The type or namespace name 'Twilio' could not be found

So, you have finally joined the exciting new world of ASP.NET 5. 
Congratulations!  And welcome!

You may be running Visual Studio 2015 - or maybe Visual Studio Code - and creating new DNX projects and ASP.NET 5 applications that target the latest .NET Framework 4.6 or .NET Core 5, running on DNX, and compiling with RyuJIT or the Roslyn platform.
Good for you!

So, let's say that you writing a DNX project that makes use of the Twilio SMS API Client.
If you're reading this blog, then you probably already know that Twilio SMS is an API that lets you programmatically send, receive and track text messages globally.

Your code, in simple terms, will look like the one below

Your logic looks solid. 
You instantiate your Twilio client with your Account Sid and Auth Token, and you invoke the SendMessage service by passing in the appropriate parameters.

So why then, does your build fail and the Error List pane show the error below?

The type or namespace name 'Twilio' could not be found (are you missing a using directive or an assembly reference?)

How can this be? 
You already have the correct Using statement.
And you have already installed the Twilio package through NuGet.

Well, what's happening here can be explained by looking at your "project.json" file.

Here, we see that both the "dnx451" (.Net Framework) and "dnxcore50" (.Net Core 5) are being targeted.
However, .Net Core 5 does NOT support the Twilio SDK.  Ergo the "type or namespace" error.

There are a couple of ways to resolve this issue.
The first is to simply remove the "dnxcore50" from the list of targeted frameworks in the project.json file.

This is a simple and straightforward solution.
However, it does exclude the .NET Core 5 framework from your project altogether, and that may not be your intent.

The second alternative is something of a "shim". 
Using a compiler directive, you can exclude any code that "dnxcore50" does not support.

In this way, you are still targeting the .NET Core 5 framework, but at the same time,
your app does not break on any code that the framework does not support.

Happy .Net coding!