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\.

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).

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.

@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?

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

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

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 🔥

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.

Any updates on this issue?

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

@ChrisMcKee In core MSBuild or full MSBuild?

It works in vs for me, but not calling dotnet build manually.

@Thealexbarney Windows, so full. I'm not sure you could target .net 2 on non-win platforms as it would lack the required build targets. 2/3.5/4/4.5/4.6 predating it all. You're back in mono-land there.

Also seeing this. Can build in VS2017, but dotnet build gives:

MSB3644: The reference assemblies for framework ".NETFramework,Version=v3.5" were not found.

@drewnoakes use msbuild

I'm getting the error with .NET 2.0. I'm trying to do dotnet pack. Is there a workaround for packaging NuGet packages that will work?

@JoshClose you can directly call

msbuild /t:Pack /p:Configuration=Release

For version suffixes, add /p:VersionSuffix=beta-123 (however, note NuGet/Home#4337)

How can I change the package output location? Sorry, I'm no too familiar with msbuild.

/p:PackageOutputPath=path\to\dir\

I got this working by adding the following to the .csproj file:

<PropertyGroup>
  <FrameworkPathOverride Condition="'$(TargetFramework)' == 'net35'">$(MSBuildProgramFiles32)\Reference Assemblies\Microsoft\Framework\.NETFramework\v3.5\Profile\Client</FrameworkPathOverride>
</PropertyGroup>

Unfortunately, the "normal" path C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v3.5 doesn't work in this case because it doesn't have a copy of mscorlib.dll.

Update

This can now be addressed using the Microsoft.NETFramework.ReferenceAssemblies NuGet package. Click through to the "Project Website" for documentation.

Yeah, that doesn't work for me, as there is no 3.5 version of System.Web available anywhere in that folder structure. 😞

@bradwilson Did you ever find a resolution for this? I'm in the same boat.

@JohnHBrock No. There is no solution.

@bradwilson This seems to work OK, but I haven't had time to thoroughly test it:

  <ItemGroup>
    <Reference Condition=" '$(TargetFramework)' == 'net35' " Include="System.Web" HintPath="$(WINDIR)\Microsoft.NET\Framework64\v2.0.50727\System.Web.dll"  />
  </ItemGroup>

This includes System.Web.dll as a dependency in the overall project by pulling it from the .NET 2.0 directory, and, combined with the @NightOwl888 workaround, seems to make dotnet build -f net35 succeed.

EDIT: A possible problem I've found with this approach: There are two versions of System.Web.dll under $(WINDIR)\Microsoft.NET\, one under Framework64\v2.0.50727\ (built for x64) and another under Framework\v2.0.50727\ (built for x86). In other words, System.Web.dll isn't built for Any CPU (aka MSIL). So if your project needs to target Any CPU, the lack of an Any CPU System.Web.dll poses a problem.

@JohHBrock have you tried to copy files and directories from 'C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\v3.5*' to 'C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework.NETFramework\v3.5'?

The plan to address this is to use an updated Microsoft.NETFramework.ReferenceAssemblies package that delivers .NET 3.5 once that exists (dotnet/installer#2022). That already works for .NET 2.0.

Since this can be done without MSBuild changes, I'm going to close this.