Trying to use .NET core app in desktop app

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.NET

C#3.5

I have 2 problem.
In short:
1. problem 1, use the .NET core from a full framework app
2. write a .NET Core app which target platform specific code

Problem 1
see "what I have tried"

Problem 2
I don't have any code to show here.
But at work we target Android, iOS and .NET SDK using PCL. I was wondering if can write a .NET Core library that somehow get to use platform specific implementation of whatever feature... I am not sure how to go about it....

What I have tried:

Problem 1:
I have 2 very simple projects.

A .NET Core library as follow (whole source code)

namespace ClassCoreLib1
{
    public static class Class1
    {
        public static string Answer() { return "42"; }
    }
}

and a .NET 4.5.2 console app as follow (whole source code)

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            var s = ClassCoreLib1.Class1.Answer();
            Console.WriteLine(s);
        }
    }
}

Now I try, as you can see to use the .NET Core library with the full .NET framework console app

I did modified the .NET Core library's project.json as follow, I added:

"net451": {
  "dependencies": {
    "Microsoft.NETCore.Platforms": "1.0.1"
  }

And it did create a .NET output folder and when I study the .DLL inside it it seems to depends on mscorlib from the GAC, .NET 4.5.1, so it should be right, I think...

Now... if I copy the .DLL in the net452 output folder outside the ClassCoreLib folder.. then I am able to link against it and run! Yahoo! :)

However I can't compile by linking against the project or even directly referencing the .dll by its path "..\..\src\ClassCoreLib1\bin\Debug\net451\ClassCoreLib1.dll" fails to compile...

Summary
Having to copy the .dll after each successfull build is quite unproductive and error prone.... I wonder if there is a better solution?