Reintroduced simplified Ioc class

Author: Sergio0694

Microsoft.Toolkit.Mvvm/DependencyInjection/Ioc.cs

@@ -0,0 +1,111 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Threading;
+
+#nullable enable
+
+namespace Microsoft.Toolkit.Mvvm.DependencyInjection
+{
+    /// <summary>
+    /// A type that facilitates the use of the <see cref="IServiceProvider"/> type.
+    /// The <see cref="Ioc"/> provides the ability to configure services in a singleton, thread-safe
+    /// service provider instance, which can then be used to resolve service instances.
+    /// The first step to use this feature is to declare some services, for instance:
+    /// <code>
+    /// public interface ILogger
+    /// {
+    ///     void Log(string text);
+    /// }
+    /// </code>
+    /// <code>
+    /// public class ConsoleLogger : ILogger
+    /// {
+    ///     void Log(string text) => Console.WriteLine(text);
+    /// }
+    /// </code>
+    /// Then the services configuration should then be done at startup, by calling the <see cref="ConfigureServices"/>
+    /// method and passing an <see cref="IServiceProvider"/> instance with the services to use. That instance can
+    /// be from any library offering dependency injection functionality, such as Microsoft.Extensions.DependencyInjection.
+    /// For instance, using that library, <see cref="ConfigureServices"/> can be used as follows in this example:
+    /// <code>
+    /// Ioc.Default.ConfigureServices(
+    ///     new ServiceCollection()
+    ///     .AddSingleton&lt;ILogger, Logger&gt;()
+    ///     .BuildServiceProvider());
+    /// </code>
+    /// Finally, you can use the <see cref="Ioc"/> instance (which implements <see cref="IServiceProvider"/>)
+    /// to retrieve the service instances from anywhere in your application, by doing as follows:
+    /// <code>
+    /// Ioc.Default.GetService&lt;ILogger&gt;().Log("Hello world!");
+    /// </code>
+    /// </summary>
+    public sealed class Ioc : IServiceProvider
+    {
+        /// <summary>
+        /// Gets the default <see cref="Ioc"/> instance.
+        /// </summary>
+        public static Ioc Default { get; } = new Ioc();
+
+        /// <summary>
+        /// The <see cref="IServiceProvider"/> instance to use, if initialized.
+        /// </summary>
+        private volatile IServiceProvider? serviceProvider;
+
+        /// <inheritdoc/>
+        public object? GetService(Type serviceType)
+        {
+            // As per section I.12.6.6 of the official CLI ECMA-335 spec:
+            // "[...] read and write access to properly aligned memory locations no larger than the native
+            // word size is atomic when all the write accesses to a location are the same size. Atomic writes
+            // shall alter no bits other than those written. Unless explicit layout control is used [...],
+            // data elements no larger than the natural word size [...] shall be properly aligned.
+            // Object references shall be treated as though they are stored in the native word size."
+            // The field being accessed here is of native int size (reference type), and is only ever accessed
+            // directly and atomically by a compare exchange instruction (see below), or here. We can therefore
+            // assume this read is thread safe with respect to accesses to this property or to invocations to one
+            // of the available configuration methods. So we can just read the field directly and make the necessary
+            // check with our local copy, without the need of paying the locking overhead from this get accessor.
+            IServiceProvider? provider = this.serviceProvider;
+
+            if (provider is null)
+            {
+                ThrowInvalidOperationExceptionForMissingInitialization();
+            }
+
+            return provider!.GetService(serviceType);
+        }
+
+        /// <summary>
+        /// Initializes the shared <see cref="IServiceProvider"/> instance.
+        /// </summary>
+        /// <param name="serviceProvider">The input <see cref="IServiceProvider"/> instance to use.</param>
+        public void ConfigureServices(IServiceProvider serviceProvider)
+        {
+            IServiceProvider? oldServices = Interlocked.CompareExchange(ref this.serviceProvider, serviceProvider, null);
+
+            if (!(oldServices is null))
+            {
+                ThrowInvalidOperationExceptionForRepeatedConfiguration();
+            }
+        }
+
+        /// <summary>
+        /// Throws an <see cref="InvalidOperationException"/> when the <see cref="IServiceProvider"/> property is used before initialization.
+        /// </summary>
+        private static void ThrowInvalidOperationExceptionForMissingInitialization()
+        {
+            throw new InvalidOperationException("The service provider has not been configured yet");
+        }
+
+        /// <summary>
+        /// Throws an <see cref="InvalidOperationException"/> when a configuration is attempted more than once.
+        /// </summary>
+        private static void ThrowInvalidOperationExceptionForRepeatedConfiguration()
+        {
+            throw new InvalidOperationException("The default service provider has already been configured");
+        }
+    }
+}