Add GetKeyedOrDefault extension to IServiceProvider

src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/src/ServiceProviderKeyedServiceExtensions.cs

@@ -31,6 +31,20 @@ public static class ServiceProviderKeyedServiceExtensions
             throw new InvalidOperationException(SR.KeyedServicesNotSupported);
         }
 
+        /// <summary>
+        /// Get service of type <typeparamref name="T"/> from the <see cref="IServiceProvider"/>.
+        /// </summary>
+        /// <typeparam name="T">The type of service object to get.</typeparam>
+        /// <param name="provider">The <see cref="IServiceProvider"/> to retrieve the service object from.</param>
+        /// <param name="serviceKey">An object that specifies the key of service object to get.</param>
+        /// <returns>A service object of type <typeparamref name="T"/> or the default value of <typeparamref name="T"/> if there is no such service.</returns>
+        public static T GetKeyedOrDefaultService<T>(this IServiceProvider provider, object? serviceKey) where T : class
+        {
+            ArgumentNullException.ThrowIfNull(provider);
+
+            return provider.GetKeyedService<T>(serviceKey) ?? provider.GetRequiredService<T>();
+        }
+
         /// <summary>
         /// Get service of type <paramref name="serviceType"/> from the <see cref="IServiceProvider"/>.
         /// </summary>

src/libraries/Microsoft.Extensions.DependencyInjection.Specification.Tests/src/KeyedDependencyInjectionSpecificationTests.cs

@@ -1222,5 +1222,33 @@
             }
         }
 #endif
+
+        [Fact]
+        public void ResolveKeyedOrDefault()
+        {
+            var defaultService = new Service();
+            var service1 = new Service();
+            var serviceCollection = new ServiceCollection();
+            serviceCollection.AddSingleton<IService>(defaultService);
+            serviceCollection.AddKeyedSingleton<IService>("service1", service1);
+
+            var provider = CreateServiceProvider(serviceCollection);
+
+            // Key exists, should return keyed service
+            Assert.Same(service1, provider.GetKeyedOrDefaultService<IService>("service1"));
+
+            // Key does not exist, should return default (non-keyed) service
+            Assert.Same(defaultService, provider.GetKeyedOrDefaultService<IService>("notFoundKey"));
+        }
+
+        [Fact]
+        public void ResolveKeyedOrDefault_Throws_WhenBothMissing()
+        {
+            var serviceCollection = new ServiceCollection();
+            var provider = CreateServiceProvider(serviceCollection);
+
+            // Both keyed and default services are missing, should throw
+            Assert.Throws<InvalidOperationException>(() => provider.GetKeyedOrDefaultService<IService>("service1"));
+        }
     }
 }