using Microsoft.Extensions.DependencyInjection;

​

// 1. Create the service collection.

var services = new ServiceCollection();

services.AddSingleton<sub>();

​

// 3. Build the service provider from the service collection.

var serviceProvider = services.BuildServiceProvider();

​

serviceProvider.GetRequiredService<sub>().Write();

​

public interface ITestClass {

    public void Write() => System.Console.WriteLine("Iwork");

}

​

public class sub(ITestClass itc = null){

    public void Write() {  if (itc != null) itc.Write(); else System.Console.WriteLine("No dep");}

}