ExtractedDependencies.cs

using System;
using System.Collections.Generic;
namespace DI2
{
    using MonApp;
    using MonApp.Adapters;

    class Program
    {
        static void Main(string[] args)
        {

            Test();

            var userAdapter = new UserFromDb();
            var students = new Students(userAdapter);

            var count = students.CountStudents();
            Console.WriteLine($" {count} students");
            var first = students.First();
            Console.WriteLine($" le premier est {first}");
        }

        static void Test()
        {
            var fakeUsers = new FakeUser();
            var students = new Students(fakeUsers);
            var count = students.CountStudents();
            var first = students.First();
            Assert(count,5);
            Assert(first,"a");
        }

        static void Assert<T>(T value, T expected)
        {
            if(value.Equals(expected)) Console.WriteLine("ok") ;
            else {
                Console.WriteLine("fail!");
            }
        }
    }
    public class FakeUser : IUser 
    {
        public IEnumerable<string> AllUsers() => new[]{"a","b","c","d","e"};
    }
}

namespace MonApp
{
    using System.Linq;
    using System.Collections.Generic;

    public class Students
    {
        private IUser users;
        
        
        public Students(IUser users)
        {
            this.users = users;
        }
        public int CountStudents()
        {
            var students = users.AllUsers();
            return students.Count();
        }

        public string First()
        {
            var students = users.AllUsers();
            var first = students.First();
            return first;
        }
    }

    public interface IUser
    {
           IEnumerable<string> AllUsers ();
    }
}

namespace MonApp.Adapters
{
    using MonApp;
    using MaLibExterne;
    public class UserFromDb : IUser
    {
        private UserDB dB;
        public UserFromDb()
        {
            dB = new UserDB();
        }
        public IEnumerable<string> AllUsers ()
        => dB.AllUsers();
    }
}

namespace MaLibExterne
{
    public class UserDB
    {
        public IEnumerable<string> AllUsers()
        => new string[]{ "Jimmy","John","Paul"};
    }
}