Microsoft Bot Framework swears at the $ symbol in the default code, why?

Question

C++ / C#

Studio 2013 Pro
When creating a project in the Microsoft Bot Framework, the code generated is
int length = (message.Text ?? string.Empty).Length;
// return our reply to the user
return message.CreateReplyMessage($"You sent {length} characters");
When compiling, it swears at the $ symbol.
What is the problem, I understand that this is most likely an argument for the CreateReplyMessage function, but I don’t understand why it swears.

Answer 1

Change to string format.

Answer 2

The very first line in Google: stackoverflow.com/questions/27093908/how-to-enable...
After installing the nugget, VS can highlight the code as an error, but it still builds.