New .csproj cannot reference GAC assemblies

From @jnm2 on March 13, 2017 20:31

You can no longer reference some GAC assemblies without specifying a HintPath, at which point there isn’t much point in using the GAC at all.

Class1.cs:

using System;
using Microsoft.SolverFoundation.Services;

public class Class1
{
    public static readonly Type X = typeof(ISolver);
}

Old .csproj, working:

<Project ToolsVersion="15.0">
  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" />
  <PropertyGroup>
    <OutputType>Library</OutputType>
    <TargetFrameworkVersion>v4.6.2</TargetFrameworkVersion>
    <OutputPath>bin\$(Configuration)\</OutputPath>
  </PropertyGroup>
  <ItemGroup>
    <Reference Include="Microsoft.Solver.Foundation" />
  </ItemGroup>
  <ItemGroup>
    <Compile Include="*.cs" />
  </ItemGroup>
  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
</Project>

New .csproj fails to find the reference:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net462</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <Reference Include="Microsoft.Solver.Foundation" />
  </ItemGroup>

</Project>

This also does not work: <Reference Include="Microsoft.Solver.Foundation, Version=3.0.2.10889, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />.

Why is this broken?

Copied from original issue: dotnet/roslyn-project-system#1739

[davkean]

From @jnm2 on March 13, 2017 23:11

Adding this to the bottom of the new csproj and reopening the solution fixes all problems:

  <!-- Workaround for https://github.com/dotnet/roslyn-project-system/issues/1739 -->
  <PropertyGroup>
    <AssemblySearchPaths>$(AssemblySearchPaths);{GAC}</AssemblySearchPaths>
  </PropertyGroup>

[davkean]

In SDK-based projects {GAC} is no longer part of the search paths. GAC is a runtime-lookup location, not a build-time lookup location. This is a deliberate change - it resulted in a huge number of bugs where projects would accidentally pick up the wrong dependencies on build machines because a targeting pack or SDK wasn’t installed. I suspect when you add a version number, you are resolving it from a build-time location such as {AssemblyFoldersEx} instead of the GAC. Looking at the build log would help us figure that out.

As you found, the workaround is to explicitly add {GAC} to the set of search paths.