[Question] Using UWP APIs from .NET Standard

[Neme12]

I have a PCL targeting .NET Framework 4.6 & UWP 10.0.10240 that I'm trying to convert to .NET Standard. It is actually used to share code that uses some UWP APIs that are available on desktop as well. I currently have these references in the project file:

<Reference Include="System.Runtime.WindowsRuntime">
  <HintPath>$(MSBuildProgramFiles32)\Reference Assemblies\Microsoft\Framework\.NETCore\v4.5\System.Runtime.WindowsRuntime.dll</HintPath>
</Reference>
<Reference Include="Windows">
  <HintPath>$(MSBuildProgramFiles32)\Windows Kits\10\UnionMetadata\Windows.winmd</HintPath>
</Reference>

Based on the target frameworks, I figured that my desired version of .NET Standard would be 1.3. However I'm getting compilation errors about missing mscorlib whenever I use any of the UWP APIs.

This is the new csproj:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netstandard1.3</TargetFramework>
  </PropertyGroup>
  <ItemGroup>
    <Reference Include="System.Runtime.WindowsRuntime">
      <HintPath>$(MSBuildProgramFiles32)\Reference Assemblies\Microsoft\Framework\.NETCore\v4.5\System.Runtime.WindowsRuntime.dll</HintPath>
    </Reference>
    <Reference Include="Windows">
      <HintPath>$(MSBuildProgramFiles32)\Windows Kits\10\UnionMetadata\Windows.winmd</HintPath>
    </Reference>
  </ItemGroup>
</Project>

and a small example:

using Windows.Storage;

class C
{
    void M()
    {
        var folder = KnownFolders.PicturesLibrary; // error CS0012: The type 'Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.
    }
}

I noticed that if I target netstandard2.0 instead, the compilation succeeds. But since I'm trying to target all versions of UWP, I can't afford that. Is it impossible to use these APIs from 1.3? Are PCLs the only option? Thank you (Sorry if this is not the appropriate repo)

[weshaggard]

Unfortunately there isn't any first class supported way of consuming UWP APIs in PCL or .NET Standard projects today. The approach of referencing System.Runtime.WindowsRuntime from another target framework is a workaround at best and isn't guaranteed to work.

I've not tested this but you might get pretty far by referencing the nuget package https://www.nuget.org/packages/System.Runtime.WindowsRuntime/.

[Neme12]

@weshaggard

Unfortunately there isn't any first class supported way of consuming UWP APIs in PCL or .NET Standard projects today.

Sorry if I wasn't clear enough. I had been consuming UWP APIs from PCLs for quite some time by adding the references in the first snippet, and everything worked properly. The problem was that this didn't work in .NET Standard which I wanted to migrate to from PCLs. But now I was able to make it work by adding a package reference to Microsoft.NETCore.Portable.Compatibility, so the issue is resolved.

The approach of referencing System.Runtime.WindowsRuntime from another target framework is a workaround at best and isn't guaranteed to work.

The problem wasn't just with System.Runtime.WindowsRuntime, the APIs from winmd files didn't work either (until I added Microsoft.NETCore.Portable.Compatibility).

I've not tested this but you might get pretty far by referencing the nuget package nuget.org/packages/System.Runtime.WindowsRuntime.

Thanks, I'll use that instead of an assembly reference. But this package wasn't made for this scenario, only for being consumed from UWP or desktop, is that right? So what's the purpose of it? What benefits does it have over an assembly reference, which seems to be the recommended way by reading the docs:
https://docs.microsoft.com/en-us/windows/uwp/porting/desktop-to-uwp-enhance

EDIT: Actually, using the System.Runtime.WindowsRuntime NuGet package doesn't work. I'm getting an exception at runtime

System.IO.FileNotFoundException: 'Could not load file or assembly 'System.Runtime.WindowsRuntime, Version=4.0.11.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' or one of its dependencies. The system cannot find the file specified.'

I'll keep using the references, which seem to be working just fine.

[Neme12]

I actually do this for all UWP libraries even if they're not shared and only used from UWP so that I can use a regular .NET project and most importantly a regular .NET unit test project as opposed to the UWP one, which not only requires an app package, but also runs in a window:

I don't think I have to explain how slow and cumbersome that is. I'm happy I can avoid all that pain (and stick to .NET wherever possible).

[weshaggard]

I didn't release we had documented https://docs.microsoft.com/en-us/windows/uwp/porting/desktop-to-uwp-enhance, while that helps folks get something working it is still only a weak foundation but it is the best we have at this point.

Yes Microsoft.NETCore.Portable.Compatibility does help in some of the older PCL and .NET Standard projects because it has an mscorlib ship which is necessary to correctly resolve any of the core types from Windows.winmd.

I'm glad that you got your project working and I'm hopefully we will improve the tooling story around this in the future.