Unity3 simple C# self-written DLL

This is my DLL:

using System;
public class Class1
{
}

I put this in my Unity project's project folder and I get the following error:

Internal compiler error. See the console log for more information. output was: Unhandled Exception: System.TypeLoadException: Could not load type 'System.Runtime.Versioning.TargetFrameworkAttribute' from assembly 'test'.

Thanks in advance !

Make sure you compile the DLL in a .NET FrameWork Unity can handle.

doesnt need the " : MonoBehaviour" herency?

using UnityEngine;
using System;
public class Class1 : MonoBehaviour
{
}

i think thats the problem