Refactor static IServiceProvider initialization

Refactor static IServiceProvider initialization to use a thread-safe Lazy<IServiceProvider>, ensuring lazy creation and improved efficiency. Replace `Services` and `Provider` fields with `_staticConfigure` delegate for dynamic service collection configuration.

Update `BuildStaticClient` methods to use `Interlocked.CompareExchange` for safe initialization of `_staticConfigure`. Modify `Create` method to retrieve the provider via `LazyProvider`.

Mark `BuildStaticClient` and `Create` methods as obsolete, encouraging the use of local service collections. Adjust exception messages and documentation to reflect these changes.

src/ReC.Client/ReCClient.cs

@@ -85,19 +85,28 @@ namespace ReC.Client
         }
 
         #region Static
-        private static readonly IServiceCollection Services = new ServiceCollection();
-
 #if NET8_0_OR_GREATER
-        private static IServiceProvider? Provider = null;
+        private static Action<IServiceCollection>? _staticConfigure = null;
 #else
-        private static IServiceProvider Provider = null;
+        private static Action<IServiceCollection> _staticConfigure = null;
 #endif
 
+        private static readonly Lazy<IServiceProvider> LazyProvider = new Lazy<IServiceProvider>(() =>
+        {
+            var configure = _staticConfigure
+                ?? throw new InvalidOperationException("Static Provider is not built. Call BuildStaticClient first.");
+
+            var services = new ServiceCollection();
+            configure(services);
+            return services.BuildServiceProvider();
+        }, System.Threading.LazyThreadSafetyMode.ExecutionAndPublication);
+
         /// <summary>
         /// Configures and builds the static <see cref="IServiceProvider"/> for creating <see cref="ReCClient"/> instances.
         /// </summary>
         /// <remarks>
         /// This method should only be called once during application startup.
+        /// The underlying <see cref="IServiceProvider"/> is created lazily and thread-safely on first access via <see cref="Create"/>.
         /// </remarks>
         /// <param name="apiUri">The base URI of the ReC API.</param>
         /// <param name="configureOptions">An optional callback to configure <see cref="ReCClientOptions"/>.</param>
@@ -109,11 +118,9 @@ namespace ReC.Client
         public static void BuildStaticClient(string apiUri, Action<ReCClientOptions>? configureOptions = null)
 #endif
         {
-            if(Provider != null)
+            Action<IServiceCollection> configure = services => services.AddRecClient(apiUri, configureOptions);
+            if (System.Threading.Interlocked.CompareExchange(ref _staticConfigure, configure, null) != null)
                 throw new InvalidOperationException("Static Provider is already built.");
-
-            Services.AddRecClient(apiUri, configureOptions);
-            Provider = Services.BuildServiceProvider();
         }
 
         /// <summary>
@@ -121,6 +128,7 @@ namespace ReC.Client
         /// </summary>
         /// <remarks>
         /// This method should only be called once during application startup.
+        /// The underlying <see cref="IServiceProvider"/> is created lazily and thread-safely on first access via <see cref="Create"/>.
         /// </remarks>
         /// <param name="configureClient">An action to configure the <see cref="HttpClient"/>.</param>
         /// <param name="configureOptions">An optional callback to configure <see cref="ReCClientOptions"/>.</param>
@@ -132,25 +140,20 @@ namespace ReC.Client
         public static void BuildStaticClient(Action<HttpClient> configureClient, Action<ReCClientOptions>? configureOptions = null)
 #endif
         {
-            if (Provider != null)
+            Action<IServiceCollection> configure = services => services.AddRecClient(configureClient, configureOptions);
+            if (System.Threading.Interlocked.CompareExchange(ref _staticConfigure, configure, null) != null)
                 throw new InvalidOperationException("Static Provider is already built.");
-
-            Services.AddRecClient(configureClient, configureOptions);
-            Provider = Services.BuildServiceProvider();
         }
 
         /// <summary>
         /// Creates a new <see cref="ReCClient"/> instance using the statically configured provider.
         /// </summary>
         /// <returns>A new instance of the <see cref="ReCClient"/>.</returns>
-        /// <exception cref="InvalidOperationException">Thrown if <see cref="BuildStaticClient(string)"/> has not been called yet.</exception>
+        /// <exception cref="InvalidOperationException">Thrown if <see cref="BuildStaticClient(string, Action{ReCClientOptions})"/> has not been called yet.</exception>
         [Obsolete("Use a local service collection instead of the static provider.")]
         public static ReCClient Create()
         {
-            if (Provider == null)
+            return LazyProvider.Value.GetRequiredService<ReCClient>();
-                throw new InvalidOperationException("Static Provider is not built. Call BuildStaticClient first.");
-
-            return Provider.GetRequiredService<ReCClient>();
         }
         #endregion
     }