNuPkg] [][BuildTimeNuPkg] [][ToolNuPkg]

Background & Motivations

Before we dive into the finer points of Code Generation, it is necessary to discuss a couple of key differences separating this effort from the original effort upon which our inspiration was derived.

First and foremost, kudos to Andrew Arnott and folks for the original effort. We have been able to generate code in most circumstances, but lately there are a couple of requirements we had in which CodeGeneration.Roslyn just was not going to work for us. This is the primary motivation for our recasting of the original effort, distilling just the code generation facilitation and getting out of the way of the code generation author as quickly as possible. We do this in a couple of ways:

1. First, Code.Generation.Roslyn is focused on just that: facilitating code generation. What do we mean by that? To be clear, we found it to be extraneous, and a bit of a distraction, to consider whether the annotated request originated in a class, struct, in what name spaces, and so on. Instead, once we discover the annotation, we simply relay that to the code generation author and step out of the way as early as possible. We do this by requiring code be generated at the CompilationUnitSyntax level as contrasted with the MemberDeclarationSyntax level.
2. Secondly, we wanted to generated code triggered by Assembly Attributes, not just Member, i.e. class, struct, interface, etc, Attributes.
3. Thirdly, a tertiary goal of ours was to allow for custom Preamble Text to be delivered into the engine. This is required to be properly formatted, however, the engine itself will append a trailing Carriage Return Line Feed symbol as appropriate. The default, of course, is a predetermined Preamble Text.
4. Last but not least, we were really not happy with the level of unit testing presented in the original effort. We found the examples to be somewhat contrived and academic in nature, whereas we wanted to present some examples that might prove at least somewhat value-added. We were also successful, we believe, in compiling an end-to-end integration test which, short of subscribing to [NuGet packages][NuPkg] themselves, demonstrates that the approach does in fact work.

There are a couple of other nuances.

1. Chiefly concerning our package versioning strategy. Whereas the original work would keep delivered versions aligned, we make no promises concerning such alignment. In the macro, we make an effort for major versions to keep in sync with each other, so you can leverage NuGet version ranges, i.e. [1,2), however, concerning minor, and especially patch, build, etc, elements, these values can migrate from release to release depending on which packages required an update.

With that said, let us reconsider what it means to Generate Code using Roslyn.

Overview

Assists in performing Roslyn-based code generation during a build. This includes design-time support, such that code generation can respond to changes made in hand-authored code files by generating new code that shows up to Intellisense as soon as the file is saved to disk.

Table of Contents

• How to write your own code generator
• Apply code generation
• [Developing your code generator][]
• [Packaging your code generator for others' use][]

How to write your own code generator

In this walkthrough, we will define a code generator that replicates any class your code generation attribute is applied to, but with a suffix appended to its name.

Prerequisites

• .NET Core SDK v2.1+. If you do not have v2.1+ there will be cryptic error messages (see #111).
• .NET Core SDK v2.1.500 specifically for building this project

Define code generator

This must be done in a library that targets netstandard2.0 or net461 (or any netcoreapp2.1-compatible target). Your generator cannot be defined in the same project that will have code generated for it because code generation runs prior to the target project compilation.

Install the [Code.Generation.Roslyn][NuPkg] NuGet Package.

Define the generator class in a class library targeting netstandard2.0 (note: constructor accepting AttributeData parameter is required). We have provided a couple of base classes for you, DocumentCodeGenerator and AssemblyCodeGenerator from which you may derive, depending on your code generation requirements, which ultimately implement either IDocumentCodeGenerator or IAssemblyCodeGenerator, respectively; however, in this case, we will be focused on DocumentCodeGenerator.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.CodeAnalysis.CSharp;

namespace Code.Generation.Roslyn.Generators
{
    using Microsoft.CodeAnalysis;
    using Microsoft.CodeAnalysis.CSharp.Syntax;

    public class ImplementCloneableInterfaceGenerator : DocumentCodeGenerator
    {
        public ImplementCloneableInterfaceGenerator(AttributeData attributeData) : base(attributeData) { }

        public override Task GenerateAsync(DocumentTransformationContext context, IProgress<Diagnostic> progress, CancellationToken cancellationToken)
        {
            // See the example code for the complete example.
            IEnumerable<CodeGeneratorDescriptor> Generate()
            {
                // ...
            }

            void RunGenerate()
            {
                foreach (var d in Generate())
                {
                    Descriptors.Add(d);
                //  ^^^^^^^^^^^^^^^ This is the key, Descriptors is provided by the base class, your Code
                //                  Generator must simply populate the collection as a result of the Task.
                }
            }

            return Task.Run(RunGenerate, cancellationToken);
        }
    }
}

For brevity, feel free to review the example source in our generators test assembly.

Define attribute

In order to activate your code generator, you need to define an attribute that will ultimately annotate the asset used to trigger the code generation. This attribute may be defined in the same assembly as defines your code generator, but since your code generator must be defined in a netcoreapp2.1-compatible library, this may limit which projects can apply your attribute. So define your attribute in another assembly if it must be applied to projects that target older platforms.

If your attributes are in their own project, you must install the [Code.Generation.Roslyn.Attributes][AttrNuPkg] package to your attributes project.

Define your attribute class. For sake of example, we will assume that the attributes are defined in the same netstandard2.0 project that defines the generator which allows us to use the more convenient typeof syntax when declaring the code generator type. If the attributes and code generator classes were in separate assemblies, you must specify the assembly-qualified name of the generator type as a string instead.

using System;
using System.Diagnostics;

namespace Code.Generation.Roslyn.Generators
{
    [AttributeUsage(AttributeTargets.Class, Inherited = false, AllowMultiple = true)]
    [CodeGenerationAttribute(typeof(ImplementCloneableInterfaceGenerator))]
    [Conditional("CodeGeneration")]
    public class ImplementCloneableInterfaceAttribute : TestAttributeBase
    {
    }
}

The [Conditional("CodeGeneration")] attribute is not necessary, but it will prevent the attribute from persisting in the compiled assembly that consumes it, leaving it instead as just a compile-time hint to code generation, and allowing you to not ship with a dependency on your code generation assembly.

Apply code generation

The attribute may not be applied in the same assembly that defines the generator. This is because the code generator must be compiled in order to execute before compiling the project that applies the attribute. Also, the consuming project (where the code will be generated) must use SDK-style csproj, which implies using VS2017+ or dotnet CLI tooling (VS Code with omnisharp, for example).

Applying code generation is incredibly simple. Just add the attribute on any type or member supported by the attribute and generator you wrote. Note you will need to add a project reference to the project that defines the attribute.

namespace Foo
{
    [Code.Generation.Roslyn.GeneratorsImplementCloneableInterface]
    public class Bar
    {
    }
}

Install the [Code.Generation.Roslyn.BuildTime]