Write your first .NET Core Library

24 May 2016

There has been a recent surge in information and all new things .NET Core, mostly due to the release of RC2. There is a lot of great excitement surrounding .NET Core these days, and I think it is much warranted. It is turning out to be a great tool and platform, and I can personally say that I’m enjoying .NET Core already.

One of the things I’ve recently been playing around with is writing a library that is meant to be consumed by .NET Core. I love the idea of modularity and units of single-responsibility, no matter if we’re talking about npm in the JavaScript world, nuget in the .NET world, or whatever medium you use to get reusable code (usually from a package manager).

A couple of days ago, this reference was added to show exactly this: How to write libraries for .NET Core and many other things that were pertinent for the topic. When I read this I really wanted to play with creating my own libraries, as I think this is going to be a large advantage of the .NET Core ecosystem.

The ability to quickly create reusable content is also a necessary feature in the ecosystem, and after a little wrestling with code and configuration I think I have it down to a science how to create a reusable .NET Core library and packaged as a nuget package.

Prerequisites (SDK and Tools)

I love an environment where blank machine to producitivity is quick and easy, and .NET Core is right up there. So what do you need? Well, you need the SDK and tools (navigate to your platform of choice). And of course you need an editor/IDE as well. My personal favorite for .NET Core, node.js, and other text requirements (I’m currently using it to write this blog post!) is Visual Studio Code. Once you have the SDK and CLI tools, as well as an editor (any will work, I just personally recommend VS Code), you should be ready to go.

First steps…

First thing’s first. We need to create the base component for the library by running dotnet new.

$ mkdir ILoveMath
$ cd ILoveMath
$ dotnet new

Library-ify it

When you run dotnet new it is perfect… for writing a console application. What we need to do is turn this into a library instead of a console app.

namespace ILoveMath
  public class DoMathThings
    public int AddTwoNumbers(int num1, int num2)
      return num1 + num2;
  "version": "1.0.0-*",
  "dependencies": {
    "NETStandard.Library": "1.5.0-rc2-24027"
  "frameworks": {
    "netstandard1.5": {}
$ dotnet restore
$ dotnet build

Consume the library

Without a doubt, we’ll want to consume this package, so now that the nuget package is added locally I can just create a .NET Core app to consume it.

$ mkdir MathApp
$ cd MathApp
$ dotnet new

I open up my new app and change the project.json config file to have a dependency on the new library by adding "ILoveMath": "1.0.0" to the dependencies section.

I change my Program.cs source code to reference my new library…

using System;
using ILoveMath;

namespace ConsoleApplication
    public class Program
        public static void Main(string[] args)
            DoMathThings mathThings = new DoMathThings();
            int result = mathThings.AddTwoNumbers(1, 2);
            Console.WriteLine("Added two numbers: {0}", result);

And of course the expected output of running dotnet run on this app is Added two numbers: 3.

There it is! I have created a .NET Core library and consumed it from a test app locally.