Examples for https://arlobelshee.com/decoupled-design/

Decoupled Design.cs

[TestClass]
public class Tests
{
    [TestMethod]
    public void TestProgram()
    {
        foreach (var main in new[] {
                DirectCall.Program.Main1,
                DependencyInjection.Program.Main2,
                EventBased.Program.Main3,
                EventBased.Program.Main4,
            })
        {
            using var output = new StringWriter();
            Console.SetOut(output);

            main();

            Assert.AreEqual("Hello, World!", output.ToString());
        }

    }

    [TestMethod]
    public void TestConfigure()
    {
        var (aClass, foo) = EventBased.Program.Configure();

        Assert.AreEqual(
            foo.Bar,
            aClass.OnBar?.GetInvocationList().Single()
            );
    }
}

namespace DirectCall
{
    class Foo { public void Bar(string message) { Console.Write(message); } }
    class AClass
    {
        private readonly Foo _foo = new Foo();

        public void Do()
        {
            this._foo.Bar("Hello, World!");
        }
    }

    static class Program
    {
        public static void Main1()
        {
            var aClass = new AClass();
            aClass.Do();
        }
    }
}

namespace DependencyInjection
{
    internal interface IFoo
    {
        void Bar(string message);
    }

    class Foo : IFoo { public void Bar(string message) { Console.Write(message); } }

    class AClass
    {
        private readonly IFoo _foo;

        public AClass(IFoo foo)
        {
            this._foo = foo;
        }

        public void Do()
        {
            this._foo.Bar("Hello, World!");
        }
    }

    static class Program
    {
        public static void Main2()
        {
            var aClass = new AClass(new Foo());
            aClass.Do();
        }
    }
}

namespace EventBased
{
    class Foo { public void Bar(string message) { Console.Write(message); } }
    class AClass
    {
        public Action<string>? OnBar;

        public void Do()
        {
            this.OnBar?.Invoke("Hello, World!");
        }
    }

    static class Program
    {
        public static void Main3()
        {
            var foo = new Foo();
            var aClass = new AClass();
            aClass.OnBar += foo.Bar;

            aClass.Do();
        }

        public static (AClass, Foo) Configure()
        {
            var foo = new Foo();
            var aClass = new AClass();
            aClass.OnBar += foo.Bar;
            return (aClass, foo);
        }

        public static void Main4()
        {
            var (foo, aClass) = Configure();
            foo.Do();
        }
    }
}