Build error when referencing .NET Framework 4.5 class libraries

[Daniel15]

Repro:

1. Create a new ASP.NET 5 project using the empty template
2. Modify project.json so it only targets .NET 4.5.1 (dnx451 in frameworks section)
3. Add a new class library targeting .NET Framework 4.5 to the solution
4. Add a reference to the class library in the website (through Visual Studio Add References UI)
5. Modify the website's Startup.cs to use a class from the class library (eg. var foo = new ClassLibrary1.Class1();)
6. Attempt to build the solution

This results in error CS0246 ("The type or namespace name 'ClassLibrary1' could not be found (are you missing a using directive or an assembly reference?)") in Visual Studio. However, this just appears to be a design-time issue, as the site does actually execute properly and there are no errors in the "Output" tab.

Note: This appears to be specific to .NET Framework 4.5. Switching the class library to use .NET Framework 4.0 makes it work as expected (no build errors)

Update: Okay I think I figured this out. My repro steps weren't 100% correct and I actually missed a step when I wrote them out. The issue is that when you change the target framework version for an assembly (such as a Class Library), its corresponding project.json in the wrap directory is not updated (eg. to change net40 to net45). I encountered this since I changed a library from .NET Framework 4.0 to .NET Framework 4.5.

Workaround: Manually update the project.json files, or delete the wrap directory and re-add the reference in Visual Studio.

[sayedihashimi]

@BillHiebert thoughts?

[Daniel15]

Did some more testing, and it looks like using .NET Framework 4.6 in both the site and the class library appears to work, as does using .NET Framework 4.0 in the class library combined with .NET Framework 4.5 in the website. It just seems to be an issue when using .NET Framework 4.5 in the class library.

Another related bug while I was testing this: Selecting ".NET Framework 4.6" in the "New Project" dialog in VS2015 still sets dnx451 in project.json, I needed to manually change it to dnx46

Edit: Now I'm not so sure - Using .NET Framework 4.6 both worked in my small test project, but didn't work in my real larger site, I'm still seeing a lot of compilation "errors" in VS that aren't really errors:

(notice it says "Build Succeeded" at the bottom. The site does work properly if I run it)

[Daniel15]

Okay I think I figured this out. My repro steps weren't 100% correct and I actually missed a step when I wrote them out. The issue is that when you change the target framework version for an assembly (such as a Class Library), its corresponding project.json in the wrap directory is not updated (eg. to change net40 to net45). I encountered this since I changed a library from .NET Framework 4.0 to .NET Framework 4.5.

Workaround: Manually update the project.json files, or delete the wrap directory and re-add the reference in Visual Studio.

[sayedihashimi]

TFS:149636
I've added this to our internal TFS, we will reply back here soon.

[webtools-bot]

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[Daniel15]

This works fine in VS2017 with the latest tooling