Enable Roslyn multi-targeting in the MVVM Toolkit generator

Author: Sergio0694

CommunityToolkit.Mvvm.SourceGenerators/CommunityToolkit.Mvvm.SourceGenerators.csproj

@@ -5,6 +5,26 @@
     <IsPackable>false</IsPackable>
   </PropertyGroup>
 
+  <!--
+    The MVVM Toolkit source generators rely on Roslyn multi-targeting to support multiple versions of the Microsoft.CodeAnalysis.CSharp package.
+    This package reference actually needs a matching version of Roslyn to be available for consumers of the source generator, so if we always
+    used the latest version, the MVVM Toolkit would just fail to load for users of eg. an older version of Visual Studio. Thankfully, Roslyn
+    supports analyzers that bundle multiple versions in the same NuGet package, each in a subfolder with a name matching the Roslyn version.
+    To leverage this, this project receives the MvvmToolkitSourceGeneratorRoslynVersion property as input, so that the MVVM Toolkit can build
+    it multiple times with multiple versions during packing, to then extract each .dll and copy it to the right NuGet package folder.
+  -->
+  <PropertyGroup>
+    
+    <!-- When the property is not set (which will not happen when creating a NuGet package anyway), just default to 4.0.1 -->
+    <MvvmToolkitSourceGeneratorRoslynVersion Condition="'$(MvvmToolkitSourceGeneratorRoslynVersion)' == ''">4.0.1</MvvmToolkitSourceGeneratorRoslynVersion>
+    
+    <!-- The output path will end up being eg. "bin\Debug\roslyn4.0\netstandard2.0" (the trailing "netstandard2.0" is added automatically) -->
+    <OutputPath>bin\$(Configuration)\roslyn$(MvvmToolkitSourceGeneratorRoslynVersion.Substring(0, 3))</OutputPath>
+    
+    <!-- Also define a "ROSLYN_<MAJOR>_<MINOR>" build constant, so the generator code can multi-target whenever needed and add any required polyfills -->
+    <DefineConstants>$(DefineConstants);ROSLYN_$(MvvmToolkitSourceGeneratorRoslynVersion.Substring(0, 3).Replace('.', '_'))</DefineConstants>
+  </PropertyGroup>
+
   <ItemGroup>
     <Compile Remove="EmbeddedResources\INotifyPropertyChanged.cs" />
     <Compile Remove="EmbeddedResources\NotNullAttribute.cs" />
@@ -32,7 +52,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="4.0.1" PrivateAssets="all" Pack="false" />
+    <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="$(MvvmToolkitSourceGeneratorRoslynVersion)" PrivateAssets="all" Pack="false" />
   </ItemGroup>
 
   <ItemGroup>

CommunityToolkit.Mvvm/CommunityToolkit.Mvvm.csproj

@@ -40,9 +40,14 @@
     <IsTrimmable>true</IsTrimmable>
   </PropertyGroup>
 
-  <!-- Source generator project reference for packing -->
+  <!-- Reference the various multi-targeted versions of the source generator project (one per Roslyn version) -->
   <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
-    <ProjectReference Include="..\CommunityToolkit.Mvvm.SourceGenerators\CommunityToolkit.Mvvm.SourceGenerators.csproj" ReferenceOutputAssembly="false" />
+    <ProjectReference Include="..\CommunityToolkit.Mvvm.SourceGenerators\CommunityToolkit.Mvvm.SourceGenerators.csproj" ReferenceOutputAssembly="false">
+      <Properties>MvvmToolkitSourceGeneratorRoslynVersion=4.0.1</Properties>
+    </ProjectReference>
+    <ProjectReference Include="..\CommunityToolkit.Mvvm.SourceGenerators\CommunityToolkit.Mvvm.SourceGenerators.csproj" ReferenceOutputAssembly="false">
+      <Properties>MvvmToolkitSourceGeneratorRoslynVersion=4.3.0</Properties>
+    </ProjectReference>
   </ItemGroup>
 
   <ItemGroup Label="Package">
@@ -53,11 +58,12 @@
     <None Include="CommunityToolkit.Mvvm.targets" PackagePath="build\netstandard2.0" Pack="true" />
     <None Include="CommunityToolkit.Mvvm.targets" PackagePath="build\netstandard2.1" Pack="true" />
 
-    <!-- Pack the source generator to the right package folder -->
-    <None Include="..\CommunityToolkit.Mvvm.SourceGenerators\bin\$(Configuration)\netstandard2.0\CommunityToolkit.Mvvm.SourceGenerators.dll"
-          PackagePath="analyzers\dotnet\roslyn4.0\cs"
-          Pack="true"
-          Visible="false" />
+    <!--
+      Pack the source generator to the right package folders (each matching the target Roslyn version).
+      Roslyn will automatically load the highest version compatible with Roslyn's version in the SDK.
+    -->
+    <None Include="..\CommunityToolkit.Mvvm.SourceGenerators\bin\$(Configuration)\roslyn4.0\netstandard2.0\CommunityToolkit.Mvvm.SourceGenerators.dll" PackagePath="analyzers\dotnet\roslyn4.0\cs" Pack="true" Visible="false" />
+    <None Include="..\CommunityToolkit.Mvvm.SourceGenerators\bin\$(Configuration)\roslyn4.3\netstandard2.0\CommunityToolkit.Mvvm.SourceGenerators.dll" PackagePath="analyzers\dotnet\roslyn4.3\cs" Pack="true" Visible="false" />
   </ItemGroup>
 
 </Project>