fix: Show error message when incompatible winappsdk to net7.0 with uno is found

Author: jeromelaban

build/uno.winui.common.targets

@@ -26,6 +26,16 @@
 			   Condition="'$(PkgUno_UI)'!='' and '$(PkgUno_WinUI)'!=''" />
 	</Target>
 
+	<!--
+	Include a marker for _FindInvalidWinAppSDKUnoPlatformReference to determine if a net7.0-only project is referenced
+	from a WinAppSDK project.
+	-->
+	<Target Name="_UnoUpdateAdditionalProperties" BeforeTargets="GetTargetFrameworksWithPlatformForSingleTargetFramework">
+		<ItemGroup>
+			<AdditionalTargetFrameworkInfoProperty Include="_IsUnoPlatform" />
+		</ItemGroup>
+	</Target>
+	
 	<Import Project="$(SourceGeneratorBasePath)Uno.UI.SourceGenerators.props" />
 	<Import Project="$(UnoUIMSBuildTasksTargetPath)Uno.UI.Tasks.targets" />
 

build/uno.winui.winappsdk.targets

@@ -64,4 +64,28 @@
 
 	</Target>
 
+	<Target Name="_FindInvalidWinAppSDKUnoPlatformReference"
+			BeforeTargets="ResolveAssemblyReferences"
+			Condition="'$(UnoDisableUNOB0002Validation)'==''">
+
+		<PropertyGroup>
+			<_UnoPlatformProperty>_IsUnoPlatform=true</_UnoPlatformProperty>
+		</PropertyGroup>
+
+		<!--
+		Determine if any ProjectReference contains AdditionalProperties which contains _IsUnoPlatform, a property
+		defined only when WinAppSDK is not included.
+
+		This scenario can happen when a WinAppSDK project is referencing a net7.0-only project which contains a reference to
+		Uno.WinUI, which is not compatible with WinAppSDK. Fixing this requires adding a netX.0-windows10.x target to the project.
+		-->
+		<Error Code="UNOB0002"
+			   HelpLink="https://aka.platform.uno/UNOB0002"
+			   Text="Project %(TargetPathWithTargetPlatformMoniker.MSBuildSourceProjectFile) contains a reference to Uno Platform but does not contain a WinAppSDK compatible target framework. https://aka.platform.uno/UNOB0002"
+			   Condition="
+				'%(TargetPathWithTargetPlatformMoniker.MSBuildSourceProjectFile)'!=''
+				and '%(TargetPathWithTargetPlatformMoniker.AdditionalPropertiesFromProject)'!='' 
+				and '%(TargetPathWithTargetPlatformMoniker.AdditionalPropertiesFromProject)'!='%(TargetPathWithTargetPlatformMoniker.AdditionalPropertiesFromProject.Replace($(_UnoPlatformProperty),))'" />
+	</Target>
+
 </Project>

doc/articles/get-started-dotnet-new.md

@@ -1,3 +1,6 @@
+---
+uid: GetStarted.dotnet-new
+---
 # dotnet new templates for Uno Platform
 
 The Uno Platform provides a set of command-line templates to create cross-platform applications.

doc/articles/guides/how-to-create-control-libraries.md

@@ -1,3 +1,6 @@
+---
+uid: Guide.HowTo.Create-Control-Library
+---
 # How to Create Control Libraries
 
 Uno Platform, like WinUI and UWP, supports Control Libraries. Control Libraries are a way to reuse UI components across multiple projects, either inside the solution or by using NuGet to distribute to other projects.

doc/articles/uno-build-error-codes.md

@@ -0,0 +1,20 @@
+---
+uid: Build.Solution.error-codes
+---
+# Uno Build error codes
+
+## UNOB0001: Cannot build with both Uno.WinUI and Uno.UI NuGet packages referenced
+
+This error code means that a project has determined what both `Uno.WinUI` and `Uno.UI` packages are referenced.
+
+To fix this issue, you may be explicitly referencing `Uno.UI` and `Uno.WinUI` in your `csproj`, or you may be referencing a NuGet package which is incompatible with your current project's configuration.
+
+For instance, if your project references `Uno.WinUI`, and you try to reference `SkiaSharp.View.Uno`, you will get this error. To fix it, you'll need to reference `SkiaSharp.View.Uno.WinUI` instead.
+
+## UNOB0002: Project XX contains a reference to Uno Platform but does not contain a WinAppSDK compatible target framework.
+
+This error code means that a WinAppSDK project is referencing a project in your solution which is not providing a `net6.0-windows10.xx` TargetFramework.
+
+This can happen if a project contains only a `net7.0` TargetFramework and has a NuGet reference to `Uno.WinUI`.
+
+To fix this, it is best to start from a `Cross Platform Library` project template provided by the Uno Platform [visual studio extension](xref:Guide.HowTo.Create-Control-Library), or using [`dotnet new`](xref:GetStarted.dotnet-new).