In this article I will explain with an example, how to use the IConfiguration in ASP.Net Core.
Microsoft has replaced System.Configuration class with IConfiguration interface in .Net Core 2.0 onwards and this article will illustrate how to use IConfiguration interface in ASP.Net Core.
What is IConfiguration
The IConfiguration is an interface for .Net Core 2.0.
The IConfiguration interface need to be injected as dependency in the Controller and then later used throughout the Controller.
The IConfiguration interface is used to read Settings and Connection Strings from AppSettings.json file.
You will need to import the following namespace.
using Microsoft.Extensions.Configuration;
Adding the AppSettings.json file
In order to add AppSettings.json file, right click on the Project in Solution Explorer. Then click Add, then New Item and then choose App Settings File option (shown below) and click Add button.
Using IConfiguration in ASP.Net Core
Once the File is created, it will have a DefaultConnection, below that a new Connection String entry is added.
 "ConnectionStrings": {
    "DefaultConnection": "Server=(localdb)\\MSSQLLocalDB;Database=_CHANGE_ME;Trusted_Connection=True;MultipleActiveResultSets=true",
    "MyConn": "Data Source=.\\SQL2017;Initial Catalog=AjaxSamples;Integrated Security=true"
Reading Connection String from AppSettings.json file using IConfiguration interface
In the below example, the IConfiguration is injected in the Controller and assigned to the private property Configuration.
Then inside the Controller, the Connection String is read from the AppSettings.json file using the GetConnectionString function.
public class HomeController : Controller
    private IConfiguration Configuration;
    public HomeController(IConfiguration _configuration)
        Configuration = _configuration;
    public IActionResult Index()
        string connString = this.Configuration.GetConnectionString("MyConn");
        return View();
Using IConfiguration in ASP.Net Core