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.
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.
This must be done in a library that targets netstandard1.3 or net46 (or later). 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.
You may need to define this property in your .NET SDK netstandard project to
workaround the problem with the Microsoft.Composition
NuGet package:
<PackageTargetFallback>$(PackageTargetFallback);portable-net45+win8+wp8+wpa81;</PackageTargetFallback>
Define the generator class:
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);
}
}
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 netstandard1.3 or net46 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
netstandard1.3 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)]
[CodeGenerationAttribute(typeof(DuplicateWithSuffixGenerator))]
[Conditional("CodeGeneration")]
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.
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.
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.
[DuplicateWithSuffix("A")]
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 can then consume the generated code at design-time:
[Fact]
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.
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.
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.
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 0.4.6 of this project, your .nuspec file would include this tag:
<dependency id="CodeGeneration.Roslyn.BuildTime" version="0.4.6" />
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="http://schemas.microsoft.com/developer/msbuild/2003">
<ItemGroup>
<GeneratorAssemblySearchPaths Include="$(MSBuildThisFileDirectory)..\tools" />
</ItemGroup>
</Project>
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:
<ItemGroup>
<PackageReference Include="YourCodeGenPackage" Version="1.2.3" PrivateAssets="all" />
<DotNetCliToolReference Include="dotnet-codegen" Version="0.4.6" />
</ItemGroup>
Also, any consumer must have a nuget.config file with at least this content:
<?xml version="1.0" encoding="utf-8"?>
<configuration>
<packageSources>
<add key="api.nuget.org" value="https://api.nuget.org/v3/index.json" />
<add key="corefxlab" value="https://dotnet.myget.org/F/dotnet-corefxlab/api/v3/index.json" />
</packageSources>
</configuration>
Make sure that the DotNetCliToolReference version matches the version of the
CodeGeneration.Roslyn
package your package depends on.