Cannot find reference assemblies for .NET 3.5 or lower using core msbuild

[nguerrera]

dotnet/sdk supports targeting .NET 3.5 just fine if you use desktop msbuild, but it fails to find the reference assemblies using core msbuild. Both desktop and core work fine for NET4.0+.

Steps to reproduce

Create a new project with dotnet new
Change the TargetFramework to net35 or lower
Add <RuntimeIdentifier>win10-x64</RuntimeIdentifier> to the .csproj
dotnet restore
dotnet build

I've been seeing this issue since the move to msbuild. Targeting .NET 3.5 and lower works in the Preview 2 release SDK. All the relevant targeting packs are installed.

Expected behavior

A successful build

Actual behavior

Fails with the error:
... error MSB3644: The reference assemblies for framework ".NETFramework,Version=v3.5" were not found. To resolve this, install the SDK or Targeting Pack for this framework version or retarget your application to a version of the framework for which you have the SDK or Targeting Pack installed. ...

Environment data

dotnet --info output:

.NET Command Line Tools (1.0.0-preview3-004056)

Product Information:
Version: 1.0.0-preview3-004056
Commit SHA-1 hash: ccc4968

Runtime Environment:
OS Name: Windows
OS Version: 10.0.14393
OS Platform: Windows
RID: win10-x64

Moved from https://github.com/dotnet/cli/issues/4626 for @Thealexbarney.

Moved from dotnet/sdk#369 for @piotrpMSFT

@rainersigwald

[dsplaisted]

I think the problem is that there aren't any reference assemblies for .NET Framework 3.5. The only thing in my .NETFramework\3.5 reference assemblies folder is Profile\Client. When I target .NET 3.5 from a Class Library, the references are coming from C:\Windows\Microsoft.NET\Framework\v2.0.50727\.

[rainersigwald]

This comes down to code in FrameworkLocationHelper.GetPathToDotNetFramework that is disabled on .NET Core: https://github.com/Microsoft/msbuild/blob/63cf735deb821969cb096056c0c429c78cc76fbb/src/Shared/FrameworkLocationHelper.cs#L1308-L1310

There are some (probably avoidable) registry calls there, but the biggest obstacle seems to be that finding the v3.5 (or whatever) runtime depends on locating the current runtime (Path.GetDirectoryName(typeof(object).Module.FullyQualifiedName)) and looking around it to find the desired version of the framework. That won't work on Core where object comes from somewhere else entirely.

There may be another way to locate a framework (looks like HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\NET Framework Setup\NDP\v3.5 has an InstallPath value that looks promising) but we'd have to figure out the right mechanism (and test the heck out of it).

[KevinH-MS]

What's the work-around for people who need to build such projects on VS 2017? I just ran into this today, and the only thing I could figure out was to compile on a separate machine with only VS 2015 installed.

[rainersigwald]

@KevinH-MS It sounds like you're seeing something different from this issue--as originally reported, projects build fine using MSBuild.exe. Can you describe your problem in more detail?

[KevinH-MS]

I see. I think that's the answer to my question... 😄

(don't use dotnet build -f net35, use msbuild /p:TargetFramework=net35)

[ChrisMcKee]

Issues a pain when you're trying to generate a multi-target nuget package;
prior to the great cheese moving of 2016 I had no issues building a package that targeted 2/3.5/4/4.5/etc
Is this likely to be sorted in the next few days when VS17s released? Seems ambitious...

Admittedly I'm presuming this is msbuild related as it came with RC4 / VS2017 changes and the issue in the error being thrown is the same as the op (albeit for every version specified).

<PropertyGroup>
      ...  <TargetFrameworks>net20;net35;net40;net45;net451;net452;net462;netstandard1.3;netstandard1.6;netstandard1.6.1;netcoreapp1.1</TargetFrameworks>
    ...

etc https://github.com/BcryptNet/bcrypt.net/blob/master/src/BCrypt.Net/BCrypt.Net.csproj

Hell, it might be that after all these changes and dotnet versions I need to set fire to my PC and start afresh to get things to build 🔥

[Thealexbarney]

Right now the only options I see are:

1. Stay on the old project.json system. (Can't use VS2017 as an IDE)
2. Migrate to msbuild and be dependent on the full msbuild. (Can't build with dotnet SDK)
3. Drop <= net35 support.

I guess legacy support is a lower priority than other things for now.

Ideally (IMO) there would be targeting packs on NuGet for each framework version, making it easier to target .NET Framework from the dotnet SDK.

Our use case at work: targeting net20 is important for us as we have to make some of our libraries work on old Windows Server 2003 systems. We can't upgrade them right now. And we don't want to install anything (say, newer .NET framework versions) to avoid risks of breaking the systems.

We love dotnet tooling, but while this issue is not fixed, we'll have to stay on the full msbuild for this use case.

[schotime]

Any updates on this issue?

[ChrisMcKee]

@schotime I've been using RTW since it came out last week and this seems to be resolved.