Force-downgrade Microsoft.OpenApi at runtime to workaround microsoft/OpenAPI.NET#2967 and domaindrivendev/Swashbuckle.AspNetCore#4065

Author: bkoelman

Directory.Build.props

@@ -70,4 +70,14 @@
   <ItemGroup>
     <PackageReference Include="JetBrains.Annotations" Version="2026.2.*" PrivateAssets="All" />
   </ItemGroup>
+
+  <ItemGroup>
+    <!--
+      Workaround: Swashbuckle 11.x (used on net10.0/net11.0) was compiled against Microsoft.OpenApi 3.9.0.
+      CS1705 prevents compiling against a lower version, so we compile against 3.9.0 and overwrite
+      the output DLL with 3.7.0 at runtime. PackageDownload ensures 3.7.0 is in the NuGet cache.
+      See the UseOpenApi370AtRuntime target in Directory.Build.targets.
+    -->
+    <PackageDownload Include="Microsoft.OpenApi" Version="[3.7.0]" />
+  </ItemGroup>
 </Project>

Directory.Build.targets

@@ -1,4 +1,22 @@
 <Project>
+  <!--
+    Workaround: Swashbuckle 11.x (used on net10.0/net11.0) was compiled against Microsoft.OpenApi 3.9.0, so
+    CS1705 prevents us from downgrading it at compile time. We compile against 3.9.0 and then overwrite
+    the output DLL with 3.7.0 so the runtime uses the desired version. The Exists() condition limits this
+    to executable/test projects (they copy transitive package DLLs to output; library projects don't).
+    PackageDownload in Directory.Build.props ensures the 3.7.0 DLL is present in the NuGet cache.
+  -->
+  <Target Name="UseOpenApi370AtRuntime" AfterTargets="Build"
+      Condition="('$(TargetFramework)' == 'net11.0' OR '$(TargetFramework)' == 'net10.0') AND Exists('$(OutputPath)Microsoft.OpenApi.dll')">
+    <PropertyGroup>
+      <_NuGetPackagesRoot Condition="'$(NUGET_PACKAGES)' != ''">$(NUGET_PACKAGES)</_NuGetPackagesRoot>
+      <_NuGetPackagesRoot Condition="'$(_NuGetPackagesRoot)' == ''">$(UserProfile)\.nuget\packages</_NuGetPackagesRoot>
+      <_OpenApi370Dll>$(_NuGetPackagesRoot)\microsoft.openapi\3.7.0\lib\net8.0\Microsoft.OpenApi.dll</_OpenApi370Dll>
+    </PropertyGroup>
+    <Copy SourceFiles="$(_OpenApi370Dll)" DestinationFolder="$(OutputPath)" OverwriteReadOnlyFiles="true"
+        Condition="Exists('$(_OpenApi370Dll)')" />
+  </Target>
+
   <Target Name="DisableCompileTimeOpenApiXmlGenerator" BeforeTargets="CoreCompile">
     <ItemGroup>
       <Analyzer Remove="@(Analyzer)" Condition="'%(Filename)' == 'Microsoft.AspNetCore.OpenApi.SourceGenerators'" />