Feat: Add AddServiceIfNotExists extension for IServiceCollection

Introduces the AddServiceIfNotExists method to register services only if they are not already registered. Includes support for specifying lifetimes and prevents duplicate registrations. Comprehensive tests are added to validate behavior for various lifetimes, ensuring robustness and error handling.

Author: AnnaSasDev

src/CodeOfChaos.Extensions.DependencyInjection/IServiceCollectionExtensions.cs

@@ -0,0 +1,31 @@
+// ---------------------------------------------------------------------------------------------------------------------
+// Imports
+// ---------------------------------------------------------------------------------------------------------------------
+// ReSharper disable once CheckNamespace
+namespace Microsoft.Extensions.DependencyInjection;
+// ---------------------------------------------------------------------------------------------------------------------
+// Code
+// ---------------------------------------------------------------------------------------------------------------------
+public static class IServiceCollectionExtensions {
+    public static IServiceCollection AddServiceIfNotExists<TService, TImplementation>(
+        this IServiceCollection services,
+        ServiceLifetime lifetime = ServiceLifetime.Scoped)
+        where TService : class
+        where TImplementation : class, TService
+    {
+        if (services.FirstOrDefault(x => x.ServiceType == typeof(TService)) is not null) return services;
+
+        switch (lifetime) {
+            case ServiceLifetime.Singleton: services.AddSingleton<TService, TImplementation>();
+                break;
+            case ServiceLifetime.Scoped: services.AddScoped<TService, TImplementation>();
+                break;
+            case ServiceLifetime.Transient: services.AddTransient<TService, TImplementation>();
+                break;
+            default: throw new ArgumentOutOfRangeException(nameof(lifetime), lifetime, null);
+        }
+
+        return services;
+    }
+
+}

tests/Tests.CodeOfChaos.Extensions.DependencyInjection/AddServiceIfNotExistsTests.cs

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+// ---------------------------------------------------------------------------------------------------------------------
+// Imports
+// ---------------------------------------------------------------------------------------------------------------------
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Tests.CodeOfChaos.Extensions.DependencyInjection;
+// ---------------------------------------------------------------------------------------------------------------------
+// Code
+// ---------------------------------------------------------------------------------------------------------------------
+public class AddServiceIfNotExistsTests {
+    [Test]
+    public async Task AddServiceIfNotExists_WhenServiceDoesNotExist_AddsWithDefaultScoped() {
+        // Arrange
+        var services = new ServiceCollection();
+        
+        // Act
+        services.AddServiceIfNotExists<ITestService, TestService>();
+        ServiceDescriptor? descriptor = services.FirstOrDefault(x => x.ServiceType == typeof(ITestService));
+        
+        // Assert
+        await Assert.That(descriptor).IsNotNull();
+        await Assert.That(descriptor!.ImplementationType).IsEqualTo(typeof(TestService));
+        await Assert.That(descriptor.Lifetime).IsEqualTo(ServiceLifetime.Scoped);
+    }
+
+    [Test]
+    [Arguments(ServiceLifetime.Singleton)]
+    [Arguments(ServiceLifetime.Scoped)]
+    [Arguments(ServiceLifetime.Transient)]
+    public async Task AddServiceIfNotExists_WithSpecificLifetime_AddsServiceCorrectly(ServiceLifetime lifetime) {
+        // Arrange
+        var services = new ServiceCollection();
+        
+        // Act
+        services.AddServiceIfNotExists<ITestService, TestService>(lifetime);
+        ServiceDescriptor? descriptor = services.FirstOrDefault(x => x.ServiceType == typeof(ITestService));
+        
+        // Assert
+        await Assert.That(descriptor).IsNotNull();
+        await Assert.That(descriptor!.Lifetime).IsEqualTo(lifetime);
+    }
+
+    [Test]
+    public async Task AddServiceIfNotExists_WhenServiceExists_DoesNotAddDuplicate() {
+        // Arrange
+        var services = new ServiceCollection();
+        services.AddScoped<ITestService, TestService>();
+        int initialCount = services.Count;
+        
+        // Act
+        services.AddServiceIfNotExists<ITestService, TestService>();
+        
+        // Assert
+        await Assert.That(services.Count).IsEqualTo(initialCount);
+    }
+
+    [Test]
+    public async Task AddServiceIfNotExists_WithInvalidLifetime_ThrowsException() {
+        // Arrange
+        var services = new ServiceCollection();
+        
+        // Act & Assert
+        await Assert.ThrowsAsync<ArgumentOutOfRangeException>(() => {
+            services.AddServiceIfNotExists<ITestService, TestService>((ServiceLifetime)999);
+            return Task.CompletedTask;
+        });
+    }
+
+    // Test interfaces and classes
+    private interface ITestService { }
+    private class TestService : ITestService { }
+}