未定义或导入预定义类型“System.ValueTuple`2”

Question

I have a .NET solution file with about 20 C# projects. All of them are targeting .NET Framework 4.6.2. Within a couple of projects, I can easily write var t = ("hello", "world"); and the compiler is happy. But within some projects of the same solution, the compilation fails with Predefined type System.ValueTuple`2 is not defined or imported for the exact same piece of code.

I've already checked the following things:

• There is no mscorlib.dll anywhere in the solution or project directories.
• bin and obj have been deleted several times.
• Problem can be reproduced on a colleagues computer.
• Not a single project is using the NuGet package System.ValueTuple .
• I did a text search across all *.csproj files for ValueTuple without any match.

I know that there are dozens of articles related to this error. But most of them are just saying: install the NuGet package or update your .NET Framework version. Unfortunately, none of them seems to cover the problem I'm encountering. Rather than installing or upgrading anything, I'd like to understand the behavior.

Answer 1

I finally found the culprit: there is one project targeting .NET Standard 2.0. Every .NET Framework project referencing the .NET Standard project automatically gets a reference to System.ValueTuple.dll from the GAC (in my case, this is .NET Framework 4.7.2).
Each project without referencing the .NET Standard project does not have the reference. Therefore, the snippet var t = ("hello", "world"); cannot be used.