Assists in performing Roslyn-based code generation during a build.
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Roslyn-based Code Generation

Build 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.

Table of Contents

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.

Define code generator

This must be done in a library that targets netstandard1.6 or net461 (or later; net461 is supported if you have .NET Core SDK v2.0+ installed, see docs for details). Your generator cannot be defined in the same project that will have code generated for it because code generation runs before the receiving project is itself compiled.

Install the CodeGeneration.Roslyn NuGet Package.

Define the generator class (note: constructor accepting AttributeData parameter is required):

using CodeGeneration.Roslyn;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using System;
using System.Threading;
using System.Threading.Tasks;
using Validation;

public class DuplicateWithSuffixGenerator : ICodeGenerator
    private readonly string suffix;

    public DuplicateWithSuffixGenerator(AttributeData attributeData)
        Requires.NotNull(attributeData, nameof(attributeData));

        this.suffix = (string)attributeData.ConstructorArguments[0].Value;

    public Task<SyntaxList<MemberDeclarationSyntax>> GenerateAsync(TransformationContext context, IProgress<Diagnostic> progress, CancellationToken cancellationToken)
        var results = SyntaxFactory.List<MemberDeclarationSyntax>();

        // Our generator is applied to any class that our attribute is applied to.
        var applyToClass = (ClassDeclarationSyntax)context.ProcessingMember;

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

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

Define attribute

To activate your code generator, you need to define an attribute that can be applied to the class to be copied. This attribute may be defined in the same assembly as defines your code generator, but since your code generator must be defined in a netstandard1.6+ or net461+ 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 CodeGeneration.Roslyn.Attributes package to your attributes project.

Define your attribute class. For this walkthrough, we will assume that the attributes are defined in the same netstandard1.6 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 CodeGeneration.Roslyn;
using System;
using System.Diagnostics;
using Validation;

[AttributeUsage(AttributeTargets.Class, Inherited = false, AllowMultiple = true)]
public class DuplicateWithSuffixAttribute : Attribute
    public DuplicateWithSuffixAttribute(string suffix)
        Requires.NotNullOrEmpty(suffix, nameof(suffix));

        this.Suffix = suffix;

    public string Suffix { get; }

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.

public class Foo

Install the CodeGeneration.Roslyn.BuildTime package into the project that uses your attribute. You may set PrivateAssets="all" on this reference because this is a build-time only package. You must also add this item to an <ItemGroup> in the project that will execute the code generator as part of your build:

<DotNetCliToolReference Include="dotnet-codegen" Version="0.4.12" />

You should adjust the version in the above xml to match the version of this tool you are using.

You can then consume the generated code at design-time:

public void SimpleGenerationWorks()
    var foo = new Foo();
    var fooA = new FooA();

You should see Intellisense help you in all your interactions with FooA. If you execute Go To Definition on it, Visual Studio will open the generated code file that actually defines FooA, and you'll notice it's exactly like Foo, just renamed as our code generator defined it to be.

Shared Projects

When using shared projects and partial classes across the definitions of your class in shared and platform projects:

  • The code generation attributes should be applied only to the files in the shared project (or in other words, the attribute should only be applied once per type to avoid multiple generator invocations).
  • The MSBuild:GenerateCodeFromAttributes custom tool must be applied to every file we want to auto generate code from.

Developing your code generator

Your code generator can be defined in a project in the same solution as the solution with the project that consumes it. You can edit your code generator and build the solution to immediately see the effects of your changes on the generated code.

Packaging up your code generator for others' use

You can also package up your code generator as a NuGet package for others to install and use. Your NuGet package should include a dependency on the CodeGeneration.Roslyn.BuildTime that matches the version of CodeGeneration.Roslyn that you used to produce your generator. For example, if you used version 0.4.12 of this project, your .nuspec file would include this tag:

<dependency id="CodeGeneration.Roslyn.BuildTime" version="0.4.12" />

In addition to this dependency, your NuGet package should include a build folder with an MSBuild file (either a .props or a .targets file) that defines an GeneratorAssemblySearchPaths MSBuild item pointing to the folder containing your code generator assembly and its dependencies. For example your package should have a build\MyPackage.targets file with this content:

<?xml version="1.0" encoding="utf-8" ?>
<Project ToolsVersion="14.0" DefaultTargets="Build" xmlns="">
    <GeneratorAssemblySearchPaths Include="$(MSBuildThisFileDirectory)..\tools" />

Then your package should also have a tools folder that contains your code generator and any of the runtime dependencies it needs besides those delivered by the CodeGeneration.Roslyn.BuildTime package.

Your attributes assembly should be placed under your package's lib folder so consuming projects can apply those attributes.

Your consumers should depend on your package, and the required dotnet CLI tool, so that the MSBuild Task can invoke the dotnet codegen command line tool:

  <PackageReference Include="YourCodeGenPackage" Version="1.2.3" PrivateAssets="all" />
  <DotNetCliToolReference Include="dotnet-codegen" Version="0.4.12" />

Make sure that the DotNetCliToolReference version matches the version of the CodeGeneration.Roslyn package your package depends on.