Create a .NET class library with Unity3D and Visual Studio.

• Create a new Unity3D project. Call it LibTest
• Save the main scene as ‘main’
• Open the Player settings and find the Other settings, Configuration section.
• Change the Scripting Runtime to .NET 4.6 (Unity might restart)

• Create an empty GameObject and rename it to app
• With app selected, click Add Component in the inspector.
• Select New Script and call it App
• Doubleclick ‘App’ to open in Visual Studio
• Rightclick the solution and add a new Project

• Select Windows classic desktop and Class library (.NET Framework)
• Name the project LibTest.Core
• Enter the location of your Unity project.

• Click OK.
• Rename Class1 to Game

• Replace the generated code with:

namespace LibTest.Core
{
public class Game
{
private int _livesLeft = 3;
public bool IsGameOver => _livesLeft == 0;
}
}

• Rightclick LibTest.Core and click Properties

Here you see the assembly name and the target framework:

• Select Build
• Enter ..\assets in the Output path

• Press CTRL+SHIFT+B to build the solution
• Go back to Unity and the library shows up in the Project tab:

• Switch back to Visual Studio
• Open App.cs and replace the code with:

using LibTest.Core;
using UnityEngine;

public class App : MonoBehaviour
{
private Game _game = new Game();

private void Update()
{
if (_game.IsGameOver)
{

}
else
{

}
}
}

And that’s it. You can now add stuff to your Game class and access it from the main project.