appsettings

appsettings.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp3.1</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="System.Configuration.ConfigurationManager" Version="4.7.0" />
  </ItemGroup>

</Project>

Program.cs

using System;

namespace appsettings
{
    class Program
    {
        static void Main(string[] args)
        {
            System.Configuration.ConfigurationManager.AppSettings["hello"] = "world";
            Console.WriteLine("Hello World!");
            Console.WriteLine(System.Configuration.ConfigurationManager.AppSettings["hello"]);
        }
    }
}