Trying to add new Type to Dotnet Runtime from github

Hi I'm trying to modify the .NET runtime from github.com/dotnet/runtime.

I created a copy of the Dictionary class called Dictionary2 in these source code files:

C:\rt5\src\libraries\System.Collections\ref\System.Collections.cs

C:\rt5\src\libraries\System.Private.CoreLib\src\System\Collections\Generic\Dictionary.cs

When I use dot peek from Jetbrains, Dictionary2 exists in my System.Collections.dll, but when I import System.Collections.dll in my main console C# application, Dictionary2 gives me compiler errors.

Am, I missing some code somewhere?

Can you guys help me out here?

Thanks!