Using IConfiguration in C# Class Library

IMO class libraries should be agnostic to application settings data. Generally, the library consumer is the one concerned with such details. Yes, this isn't always true (e.g. if you have a class that does RSA encryption/decryption, you may want some private configuration to allow for the private key gen/storage), but for the most part, it is true.

So, in general, try to keep application settings out of the class library and have the consumer provide such data. In your comment you mention a connection string to a database. This is a perfect example of data to be kept OUT of a class library. The library shouldn't care what database it's calling to to read, just that it needs to read from one. Example below (I apologize if there's some mistakes as I am writing this on the fly from memory):

Library

Library class that uses a connection string

public class LibraryClassThatNeedsConnectionString
{
    private string connectionString;

    public LibraryClassThatNeedsConnectionString(string connectionString)
    {
        this.connectionString = connectionString;
    }

    public string ReadTheDatabase(int somePrimaryKeyIdToRead)
    {
        var result = string.Empty;

        // Read your database and set result

        return result;
    }
}

---

Application

appsettings.json

{
  "DatabaseSettings": {
    "ConnectionString": "MySuperCoolConnectionStringWouldGoHere"
  }
}

DatabaseSettings.cs

public class DatabaseSettings
{
    public string ConnectionString { get; set; }
}

Startup.cs

public class Startup
{
    public Startup(IHostingEnvironment env)
    {
        Configuration = new ConfigurationBuilder()
                        .SetBasePath(env.ContentRootPath)
                        .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
                        .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true)
                        .AddEnvironmentVariables()
                        .Build();
    }

    public IConfigurationRoot Configuration { get; }

    public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
    {
        // Setup logging
        // Configure app

    }

    public void ConfigureServices(IServiceCollection services)
    {
        // Configure services
        services.Configure<DatabaseSettings>(Configuration.GetSection("DatabaseSettings"));
        services.AddOptions();

        // Register our class that reads the DB into the DI framework
        services.AddTransient<IInterfaceForClass, ClassThatNeedsToReadDatabaseUsingLibrary>();
    }
}

Class that uses the library class to read the database

public interface IInterfaceForClass
{
    string ReadDatabaseUsingClassLibrary(int somePrimaryKeyIdToRead);
}

public class ClassThatNeedsToReadDatabaseUsingLibrary : IInterfaceForClass
{
    private DatabaseSettings dbSettings;
    private LibraryClassThatNeedsConnectionString libraryClassThatNeedsConnectionString;

    public ClassThatNeedsToReadDatabaseUsingLibrary(IOptions<DatabaseSettings> dbOptions)
    {
        this.dbSettings = dbOptions.Value;
        this.libraryClassThatNeedsConnectionString = new LibraryClassThatNeedsConnectionString(this.dbSettings.ConnectionString);
    }

    public string ReadDatabaseUsingClassLibrary(int somePrimaryKeyIdToRead)
    {
        return this.libraryClassThatNeedsConnectionString.ReadTheDatabase(somePrimaryKeyIdToRead);
    }
}

Some controller class that handles UI stuff to read from the DB

public class SomeController : Controller
{
    private readonly classThatReadsFromDb;

    public SomeController(IInterfaceForClass classThatReadsFromDb)
    {
        this.classThatReadsFromDb = classThatReadsFromDb;
    }

    // Controller methods
}

---

TL;DR

Try to avoid using application settings in a class library. Instead, have your class library be agnostic to such settings and let the consumer pass those settings in.

Edit:

I added in dependency injection into a controller class to demonstrate using dependency injection to build the class that reads from the DB. This lets the DI system resolve the necessary dependences (e.g. the DB options).

This is one way of doing it (and the best way). Another way is to inject the IOptions into the controller and manually newing up the class that reads from the DB and passing the options in (not best practice, DI is a better way to go)

This should work. Need to install package Microsoft.Extensions.Configuration.Json

 public static class Config
  {
    private static IConfiguration configuration;
    static Config()
    {
        var builder = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true);
        configuration = builder.Build();
    }

    public static string Get(string name)
    {
        string appSettings = configuration[name];
        return appSettings;
    }
}

Never used it but a quick search lead me to this...

var configuration = new Configuration();
configuration.AddJsonFile("config.json");
var emailAddress = configuration.Get("emailAddress");

Maybe you could try that.