The C# Interactive window has been made available again in Visual Studio with the first update to 2015, this time as a REPL window. You can type or paste and execute C# code, and it includes support for adding references to external DLLs and using namespaces. The window is intended for rapid prototyping C# code.
For references to additional DLLs (whether from the framework or custom) you use the #r command specifying the path to the module.
After you referenced a module you can use its namespaces and the types available there.
For a list of all commands use the #help command.