Fix PublishSingleFile not working with ClassLibrary references

build/NuSpecs/Microsoft.WindowsAppSDK.ClassLibraryPublish.targets

@@ -0,0 +1,56 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright (c) Microsoft Corporation. All rights reserved. Licensed under the MIT License. See LICENSE in the project root for license information. -->
+<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <!-- 
+    Fix for WinUI ClassLibrary references during publish operations.
+    Without this, the build process incorrectly adds the runtime identifier to the class library reference path.
+
+    This target runs during any Publish operation to ensure ClassLibrary references are handled correctly 
+    regardless of PublishSingleFile setting.
+  -->
+  <Target Name="FixClassLibraryReferencesForPublish" 
+          BeforeTargets="_ComputeResolvedFilesToPublishList">
+
+    <!-- 
+      Modify the ResolvedFileToPublish items to ensure they're using the correct paths without runtime identifiers
+      for class library references during publish operations.
+    -->
+    <ItemGroup>
+      <ResolvedFileToPublish Condition="'%(ResolvedFileToPublish.AssetType)' == 'runtime' 
+                               AND '%(ResolvedFileToPublish.RuntimeIdentifier)' != ''
+                               AND $([System.String]::Copy('%(ResolvedFileToPublish.SourcePath)').Contains('%(ResolvedFileToPublish.RuntimeIdentifier)'))">
+        <DestinationSubPath>$(PublishDir)%(ResolvedFileToPublish.DestinationSubPath)</DestinationSubPath>
+      </ResolvedFileToPublish>
+    </ItemGroup>
+
+    <!-- 
+      Check if any class library references need to have their paths corrected by looking for files
+      that should exist in the non-runtime-specific folder instead.
+    -->
+    <ItemGroup>
+      <_PublishItemsToCheck Include="@(ResolvedFileToPublish)" 
+                            Condition="'%(ResolvedFileToPublish.AssetType)' == 'runtime' 
+                                       AND '%(ResolvedFileToPublish.RuntimeIdentifier)' != ''
+                                       AND $([System.String]::Copy('%(ResolvedFileToPublish.SourcePath)').Contains('%(ResolvedFileToPublish.RuntimeIdentifier)'))" />
+
+      <_MissingRuntimeFiles Include="@(_PublishItemsToCheck)" 
+                           Condition="!Exists('%(SourcePath)')" />
+    </ItemGroup>
+
+    <!-- For any missing files, look for them in the non-runtime-specific location -->
+    <ItemGroup>
+      <_NonRIDSourcePaths Include="@(_MissingRuntimeFiles->'$([System.String]::Copy('%(SourcePath)').Replace('\%(RuntimeIdentifier)\', '\'))')">
+        <OriginalItem>%(_MissingRuntimeFiles.Identity)</OriginalItem>
+      </_NonRIDSourcePaths>
+    </ItemGroup>
+
+    <!-- Update the ResolvedFileToPublish items with the corrected SourcePath -->
+    <ItemGroup>
+      <ResolvedFileToPublish Remove="@(_MissingRuntimeFiles)" />
+      <ResolvedFileToPublish Include="@(_NonRIDSourcePaths->'%(OriginalItem)')" 
+                             Condition="Exists('%(Identity)')">
+        <SourcePath>%(Identity)</SourcePath>
+      </ResolvedFileToPublish>
+    </ItemGroup>
+  </Target>
+</Project>

build/NuSpecs/Microsoft.WindowsAppSDK.Foundation.targets

@@ -16,6 +16,9 @@
   <Import Project="$(MSBuildThisFileDirectory)Microsoft.WindowsAppSDK.AutoInitializerCommon.targets" />
   <Import Project="$(MSBuildThisFileDirectory)Microsoft.WindowsAppSDK.AutoInitializer.CS.targets" Condition="'$(WindowsAppSdkAutoInitialize)' == 'true'"/>
 
+  <!-- Import the ClassLibraryPublish fix for WinUI class library references -->
+  <Import Project="$(MSBuildThisFileDirectory)Microsoft.WindowsAppSDK.ClassLibraryPublish.targets" />
+
   <ItemGroup>
     <Compile Condition="'$(WindowsAppSDKAggregatePackage)' == 'true' and '$(WindowsAppSdkIncludeVersionInfo)'=='true'" Include="$(MSBuildThisFileDirectory)..\include\WindowsAppSDK-VersionInfo.cs" />
   </ItemGroup>

docs/Packaging/WinUI-ClassLibrary-References.md

@@ -0,0 +1,43 @@
+# WinUI ClassLibrary References During Publish
+
+## Issue
+
+When using the Windows App SDK with a project that references a WinUI class library, the build process incorrectly looks for the class library in a path that includes the runtime identifier (e.g., `win-x64`), which causes build failures. This occurs in any publish scenario, including when using `PublishSingleFile=true`.
+
+For example, if your solution structure is:
+```
+Solution
+  ├── App1 (references ClassLibrary1)
+  └── ClassLibrary1
+```
+
+When publishing App1, you might encounter errors like:
+```
+Cannot resolve Assembly or Windows Metadata file 'path\to\ClassLibrary1\bin\Release\net8.0-windows10.0.19041.0\win-x64\ClassLibrary1.dll'
+Metadata file 'path\to\ClassLibrary1\bin\Release\net8.0-windows10.0.19041.0\win-x64\ClassLibrary1.dll' could not be found
+```
+
+## Solution
+
+The fix implemented in `Microsoft.WindowsAppSDK.ClassLibraryPublish.targets` addresses this issue by correcting the reference paths during the publish process.
+
+The target does the following:
+1. Runs before the `_ComputeResolvedFilesToPublishList` target during any publish operation
+2. Identifies class library references that include runtime identifiers in their paths
+3. Checks if the files exist at the specified paths
+4. For missing files, tries to find them in the non-runtime-specific location
+5. Updates the `ResolvedFileToPublish` items with the corrected paths
+
+## Workaround (prior to this fix)
+
+If you encounter this issue before updating to a version with this fix, you can:
+
+1. Create a Blank Project
+2. Create a Blank Class Library
+3. Add Class Library Reference to Main Project
+4. Build
+5. Manually modify the Class Library output folder:
+   - Create a folder named after your runtime identifier (e.g., `win-x64`) in the output directory
+   - Copy all files from the main output directory into this new folder
+
+This mimics the file structure expected by the publish process.