Include a .targets file in NuGet packages which will select the correct analyzer assembly depending on which Roslyn version will be used to compile.

Author: eerhardt

eng/MultiTargetRoslynComponent.targets.template

@@ -0,0 +1,41 @@
+<Project>
+  <!-- Reads the version of the compiler APIs that are currently being used in order to pick the correct Roslyn components. -->
+  <Target Name="_{TargetPrefix}ResolveRoslynApiMajorVersion"
+          Condition="'$(_RoslynApiMajorVersion)' == ''">
+    <GetAssemblyIdentity AssemblyFiles="$(RoslynTargetsPath)\Microsoft.Build.Tasks.CodeAnalysis.dll">
+      <Output TaskParameter="Assemblies" ItemName="_CodeAnalysisIdentity" />
+    </GetAssemblyIdentity>
+
+    <PropertyGroup>
+      <_RoslynApiMajorVersion>$([System.Version]::Parse(%(_CodeAnalysisIdentity.Version)).Major)</_RoslynApiMajorVersion>
+    </PropertyGroup>
+  </Target>
+
+  <Target Name="_{TargetPrefix}RoslynComponentMultiTargeting" 
+          Condition="'$(SupportsRoslynComponentVersioning)' != 'true'" 
+          AfterTargets="ResolveAssemblyReferences" 
+          DependsOnTargets="_{TargetPrefix}ResolveRoslynApiMajorVersion">
+    <ItemGroup>
+      <!-- clear _CurrentAnalyzer, in case some other target is using it. -->
+      <_CurrentAnalyzer Remove="@(_CurrentAnalyzer)" />
+      <_CurrentAnalyzer Include="@(Analyzer)" Condition="'%(Analyzer.NuGetPackageId)' == '{NuGetPackageId}'" />
+    </ItemGroup>
+
+    <ItemGroup Condition="$([System.Int32]::Parse($(_RoslynApiMajorVersion))) &lt; 4">
+      <!-- Remove our analyzers targeting roslyn4.x -->
+      <Analyzer Remove="@(_CurrentAnalyzer)"
+                Condition="$([System.String]::Copy('%(_CurrentAnalyzer.Identity)').IndexOf('roslyn4')) &gt;= 0"/>
+    </ItemGroup>
+
+    <ItemGroup Condition="$([System.Int32]::Parse($(_RoslynApiMajorVersion))) &gt;= 4">
+      <!-- Remove our analyzers targeting roslyn3.x -->
+      <Analyzer Remove="@(_CurrentAnalyzer)"
+                Condition="$([System.String]::Copy('%(_CurrentAnalyzer.Identity)').IndexOf('roslyn3')) &gt;= 0"/>
+    </ItemGroup>
+
+    <ItemGroup Condition="'$({DisableSourceGeneratorPropertyName})' == 'true'">
+      <!-- Remove all our analyzers -->
+      <Analyzer Remove="@(_CurrentAnalyzer)" />
+    </ItemGroup>
+  </Target>
+</Project>

eng/packaging.targets

@@ -150,6 +150,42 @@
     </ItemGroup>
   </Target>
 
+  <PropertyGroup>
+    <MultiTargetRoslynComponentTargetsFileIntermediatePath>$(IntermediateOutputPath)MultiTargetRoslynComponent.targets</MultiTargetRoslynComponentTargetsFileIntermediatePath>
+    <IncludeMultiTargetRoslynComponentTargets Condition="'$(IncludeMultiTargetRoslynComponentTargets)' == ''">true</IncludeMultiTargetRoslynComponentTargets>
+  </PropertyGroup>
+
+  <!-- In packages that contain Analyzers, include a .targets file that will select the correct
+       analyzer given the version of Roslyn being used to compile the project. -->
+  <Target Name="IncludeMultiTargetRoslynComponentTargetsInPackage"
+          AfterTargets="IncludeAnalyzersInPackage"
+          Condition="'@(AnalyzerReference)' != '' and '$(IncludeMultiTargetRoslynComponentTargets)' == 'true'"
+          DependsOnTargets="GenerateMultiTargetRoslynComponentTargetsFile">
+    <ItemGroup>
+      <Content Include="$(MultiTargetRoslynComponentTargetsFileIntermediatePath)"
+               PackagePath="build\$(PackageId).targets"
+               Pack="True" />
+    </ItemGroup>
+  </Target>
+
+  <Target Name="GenerateMultiTargetRoslynComponentTargetsFile"
+          Inputs="$(MSBuildProjectFullPath)"
+          Outputs="$(MultiTargetRoslynComponentTargetsFileIntermediatePath)">
+    <PropertyGroup>
+      <_MultiTargetRoslynComponentTargetsTemplate>$(MSBuildThisFileDirectory)MultiTargetRoslynComponent.targets.template</_MultiTargetRoslynComponentTargetsTemplate>
+      <_MultiTargetRoslynComponentTargetPrefix>$(PackageId.Replace('.', '_'))</_MultiTargetRoslynComponentTargetPrefix>
+      <_MultiTargetRoslynComponentDisableSourceGeneratorPropertyName>Disable$(PackageId.Replace('.', ''))SourceGenerator</_MultiTargetRoslynComponentDisableSourceGeneratorPropertyName>
+      <_MultiTargetRoslynComponentDisableSourceGeneratorPropertyName>$(_MultiTargetRoslynComponentDisableSourceGeneratorPropertyName.Replace('Abstractions', ''))</_MultiTargetRoslynComponentDisableSourceGeneratorPropertyName>
+    </PropertyGroup>
+
+    <WriteLinesToFile File="$(MultiTargetRoslynComponentTargetsFileIntermediatePath)"
+                      Lines="$([System.IO.File]::ReadAllText('$(_MultiTargetRoslynComponentTargetsTemplate)')
+                                                 .Replace('{TargetPrefix}', '$(_MultiTargetRoslynComponentTargetPrefix)')
+                                                 .Replace('{NuGetPackageId}', '$(PackageId)')
+                                                 .Replace('{DisableSourceGeneratorPropertyName}', '$(_MultiTargetRoslynComponentDisableSourceGeneratorPropertyName)'))"
+                      Overwrite="true" />
+  </Target>
+
   <!-- Include a netstandard compat error if the project targets both .NETStandard and
        .NETCoreApp. This prohibits users to consume packages on an older .NETCoreApp version
        than the minimum supported one. -->