From ed8116ac09f27ce5fa834dd3ee7a185047b00388 Mon Sep 17 00:00:00 2001
From: Nikola Metulev <711864+nmetulev@users.noreply.github.com>
Date: Fri, 17 Jul 2026 18:43:20 -0700
Subject: [PATCH 1/3] Resolve Windows metadata from .NET SDK ref pack on
 SDK-less hosts

The shipped MSBuild targets default `CsWinRTWindowsMetadata` to a bare Windows
SDK version (`$(WindowsSDKVersion)`/`$(TargetPlatformVersion)`), which
`cswinrt.exe` can only resolve via the registry
(`HKLM\...\Windows Kits\Installed Roots\KitsRoot10`). On hosts with no
registered Windows SDK (clean CI agents, containers, SDK-less dev machines)
that lookup throws "Could not find the Windows SDK in the registry", cascading
into downstream (e.g. XAML) build failures.

Add `CsWinRTResolveWindowsMetadataFromSdkRefPack`, which - only when no Windows
SDK is registered and the value is still the bare-version default - repoints
`CsWinRTWindowsMetadata` at the `\winmd` folder of the
`Microsoft.Windows.SDK.NET.Ref` projection ref pack the .NET SDK already
restores for `net*-windows10.0.x` TFMs (resolved robustly via
`@(ResolvedTargetingPack)`, versioned to `$(TargetPlatformVersion)`). Registered
-SDK builds (Visual Studio, SDK-installed CI) and explicit `CsWinRTWindowsMetadata`
values are unaffected.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 4f8b6648-c42b-4fca-98a7-2025142b37bd
---
 nuget/Microsoft.Windows.CsWinRT.targets | 60 +++++++++++++++++++++++++
 nuget/readme.md                         | 10 +++++
 2 files changed, 70 insertions(+)

diff --git a/nuget/Microsoft.Windows.CsWinRT.targets b/nuget/Microsoft.Windows.CsWinRT.targets
index 9c144cf6df..78d23b2bb5 100644
--- a/nuget/Microsoft.Windows.CsWinRT.targets
+++ b/nuget/Microsoft.Windows.CsWinRT.targets
@@ -79,6 +79,66 @@ Copyright (C) Microsoft Corporation. All rights reserved.
     <Error Text="Support for .NET 6 and .NET 7 ended with C#/WinRT 2.2. For .NET 6 or .NET 7 support, use C#/WinRT version 2.2.x." />
   </Target>
 
+  <!--
+    Resolve Windows metadata from the .NET SDK projection ref pack when no Windows SDK is registered.
+
+    The default 'CsWinRTWindowsMetadata' above (from '$(WindowsSDKVersion)' or '$(TargetPlatformVersion)')
+    is a bare Windows SDK version number, e.g. '10.0.19041.0'. When that bare version is passed to
+    'cswinrt.exe' as '-input', the tool locates the metadata by reading the Windows SDK install root from
+    the registry ('HKLM\SOFTWARE\Microsoft\Windows Kits\Installed Roots\KitsRoot10'). On hosts without a
+    registered Windows SDK (clean CI agents, containers, SDK-less developer machines) that lookup throws
+    "Could not find the Windows SDK in the registry", which cascades into downstream (e.g. XAML) failures.
+
+    For modern .NET TFMs (e.g. 'net*-windows10.0.x'), the .NET SDK auto-restores the
+    'Microsoft.Windows.SDK.NET.Ref' projection ref pack, which ships the Windows metadata (.winmd) files -
+    versioned to match '$(TargetPlatformVersion)' - under its '\winmd' folder. 'cswinrt.exe' already
+    accepts '-input <folder>', so we can point 'CsWinRTWindowsMetadata' at that folder and have projection
+    and authoring work with no registered Windows SDK. Setting the property here covers both the projection
+    response file ('-input $(CsWinRTWindowsMetadata)') and the authoring source generator (which reads the
+    'CsWinRTWindowsMetadata' compiler-visible property).
+
+    This is intentionally gated so that machines with a registered Windows SDK (Visual Studio, SDK-installed
+    CI) keep their existing behavior byte-for-byte. It only takes effect when ALL of the following hold:
+      - 'CsWinRTWindowsMetadata' is still the bare-version default: it is non-empty, is not one of the
+        tool's own sentinels ('local'/'sdk'/'sdk+'), and does not point at an existing path (an explicit
+        file/folder provided by the user is left untouched).
+      - No Windows SDK is registered (the 'KitsRoot10' registry value is empty).
+      - The 'Microsoft.Windows.SDK.NET.Ref' ref pack was resolved and its '\winmd' folder exists.
+  -->
+  <Target Name="CsWinRTResolveWindowsMetadataFromSdkRefPack"
+          Condition="$(CsWinRTEnabled)"
+          DependsOnTargets="ResolveReferences"
+          BeforeTargets="CsWinRTGenerateProjection;CsWinRTSetAuthoringWinMDs;GenerateMSBuildEditorConfigFile;GenerateMSBuildEditorConfigFileCore">
+
+    <PropertyGroup>
+      <!-- Empty when no Windows SDK is registered. Mirrors the 32-bit-then-default view order used by cswinrt.exe. -->
+      <_CsWinRTKitsRoot10>$([MSBuild]::GetRegistryValueFromView('HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Kits\Installed Roots', 'KitsRoot10', null, Microsoft.Win32.RegistryView.Registry32, Microsoft.Win32.RegistryView.Registry64))</_CsWinRTKitsRoot10>
+
+      <!-- True only when the current value is a bare SDK version that needs the registry to resolve. -->
+      <_CsWinRTWindowsMetadataIsBareVersion Condition="'$(CsWinRTWindowsMetadata)' != '' and
+                                                       '$(CsWinRTWindowsMetadata)' != 'local' and
+                                                       '$(CsWinRTWindowsMetadata)' != 'sdk' and
+                                                       '$(CsWinRTWindowsMetadata)' != 'sdk+' and
+                                                       !Exists('$(CsWinRTWindowsMetadata)')">true</_CsWinRTWindowsMetadataIsBareVersion>
+    </PropertyGroup>
+
+    <ItemGroup Condition="'$(_CsWinRTKitsRoot10)' == '' and '$(_CsWinRTWindowsMetadataIsBareVersion)' == 'true'">
+      <!-- The ref pack resolves as several targeting-pack items (Windows/Xaml/CsWinRT3) that share one root; de-duplicate. -->
+      <_CsWinRTSdkNetRefPackDir Include="@(ResolvedTargetingPack->'%(Path)')"
+                               Condition="'%(ResolvedTargetingPack.NuGetPackageId)' == 'Microsoft.Windows.SDK.NET.Ref'" />
+      <_CsWinRTSdkNetRefPackDirDistinct Include="@(_CsWinRTSdkNetRefPackDir->Distinct())" />
+    </ItemGroup>
+
+    <PropertyGroup Condition="'$(_CsWinRTKitsRoot10)' == '' and '$(_CsWinRTWindowsMetadataIsBareVersion)' == 'true'">
+      <_CsWinRTSdkNetRefPackWinMDDir Condition="'@(_CsWinRTSdkNetRefPackDirDistinct)' != ''">@(_CsWinRTSdkNetRefPackDirDistinct)\winmd</_CsWinRTSdkNetRefPackWinMDDir>
+      <CsWinRTWindowsMetadata Condition="'$(_CsWinRTSdkNetRefPackWinMDDir)' != '' and Exists('$(_CsWinRTSdkNetRefPackWinMDDir)')">$(_CsWinRTSdkNetRefPackWinMDDir)</CsWinRTWindowsMetadata>
+    </PropertyGroup>
+
+    <Message Condition="'$(_CsWinRTSdkNetRefPackWinMDDir)' != '' and '$(CsWinRTWindowsMetadata)' == '$(_CsWinRTSdkNetRefPackWinMDDir)'"
+             Importance="$(CsWinRTMessageImportance)"
+             Text="C#/WinRT: no Windows SDK is registered; using Windows metadata from the .NET SDK projection ref pack: '$(_CsWinRTSdkNetRefPackWinMDDir)'." />
+  </Target>
+
   <!-- Note this runs before the msbuild editor config file is generated because that is what is used to pass properties to the source generator. -->
   <Target Name="CsWinRTSetGeneratorProperties" BeforeTargets="GenerateMSBuildEditorConfigFile;GenerateMSBuildEditorConfigFileCore">
     <PropertyGroup>
diff --git a/nuget/readme.md b/nuget/readme.md
index cc027eca67..08f1459204 100644
--- a/nuget/readme.md
+++ b/nuget/readme.md
@@ -90,6 +90,16 @@ Windows Metadata is required for all C#/WinRT projections, and can be supplied b
 
 If a Windows SDK is installed, $(WindowsSDKVersion) is defined when building from a VS command prompt.
 
+If no Windows SDK is registered on the machine (for example on clean CI agents, containers, or SDK-less
+developer machines), the default `$(WindowsSDKVersion)`/`$(TargetPlatformVersion)` value is a bare SDK
+version number that `cswinrt.exe` can only resolve through the registry, so the build would otherwise fail
+with "Could not find the Windows SDK in the registry". In that case, for modern .NET TFMs (e.g.
+`net*-windows10.0.x`), C#/WinRT automatically falls back to the Windows metadata (`.winmd`) files shipped in
+the `Microsoft.Windows.SDK.NET.Ref` projection ref pack that the .NET SDK restores (its `\winmd` folder,
+versioned to `$(TargetPlatformVersion)`). This fallback only applies when no Windows SDK is registered and
+`$(CsWinRTWindowsMetadata)` is still the bare-version default; an explicit `$(CsWinRTWindowsMetadata)` value
+(a path, or `local`/`sdk`) and registered-SDK builds are unaffected.
+
 ## Consuming and Producing 
 
 The C#/WinRT package can be used both to consume WinRT types, and to produce them (via CsWinRTComponent).  It is also possible to combine these settings and do both.  For example, a developer might want to *produce* a library that's implemented in terms of another WinRT runtime class (*consuming* it).

From 7271a49facf9f8dfd2cae83f0099747e063b2466 Mon Sep 17 00:00:00 2001
From: Nikola Metulev <711864+nmetulev@users.noreply.github.com>
Date: Fri, 17 Jul 2026 19:15:28 -0700
Subject: [PATCH 2/3] Clarify ref-pack resolution tolerates preview/suffix
 versions

Document (in the targets comment) that @(ResolvedTargetingPack) resolves the
actually-restored Microsoft.Windows.SDK.NET.Ref pack via the SDK's own
targeting-pack resolution, so it tolerates ref-pack versions that do not
string-match $(TargetPlatformVersion) (e.g. a 10.0.26100.67-preview pack for a
project targeting 10.0.26100.0) - which an exact-version lookup would miss.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 4f8b6648-c42b-4fca-98a7-2025142b37bd
---
 nuget/Microsoft.Windows.CsWinRT.targets | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/nuget/Microsoft.Windows.CsWinRT.targets b/nuget/Microsoft.Windows.CsWinRT.targets
index 78d23b2bb5..adcaa1b799 100644
--- a/nuget/Microsoft.Windows.CsWinRT.targets
+++ b/nuget/Microsoft.Windows.CsWinRT.targets
@@ -123,7 +123,14 @@ Copyright (C) Microsoft Corporation. All rights reserved.
     </PropertyGroup>
 
     <ItemGroup Condition="'$(_CsWinRTKitsRoot10)' == '' and '$(_CsWinRTWindowsMetadataIsBareVersion)' == 'true'">
-      <!-- The ref pack resolves as several targeting-pack items (Windows/Xaml/CsWinRT3) that share one root; de-duplicate. -->
+      <!--
+        The ref pack resolves as several targeting-pack items (Windows/Xaml/CsWinRT3) that share one root; de-duplicate.
+        We resolve the folder from the actually-restored targeting pack rather than scanning for a pack whose version
+        string equals '$(TargetPlatformVersion)'. The restored ref-pack version does not always string-match the TPV
+        (e.g. a '10.0.26100.67-preview' ref pack for a project targeting '10.0.26100.0'); @(ResolvedTargetingPack) goes
+        through the SDK's own targeting-pack resolution, so it tolerates preview/suffix versions that an exact-match
+        lookup would miss.
+      -->
       <_CsWinRTSdkNetRefPackDir Include="@(ResolvedTargetingPack->'%(Path)')"
                                Condition="'%(ResolvedTargetingPack.NuGetPackageId)' == 'Microsoft.Windows.SDK.NET.Ref'" />
       <_CsWinRTSdkNetRefPackDirDistinct Include="@(_CsWinRTSdkNetRefPackDir->Distinct())" />

From cc67f3aae7fea095cc8a8f21e6ca5bf78c1bf41f Mon Sep 17 00:00:00 2001
From: Nikola Metulev <nmetulev@users.noreply.github.com>
Date: Fri, 17 Jul 2026 21:24:14 -0700
Subject: [PATCH 3/3] Update nuget/Microsoft.Windows.CsWinRT.targets

Co-authored-by: Manodasan Wignarajah <manodasan@hotmail.com>
---
 nuget/Microsoft.Windows.CsWinRT.targets | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/nuget/Microsoft.Windows.CsWinRT.targets b/nuget/Microsoft.Windows.CsWinRT.targets
index adcaa1b799..be41c5620d 100644
--- a/nuget/Microsoft.Windows.CsWinRT.targets
+++ b/nuget/Microsoft.Windows.CsWinRT.targets
@@ -143,7 +143,7 @@ Copyright (C) Microsoft Corporation. All rights reserved.
 
     <Message Condition="'$(_CsWinRTSdkNetRefPackWinMDDir)' != '' and '$(CsWinRTWindowsMetadata)' == '$(_CsWinRTSdkNetRefPackWinMDDir)'"
              Importance="$(CsWinRTMessageImportance)"
-             Text="C#/WinRT: no Windows SDK is registered; using Windows metadata from the .NET SDK projection ref pack: '$(_CsWinRTSdkNetRefPackWinMDDir)'." />
+             Text="C#/WinRT: no Windows SDK is registered; using Windows metadata from the Windows SDK projection ref pack: '$(_CsWinRTSdkNetRefPackWinMDDir)'." />
   </Target>
 
   <!-- Note this runs before the msbuild editor config file is generated because that is what is used to pass properties to the source generator. -->