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When building dot net framework 4.7.2 projects (without any changes), we never see the following line:

========== Build: 0 succeeded, 0 failed, 1 up-to-date, 0 skipped ==========

The projects always Builds. We expect the project only builds after changing anything, or after issuing a 'Clean' command.

The reason from the detailed logs is:

1>Project 'Project1' is not up to date. CopyLocal reference 'C:\VSTS\Project1\bin\Debug\System.IO.dll' is missing from output location.
1> Encountered conflict between 'Reference:System.IO, Version=4.1.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL' and 'Platform:System.IO.dll'. Choosing 'Platform:System.IO.dll' because AssemblyVersion '4.1.2.0' is greater than '4.1.1.0'. (TaskId:14)

It seems like the build process decides to not copy the System.IO to the output directory; because a platform System.IO.dll exists with a greater version number.

The check for any changes in the project does not recognize this situation; so it comes to the wrong conclusion (the project is not up to date).

The check for any changes in the project should recognize the situation when a NuGet dll is not copied because a platform dll exists.

This problem costs our team a lot of valuable time. Our solution includes over 75 projects; and it takes several minutes to start a debugging session; even when nothing has changed.

To reproduce the issue:

• Create a Dot Net Framework 4.7.2 class library.
• Add System.IO NuGet package.
• Build the project.
• Change nothing and Build the project again. The expected behaviour is: the project is not build because it is up to date. But the project builds anyway.