PRI: Improve error message when referencing WinUI3 from net5.0 project

[dotMorten]

Describe the bug
The build error you receive when incorrectly targeting net5.0 instead of net5.0-windows10.0.xxxxx is very unhelpful:

Error	MSB4086	A numeric comparison was attempted on "$(TargetPlatformVersion)" that evaluates to "" instead of a number, in condition "'$(TargetPlatformVersion)' >= '10.0.11000.0'".	
ClassLibrary4	C:\Users\mn\.nuget\packages\microsoft.winui\3.0.0-preview3.201113.0\buildTransitive\Microsoft.ApplicationModel.Resources.PriGen.targets	350	

VS also shows the following tooltip error:

This issue was reported by a user in the WinUI Discord channel, and didn't know what to do at first. A better error message would have saved the person a lot of time.

Steps to reproduce the bug

1. Create a new .NET Core class library
2. Change Target Framework to net5.0
3. Add WinUI3 package
4. Compile

.csproj:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net5.0</TargetFramework>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.WinUI" Version="3.0.0-preview3.201113.0" />
  </ItemGroup>
</Project>

Expected behavior
A helpful error that informs the user that they need to retarget.

As an example, see the PR I did for RX.NET here to help users target the correct thing: dotnet/reactive#1442
(just put the .targets file in the net5.0 folder instead of net5.0-windows`

Version Info

NuGet package version:
[Microsoft.WinUI 3.0.0-preview3.201113.0]

Windows app type:

UWP	Win32
No	No

Windows 10 version	Saw the problem?
Insider Build (xxxxx)
May 2020 Update (19041)
November 2019 Update (18363)
May 2019 Update (18362)
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop
Xbox
Surface Hub
IoT

Additional context

The main problem is probably that the .targets files aren't in the proper \build\net5.0-windows10.[minversion] folder, so it gets included in every target framework. Once that has moved to a subfolder, the error should go away (as the targets aren't included), and the approach used in the PR referenced above can be used to improve the error message.

[stevenbrix]

I transferred this issue from the WinUI repo since this is an issue with the PRI gen targets. I agree that a better error message telling the user what they need to do would be better.

[riverar]

Wanted to add I also hit this with a WPF desktop application project (net5) with the PR metapackage.

[janseris]

@stevewri Any news? I encountered this error today in a MAUI Preview 14 app and I don't know what to do.

[dotMorten]

@janseris this really shouldn’t be an issue with Maui. Are you not multitargeting your Maui app?

[janseris]

@janseris this really shouldn’t be an issue with Maui. Are you not multitargeting your Maui app?

I am taking this project: https://github.com/jasho/cookbook-maui
And trying to run on Windows Machine.
No modifications to the project or source code are done.
And this appears:

When I add net6.0-windows to the .csproj file,a new error - the one from this issue appears.
I am adding it hoping I solve the previous error because I read that the list there is the list of platforms for which the MAUI project will be compiled and I thought Windows was missing. That might not be the case because the default MAUI template does not include net6.0-windows in that list.

The project 'CookBook.Mobile' ran into a problem during the last operation: A numeric comparison was attempted on "$(TargetPlatformVersion)" that evaluates to "" instead of a number, in condition "'$(TargetPlatformIdentifier)' == 'Windows' and
                                              '$(TargetPlatformVersion)' >= '8.0'".  C:\Users\janse\.nuget\packages\microsoft.windowsappsdk\1.0.0\build\Microsoft.UI.Xaml.Markup.Compiler.interop.targets You may need to reload the solution after fixing the problem. 

I have no idea what causes the issue because I checked the default MAUI Preview 14 (latest) project template .csproj contents and they seem to match the linked project .csproj MAUI project file.

[dotMorten]

@janseris ? That’s a different error than what I reported here

[janseris]

@janseris ? That’s a different error than what I reported here

A numeric comparison was attempted on "$(TargetPlatformVersion)" that evaluates to "" instead of a number, in condition "'$(TargetPlatformIdentifier)' == 'Windows' and '$(TargetPlatformVersion)' >= '8.0'".

A numeric comparison was attempted on "$(TargetPlatformVersion)" that evaluates to "" instead of a number , in condition "'$(TargetPlatformVersion)' >= '10.0.11000.0'".

I thought these two shared the Platform version number parsing issue.

[tubaplayerdis]

For me fixing this issue was in the line with net6.0-windows.
For me the application was looking for a target verion and i had to put that in my like this:
net6.0-windows10.0.19041.0

[hdufortlaumie]

I've a similar issue with WinUI and dotnet 7.
I've create a new WinUI3 project , flipping starget Framework to .Net 7 instead of .Net 6. raise this error:

2022-11-16 15:49:20 Critical Project 'App1' load failed| [MSB4086] A numeric comparison was attempted on "$(TargetPlatformVersion)" that evaluates to "" instead of a number, in condition "'$(TargetPlatformVersion)' < '10.0.18362.0'". C:\Users\hugod.nuget\packages\microsoft.windowsappsdk\1.2.221109.1\buildTransitive\Microsoft.InteractiveExperiences.Common.targets at (15:27)

I'm using Visual Studio Pro v17.4.1

[mzanm]

Same, switching target framework to dotnet 7 does not work even though it's supposed to be supported.

[francodelucchi]

For me fixing this issue was in the line with net6.0-windows. For me the application was looking for a target version and i had to put that in my like this: net6.0-windows10.0.19041.0

You saved me with this post, thanks! :-) Exactly the same problem, fixed this way!

[JohnGomes]

For me fixing this issue was in the line with net6.0-windows. For me the application was looking for a target verion and i had to put that in my like this: net6.0-windows10.0.19041.0

Yup, I had to follow a similar formula -- net{VersionNumber}.0-windows{TargetPlatformMinVersion}
net7.0-windows10.0.17763.0

[AnthonyAlexandre]

JohnGomes... you saved the day... It worked on my Windows App SDK. Now is asking to upgrade some dependency class libraries... i wil do anyways... :)