Reading AppSettings (edit)

public string NorthwindConnection
{
    get
    {    
        return this._configuration["ConnectionStrings:NorthwindDatabase"]; 
    }
}

public string GetConnectionString(string connectionName)
{
    return this._configuration.GetConnectionString(connectionName);
}

ConfigManager

using Microsoft.Extensions.Configuration;

namespace GeeksConfiguration
{
    public class GeekConfigManager : IGeekConfigManager
    {
        private readonly IConfiguration _configuration;
        public GeekConfigManager(IConfiguration configuration)
        {
            this._configuration = configuration;
        }
    }
}

NuGet

Install-Package Microsoft.Extensions.Configuration.Abstractions

Install-Package Microsoft.Extensions.Configuration

Configuration

using Microsoft.Extensions.Configuration;

namespace GeeksConfiguration
{
    public interface IGeekConfigManager 
    {
        string NorthwindConnection { get; }

        string EmailID { get; }

        string AccountKey { get; }

        string GetConnectionString(string connectionName);

        IConfigurationSection GetConfigurationSection(string Key);
    }
}

AppSettings.json

{
  "Logging": {
    "LogLevel": {
      "Default": "Warning"
    }
  },
  "ConnectionStrings": {
    "NorthwindDatabase": "Data Source=(localdb)\\ProjectsV13;Initial Catalog=Northwind;
    Integrated Security=True;Connect Timeout=30;"
  },
  "AppSeettings": {
    "EmailID": "geeks@array.com",
    "AccountKey" : "xlymrkar24arla"
  },
  "AllowedHosts": "*"
}

Class Library(.NET Standard) - Reading appsetting.json Configuration | TheCodeBuzz

Reading appsettings.json in .Net Core Class Library Using Dependency Injection : GeeksArray.com