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Roslyn-based Code Generation

Build Status GitHub Actions CI status NuGet package

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.

See who's generating code or consuming it using Cythral.CodeGeneration.Roslyn.

Instructions on development and using this project's source code are in CONTRIBUTING.md.

Table of Contents

How to write your own code generator

In this walkthrough, we will define a code generator that replicates any class (annotated with our custom attribute) with a suffix (specified in the attribute) appended to its name.

Prerequisites

Define code generator

Your generator cannot be defined in the same project that will have code generated for it. That's because code generation runs before the consuming project is itself compiled. We'll start by creating the generator. This must be done in a library that targets netcoreapp2.1. Let's create one called Duplicator:

dotnet new classlib -f netcoreapp2.1 -o Duplicator.Generators

Now we'll use an MSBuild project SDK CodeGeneration.Roslyn.Plugin.Sdk to speed up configuring our generator plugin. Edit your project file and add the <Sdk> element:

<!-- Duplicator.Generators/Duplicator.Generators.csproj -->
<Project Sdk="Microsoft.NET.Sdk">
  <!-- Add the following element above any others: -->
  <Sdk Name="CodeGeneration.Roslyn.Plugin.Sdk" Version="{replace with actual version used}"/>

  <PropertyGroup>
    <TargetFramework>netcoreapp2.1</TargetFramework>
  </PropertyGroup>

</Project>

This SDK will implicitly add a PackageReference to the corresponding version of CodeGeneration.Roslyn nuget, and set properties to make plugin build correctly. We can now write our generator:

⚠ Note: constructor is required to have exactly a single AttributeData parameter.

// Duplicator.Generators/DuplicateWithSuffixGenerator.cs
using System;
using System.Threading;
using System.Threading.Tasks;
using Cythral.CodeGeneration.Roslyn;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;

namespace Duplicator.Generators
{
    public class DuplicateWithSuffixGenerator : ICodeGenerator
    {
        private readonly string suffix;

        public DuplicateWithSuffixGenerator(AttributeData attributeData)
        {
            suffix = (string)attributeData.ConstructorArguments[0].Value;
        }

        public Task<SyntaxList<MemberDeclarationSyntax>> GenerateAsync(TransformationContext context, IProgress<Diagnostic> progress, CancellationToken cancellationToken)
        {
            // Our generator is applied to any class that our attribute is applied to.
            var applyToClass = (ClassDeclarationSyntax)context.ProcessingNode;

            // Apply a suffix to the name of a copy of the class.
            var copy = applyToClass.WithIdentifier(SyntaxFactory.Identifier(applyToClass.Identifier.ValueText + suffix));

            // Return our modified copy. It will be added to the user's project for compilation.
            var results = SyntaxFactory.SingletonList<MemberDeclarationSyntax>(copy);
            return Task.FromResult(results);
        }
    }
}

Create consuming console app

We'll consume our generator in a Reflector app:

dotnet new console -f netcoreapp2.1 -o Reflector

dotnet add Reflector reference Duplicator.Generators

Let's write a simple program that prints all types in its assembly:

// Reflector/Program.cs
using System;

namespace Reflector
{
    class Program
    {
        static void Main(string[] args)
        {
            foreach (var type in typeof(Program).Assembly.GetTypes())
                Console.WriteLine(type.FullName);
        }
    }
}

Now, when we dotnet run -p Reflector we should get:

Reflector.Program

Define attribute

To activate your code generator, you need to define an attribute with which we'll annotate the class to be copied. Install Attributes package:

dotnet add Reflector package CodeGeneration.Roslyn.Attributes

Then, define your attribute class:

// Reflector/Program.cs
using System;
using System.Diagnostics;
using Cythral.CodeGeneration.Roslyn;

namespace Reflector
{
    class Program
    {
        // ...
    }

    [AttributeUsage(AttributeTargets.Class)]
    [CodeGenerationAttribute("Duplicator.Generators.DuplicateWithSuffixGenerator, Duplicator.Generators")]
    [Conditional("CodeGeneration")]
    public class DuplicateWithSuffixAttribute : Attribute
    {
        public DuplicateWithSuffixAttribute(string suffix)
        {
            Suffix = suffix;
        }

        public string Suffix { get; }
    }
}

CodeGenerationAttribute is crucial - this tells the CG.R Tool which generator to invoke for a member annotated with our DuplicateWithSuffixAttribute. It's parameter is an assembly-qualified generator type name (incl. namespace): Full.Type.Name, Full.Assembly.Name

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.

ℹ Of course, the attribute will persist if you define compilation symbol "CodeGeneration"; we assume it won't be defined.

Apply code generation

Applying code generation is incredibly simple. Just add the attribute on any type or member supported by the attribute and generator you wrote. We'll test our Duplicator on a new Test class:

// Reflector/Program.cs
using System;
using System.Diagnostics;
using Cythral.CodeGeneration.Roslyn;

namespace Reflector
{
    [DuplicateWithSuffix("Passed")]
    class Test {}

    class Program
    {
        // ...
    }

    class DuplicateWithSuffixAttribute : Attribute
    {
        // ...
    }
}

Let's check our app again:

> dotnet run -p Reflector

Reflector.DuplicateWithSuffixAttribute

Reflector.Program

Reflector.Test

Still nothing except what we wrote. Now all that's left is to plumb the build pipeline with code generation tool; that tool will handle invoking our Duplicator at correct time during build and write generated file, passing it into the compilation.

You'll need to add a reference to CodeGeneration.Roslyn.Tool package:

dotnet add Reflector package CodeGeneration.Roslyn.Tool

Also, you need to add the following metadata to your generator project reference: OutputItemType="CodeGenerationRoslynPlugin". This will add the path to the Duplicator.Generators.dll to the list of plugins the tool runs.

This is how your project file can look like:

<!-- Reflector/Reflector.csproj -->
<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp2.1</TargetFramework>
  </PropertyGroup>
  <ItemGroup>
    <!-- This ProjectReference to the generator needs to have the OutputItemType metadata -->
    <ProjectReference Include="..\Duplicator.Generators\Duplicator.Generators.csproj"
                      OutputItemType="CodeGenerationRoslynPlugin" />
    <PackageReference Include="CodeGeneration.Roslyn.Attributes"
                      Version="{replace with actual version used}" />
    <!--
      This contains the generation tool and MSBuild targets that invoke it,
      and so can be marked with PrivateAssets="all"
    -->
    <PackageReference Include="CodeGeneration.Roslyn.Tool"
                      Version="{replace with actual version used}"
                      PrivateAssets="all" />
  </ItemGroup>
</Project>

And if all steps were done correctly:

> dotnet run -p Reflector

Reflector.DuplicateWithSuffixAttribute

Reflector.Program

Reflector.Test

Reflector.TestPassed

💡 Notice that there is a TestPassed type in the assembly now.

What's even better is that you should see that new type in IntelliSense as well! Try executing Go to Definition (F12) on it - your IDE (VS/VS Code) should open the generated file for you (it'll be located in IntermediateOutputPath - most commonly obj/).

Advanced scenarios

While the sample worked, it was also unrealistically simple and skipped many complex issues that will inevitably arise when you try to use this project in real world. What follows is a deep dive into more realistic solutions.

Most of the issues are about two things: TargetFramework and dependecies. TargetFramework/TFM/TxM (e.g. netcoreapp2.1) of the generator is restricted to an exact version and there's not a lot of wiggle room there. In contrast, projects consuming generators and their outputs will target any existing TFM. If you will try to add a P2P (project-to-project) reference to a generator (targeting netcoreapp2.1) to a business model project targeting netstandard1.0, you'll get errors.

Customize generator reference

We don't need the generator as a compile reference. However, the magic OutputItemType metadata is important - it adds a path to the generator dll to the list of plugins run by the CodeGeneration.Roslyn.Tool tool. Additionally, we want to specify that there's a build dependency of the consuming project on the generator. So we modify the reference:

<!-- Reflector/Reflector.csproj -->
  <ItemGroup>
    <ProjectReference Include="..\Duplicator\Duplicator.csproj"
      ReferenceOutputAssembly="false"
      SkipGetTargetFrameworkProperties="true"
      OutputItemType="CodeGenerationRoslynPlugin" />
  </ItemGroup>

We add two new metadata attributes:

  • ReferenceOutputAssembly="false" - this causes the compilation to not add a reference to Duplicator.Generators.dll to the ReferencePath - so the source code has no dependency and doesn't know anything about that project.
  • SkipGetTargetFrameworkProperties="true" - this prevents build tasks from checking compatibility of the generator's TFM with this project's TFM.

Now we can retarget our Reflector to any TFM compatible with Attributes package (so netstandard1.0-compatible), e.g. netcoreapp2.0 - and it should run just fine.

Multitargeting generator

It can happen that your generator project will become multi-targeting. You could need to do that to use C#8's Nullable Reference Types feature in the Duplicator; the generator has to target netcoreapp2.1 as this is the framework it'll be run in by the CG.R.Tool - on the other hand, NRT feature is only supported in newer TFMs, starting with netcoreapp3.1. So you'll do:

<!-- Duplicator.Generators/Duplicator.Generators.csproj -->
<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFrameworks>netcoreapp2.1;netcoreapp3.1</TargetFrameworks>
  </PropertyGroup>
  <!-- ... -->
</Project>

There'll be a build error, because the consumer (Reflector) doesn't know which output to use (and assign to the CodeGenerationRoslynPlugin Item). To fix that we have to use SetTargetFramework metadata. Setting it implies SkipGetTargetFrameworkProperties="true" so we can replace it.

<!-- Reflector/Reflector.csproj -->
<Project Sdk="Microsoft.NET.Sdk">
  <!-- ... -->
  <ItemGroup>
    <ProjectReference Include="..\Duplicator\Duplicator.csproj"
      ReferenceOutputAssembly="false"
      SetTargetFramework="TargetFramework=netcoreapp2.1"
      OutputItemType="CodeGenerationRoslynPlugin" />
    <!-- ... -->
  </ItemGroup>
</Project>

Package your code generator

You can also package up your code generator as a NuGet package for others to install and use. A project using CodeGeneration.Roslyn.Plugin.Sdk is automatically configured to produce a correct Plugin nuget package.

Separate out the attribute

The triggering attribute has to be available in consuming code. Your consumers can write it themselves, but it's not a good idea to require them do so. So we'll separate the attribute into another project that has TFM allowing all your consumers to reference it, for example netstandard1.0 - you're constrained only by what CodeGeneration.Roslyn.Attributes targets.

Let's call this new project Duplicator.Attributes.

dotnet new classlib -o Duplicator.Attributes dotnet add Duplicator.Attributes package CodeGeneration.Roslyn.Attributes dotnet add Reflector reference Duplicator.Attributes

Now, move the attribute definition from Reflector to our new project. Dont' forget to change the namespace. The app should work the same, except for not printing DuplicateWithSuffixAttribute type.

If you annotate your triggering attribute with [Conditional("...")], your consumers can make any references to the attribute package non-transient via PrivateAssets="all" and/or exclude from runtime assets via ExcludeAssets="runtime".

Create the metapackage

Your consumers will now have to depend (have PackageReference) on the following:

  • CodeGeneration.Roslyn.Tool tool
  • Duplicator.Attributes (your attributes package)
  • Duplicator.Generators (your generator/plugin package)

An example consuming project file would contain:

<!-- Reflector/Reflector.csproj -->
<ItemGroup>
  <PackageReference Include="Duplicator.Generators" Version="1.0.0" PrivateAssets="all" />
  <PackageReference Include="Duplicator.Attributes" Version="1.0.0" PrivateAssets="all" />
  <PackageReference Include="CodeGeneration.Roslyn.Tool"
                    Version="{CodeGeneration.Roslyn.Tool version}"
                    PrivateAssets="all" />
</ItemGroup>

➡ There's a much better approach: metapackage.

For this, we'll use CodeGeneration.Roslyn.PluginMetapackage.Sdk MSBuild project SDK in a new project called simply Duplicator, which will reference our attributes:

dotnet new classlib -o Duplicator dotnet add Duplicator reference Duplicator.Attributes

Remove Class1.cs file.

Modify project file:

  • Add <Sdk> element
  • Set NoBuild=true and IncludeBuildOutput=false
  • Add NupkgAdditionalDependency in ItemGroup
<!-- Duplicator/Duplicator.csproj -->
<Project Sdk="Microsoft.NET.Sdk">
  <!-- Add the following element above any others: -->
  <Sdk Name="CodeGeneration.Roslyn.PluginMetapackage.Sdk" Version="{replace with actual version used}"/>

  <PropertyGroup>
    <!-- Declare the TargetFramework(s) the same as in your Attributes package -->
    <TargetFramework>netstandard1.0</TargetFramework>
    <!-- This project contains no files, so building can be skipped -->
    <NoBuild>true</NoBuild>
    <!-- Since we don't build, there'll be no build output -->
    <IncludeBuildOutput>false</IncludeBuildOutput>
  </PropertyGroup>

  <ItemGroup>
    <!-- Reference your Attributes project normally -->
    <ProjectReference Include="../Duplicator.Attributes/Duplicator.Attributes.csproj" />
    <!--
      Reference your generators package by adding an item to NupkgAdditionalDependency
      with IncludeAssets="all" to flow "build" assets.
      Version used will be the PackageVersion Pack resolves,
      but you can specify Version metadata to override it.

      This is necessary to do like that, because it ensures the dependency is setup
      correctly (e.g. simple transient dependency), and skips validation of TFM (Plugin is a tool,
      it's TFM has no meaning for the consumer).
    -->
    <NupkgAdditionalDependency
        Include="Duplicator.Generators"
        IncludeAssets="all" />
  </ItemGroup>
</Project>

This project will now produce a nupkg that will not contain anything on it's own, but will "pull in" all the other dependecies consumers will need:

  • CodeGeneration.Roslyn.Tool (added implicitly by the Sdk)
  • Duplicator.Attributes
  • Duplicator.Generators

Our metapackage should be versioned in the same manner as it's dependant packages.

📋 For a sample metapackage, see MetapackageSample.

Add extra build/ content in Plugin package

CG.R.Plugin.Sdk creates custom build/PackageId.props/targets files. If you want to add custom MSBuild props/targets into NuGet package's build folder (and have it imported when package is referenced), you'll need to use PackageBuildFolderProjectImport ItemGroup, as shown in PackagedGenerator sample.

Accesss MSBuild Properties

You may access MSBuild property values of the project being generated for, by first adding the property name to the PluginRequestedProperty item list. For example, if you want to access the TargetFramework build property, you would do the following in your generator's .csproj file:

<ItemGroup>
  <PluginRequestedProperty Include="ExampleBuildProperty" />
</ItemGroup>

Then, you can access its value like this:

public Task<SyntaxList<MemberDeclarationSyntax>> GenerateAsync(TransformationContext context, IProgress<Diagnostic> progress, CancellationToken cancellationToken)
{
    var targetFramework = context.BuildProperties["TargetFramework"];
    // ...
}

ℹ For a sample generator that accesses MSBuild properties, see BuildPropsGenerator and its consuming project, BuildPropsConsumer