Example Of Dependency Injection in C#. (BTW, I'm not 100% sure :p )

ExampleOfDependencyInjection.cs

namespace SomeNamespace
{
    public interface ITest
    {
        void hello();
    }

    //NO Singleton Class, Simple One.
    public class Test1 : ITest
    {
        public void hello()
        {
            System.Console.WriteLine("hiiiiiiii");
        }
    }

    //This is Singleton Class, so this class has only one instance
    public class Test2 : ITest
    {
        private static Test2 _instance;
        public static Test2 Instance
        {
            get
            {
                if (_instance == null)
                    _instance = new Test2();

                return _instance;
            }
        }

        //Private constractor, so you can't create object of it from outsite of the class
        private Test2() { } 

        public void hello()
        {
            System.Console.WriteLine("Heyyyyyyy");
        }
    }
    
    /*`Demo` class is dependented on ITest*/
    public class Demo
    {
        private readonly ITest _test;
        
        //dependency of ITest
        public Demo(ITest t) 
        {
            _test = t;
        }

        public void foo()
        {
            //do work;
            _test.hello();
        }

    }

    public class MainClass
    {
        public static void Main()
        {
            Demo T1 = new Demo(new Test1()); //Depecency Injection
            Demo T2 = new Demo(Test2.Instance); //Depecency Injection + Singleton (One single instance through out application life cycle)

            T1.foo(); //hiiiiiiii
            T2.foo(); //Heyyyyyyy

            /*
                Demo class is decoupled from Test1 & Test2 class.
                Any class which is implemented `ITest` interface, can be injected into Demo class.
            */
        }
    }

}