title: Manually adding .NET 4.5 scripts to Unity: Potential issues?
body: <p>I've recently had to some work in Unity that required classes only available in the later versions of .NET, which Unity does not yet support. I managed to download the very few scripts that I was missing and added it to my project and it seems to work fine. What I wanted to know is why can I not just take the entire .NET source and shove it into my projects? What are the implications of doing something like that?</p>
  
  <p>EDIT: The scripts that I have added so far was the generic version of WeakReference as well as any dependencies that it needed (Contract in System.Diagnostics.Contract). </p>
  
  <p>EDIT EDIT: I basically want to know why I can't simply just the C# class files found in the later versions of .NET to Unity, which uses an older version of .NET. Is it a software issue? Is it a hardware issue? Is it because Unity's system components were built in correspondence with the earlier version of .NET?</p>