Why is this NuGet dependency missing when compiling .NET Framework project depending on .NET Standard?

I have a Visual Studio solution, with 3 projects.

The top level is a .NET Framework 4.6.1 Console App (Project A). It depends on a .NET Framework 4.6.1 Class Library (Project B). Project B depends on a .NET Standard 2.0 Class Library (Project C).

I have some code in Project C that uses System.Data.SqlClient (NuGet package version 4.6.1).

Due to the following known issue https://github.com/dotnet/sdk/issues/901 I have also added System.Data.SqlClient as a NuGet dependency to Project B (the .NET Framework Class Library).

This is Scenario 1, and when the solution is built, System.Data.SqlClient is copied to the /bin/Debug folder of Project A and the the application runs successfully.

The code for Scenario 1 is here https://github.com/JamesDunlop/TestDependencyFlowsNetStandard

However, for Scenario 2, I now ADD a project reference to Project A such that it now also directly references/depends-on Project C (i.e. the .NET Standard Class Library), as well as Project B. This mimics what I will need to do in a legacy application.

Clean, and rebuild and run. System.Data.SqlClient is now missing from the /bin/Debug folder of Project A, and at run time there is an exception "System.IO.FileNotFoundException: 'Could not load file or assembly 'System.Data.SqlClient"

Why does the System.Data.SqlClient not get copied to /bin/Debug ?

Note that I have chosen NOT to migrate the .NET Framework projects to PackageReferences in order to resolve the issue https://github.com/dotnet/sdk/issues/901, since I need to implement this in a large legacy ASP.NET solution where it is not feasible.

I would expect that adding a reference to Project C would have little effect, other than (as observed) it results in a lot more type-forwarding DLLs being copied to the /bin/Debug folder. But I would not expect System.Data.SqlClient to now be missing.

回答:

I'll repeat my comment above here, as it is considered valid as an answer.

The MSBuild log, with its build output verbosity set to level detailed, gives more insights in what happens.

Scenario 1 (A referencing B, B referencing C)

The build log shows that project A successfully resolved its System.Data.SqlClient dependency from the \bin\debug folder of project B and copies it locally.
(As project B is a .NET Framework class library, its NuGet dependencies do get copied to its bin folder.)

Dependency "System.Data.SqlClient, Version=4.5.0.1, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".
  Resolved file path is "C:\...\TestDependencyFlows.Library\bin\Debug\System.Data.SqlClient.dll".

Scenario 2 (A referencing B and C, B referencing C)

The build log mentions that project A tries to resolve its System.Data.SqlClient dependency from the NET Standard project C (and some wellknown folders), but not anymore from project B.
(Because project C is a NET Standard project, it doesn't copy its NuGetdependencies to its bin folder.)
All these attempts fail with the message that the file doesn't exist at these locations.

Dependency "System.Data.SqlClient, Version=4.5.0.1, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".
  Could not resolve this reference. Could not locate the assembly "System.Data.SqlClient, Version=4.5.0.1, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a". 
  Check to make sure the assembly exists on disk. If this reference is required by your code, you may get compilation errors.
  For SearchPath "C:\...\TestDependencyFlows.Library.NetStandard\bin\Debug\netstandard2.0".
      Considered "C:\...\TestDependencyFlows.Library.NetStandard\bin\Debug\netstandard2.0\System.Data.SqlClient.winmd", but it didn't exist.
      Considered "C:\...\TTestDependencyFlows.Library.NetStandard\bin\Debug\netstandard2.0\System.Data.SqlClient.dll", but it didn't exist.
      Considered "C:\...\TestDependencyFlows.Library.NetStandard\bin\Debug\netstandard2.0\System.Data.SqlClient.exe", but it didn't exist.
      ...