Example

Install packages:

dotnet add package Microsoft.Extensions.Configuration.Json
dotnet add package Microsoft.Extensions.Configuration.Binder
dotnet add package Microsoft.Extensions.Configuration.EnvironmentVariables # Only if using environment variables.

Assuming this appsettings.json file:

{
    "Settings": {
        "KeyOne": 1,
        "KeyTwo": true,
        "KeyThree": {
            "Message": "Oh, that's nice..."
        }
    }
}

Set the file properties:

Build action = Content
Copy to Output Directory = Copy always or Copy if newer

// Assuming this code for settings object:

public class Settings
{
    public int KeyOne { get; set; }
    public bool KeyTwo { get; set; }
    public NestedSettings KeyThree { get; set; } = null!;
}

public class NestedSettings
{
    public string Message { get; set; } = null!;
}

Hosted Approach

using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

using IHost host = Host.CreateDefaultBuilder(args).Build();

// Ask the service provider for the configuration abstraction:
IConfiguration config = host.Services.GetRequiredService<IConfiguration>();

// Get values from the config given their key and their target type.
int keyOneValue = config.GetValue<int>("KeyOne");
bool keyTwoValue = config.GetValue<bool>("KeyTwo");
string keyThreeNestedValue = config.GetValue<string>("KeyThree:Message");

// Write the values to the console.
Console.WriteLine($"KeyOne = {keyOneValue}");
Console.WriteLine($"KeyTwo = {keyTwoValue}");
Console.WriteLine($"KeyThree:Message = {keyThreeNestedValue}");

// Application code which might rely on the config could start here.

await host.RunAsync();

Non-Hosted Approach

using Microsoft.Extensions.Configuration;

// Build a config object, using env vars and JSON providers:
IConfiguration config = new ConfigurationBuilder()
    .AddJsonFile("appsettings.json")
    .AddEnvironmentVariables()
    .Build();

// Get values from the config given their key and their target type:
Settings settings = config.GetRequiredSection("Settings").Get<Settings>();

// Write the values to the console.
Console.WriteLine($"{settings.KeyOne}"); // 1
Console.WriteLine($"{settings.KeyTwo}"); // True
Console.WriteLine($"{settings.KeyThree.Message}"); // "Oh, that's nice…"

// Application code which might rely on the config could start here.