LooselyCoupledExample.cs

using System;

namespace WebApplication1
{
    public interface IDoSomething
    {
        void DoSomething(object param);
    }

    public class DoerOfSomething : IDoSomething
    {
        public void DoSomething(object param)
        {
            Console.WriteLine("I do something");
        }
    }

    public class DoerOfSomethingElse : IDoSomething
    {
        public void DoSomething(object param)
        {
            Console.WriteLine("I do something different");
        }
    }


    public class LooselyCoupled
    {
        private IDoSomething op; // loosely coupled as it's tied to abstraction (interface)

        public LooselyCoupled(IDoSomething op)
        {
            // the op dependency can be ANY concrete type that inherits from IDoSomething
            // i.e. - DoerOfSomething OR DoerOfSomethingElse can be passed in achieving polymorphism
            this.op = op;
        }

        public void PerformNeededTask() => op.DoSomething(new object());
    }
    
    public class TightlyCoupled
    {
        private DoerOfSomething doer; // tightly linked to DoerOfSomething class (concrete implementation)

        public TightlyCoupled(DoerOfSomething op) // tightly linked to DoerOfSomething
        {
            // the op dependency can ONLY be of type DoerOfSomething
            this.doer = op;
        }

        public void PerformNeededTask() => doer.DoSomething(new object());
    }
    
    public class BadCodingExample 
    {
        IDoerOfSomething op;

        public BadCodingExample()
        {
            this.op = new DoerOfSomething(); // breaks polymorphism + SOLID
        }
    }
}