Circular Dependency in DOT NET

CircularDependencyDemo.cs

// ============================================================
// Circular Dependency Example (BAD) + Fixed Example (GOOD)
// Single-file demonstration for training
// ============================================================

using System;
using Microsoft.Extensions.DependencyInjection;

namespace CircularDependencyDemo
{
    // ===============================
    // ❌ BAD EXAMPLE: Circular Dependency
    // ===============================

    public interface IAService
    {
        void DoA();
    }

    public interface IBService
    {
        void DoB();
    }

    public class AService : IAService
    {
        private readonly IBService _b;

        public AService(IBService b)
        {
            _b = b;
        }

        public void DoA()
        {
            Console.WriteLine("AService running...");
            _b.DoB();
        }
    }

    public class BService : IBService
    {
        private readonly IAService _a;

        public BService(IAService a)
        {
            _a = a;
        }

        public void DoB()
        {
            Console.WriteLine("BService running...");
            _a.DoA();
        }
    }


    // ===============================
    // ✔ GOOD EXAMPLE: Fixed (Shared Dependency)
    // ===============================

    public interface ISharedLogic
    {
        void Log(string msg);
    }

    public class SharedLogic : ISharedLogic
    {
        public void Log(string msg)
        {
            Console.WriteLine($"[SharedLogic] {msg}");
        }
    }

    public class AServiceFixed
    {
        private readonly ISharedLogic _shared;
        public AServiceFixed(ISharedLogic shared) => _shared = shared;
        public void DoSomething() => _shared.Log("AServiceFixed running...");
    }

    public class BServiceFixed
    {
        private readonly ISharedLogic _shared;
        public BServiceFixed(ISharedLogic shared) => _shared = shared;
        public void DoSomething() => _shared.Log("BServiceFixed running...");
    }


    // ===============================
    // Main Program
    // ===============================
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("=== Circular Dependency Demo ===");

            var services = new ServiceCollection();

            // Register BAD circular dependencies
            services.AddScoped<IAService, AService>();
            services.AddScoped<IBService, BService>();

            // Register GOOD implementations
            services.AddScoped<ISharedLogic, SharedLogic>();
            services.AddScoped<AServiceFixed>();
            services.AddScoped<BServiceFixed>();

            var provider = services.BuildServiceProvider();

            // -------------------------------
            // 1. Demonstrate Circular Dependency Failure
            // -------------------------------
            try
            {
                Console.WriteLine("\n--- BAD Example: Expect circular dependency error ---");
                var a = provider.GetRequiredService<IAService>(); // Will crash
                a.DoA();
            }
            catch (Exception ex)
            {
                Console.WriteLine("❌ Circular Dependency Detected:");
                Console.WriteLine(ex.Message);
            }

            // -------------------------------
            // 2. Demonstrate FIXED version
            // -------------------------------
            Console.WriteLine("\n--- GOOD Example: No circular dependency ---");

            var aFixed = provider.GetRequiredService<AServiceFixed>();
            var bFixed = provider.GetRequiredService<BServiceFixed>();

            aFixed.DoSomething();
            bFixed.DoSomething();

            Console.WriteLine("\n✓ Demo completed.");
        }
    }
}