DependencyInjection.csproj

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

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net5.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="5.0.1" />
  </ItemGroup>

</Project>

Program.cs

using System;
using Microsoft.Extensions.DependencyInjection;

namespace DependencyInjectPlayground
{
    class Program
    {
        static void Main(string[] args)
        {
            // See https://www.stevejgordon.co.uk/aspnet-core-dependency-injection-what-is-the-iservicecollection
            var serviceCollection = new ServiceCollection();

            serviceCollection.AddSingleton<ClassA>();
            serviceCollection.AddSingleton<IThing, ClassB>();
            
            // See https://www.stevejgordon.co.uk/aspnet-core-dependency-injection-what-is-the-iserviceprovider-and-how-is-it-built
            var serviceProvider = serviceCollection.BuildServiceProvider();

            Console.WriteLine("Done");
        }
    }

    public interface IThing
    {
        public void DoStuff();
    }

    public class ClassA
    {
        private readonly IThing _dependency;

        public ClassA(IThing thing) => _dependency = thing;

        public void DoWork() => _dependency.DoStuff();
    }

    public class ClassB : IThing
    {
        public void DoStuff()
        {
            // Imagine implementation
        }
    }
}