Allow operations control client to be configured via a callback (#153)

* Add new options class and provide a callback to retrieve it, rather than requiring config to be provided immediately.

* Exception message tweak.

* Use string.IsNullOrEmpty rather than a simple null check to cater for configuration where the resource Id has been left as an empty string.

Solutions/Marain.Operations.ControlClient/Marain/Operations/Client/OperationsControl/MarainOperationsControlClientOptions.cs

@@ -0,0 +1,30 @@
+// <copyright file="OperationsControlClientServiceCollectionExtensions.cs" company="Endjin Limited">
+// Copyright (c) Endjin Limited. All rights reserved.
+// </copyright>
+
+namespace Marain.Operations.Client.OperationsControl
+{
+    using System;
+
+    /// <summary>
+    /// Settings for configuring the <see cref="MarainOperationsControl"/> service.
+    /// </summary>
+    public class MarainOperationsControlClientOptions
+    {
+        /// <summary>
+        /// Gets or sets the base URL of the operations control service.
+        /// </summary>
+        public Uri OperationsControlServiceBaseUri { get; set; }
+
+        /// <summary>
+        /// Gets or sets the resource ID to use when asking the Managed Identity system for a token with which to
+        /// communicate with the service. This is typically the App ID of the application created for securing access
+        /// to the operations control service.
+        /// </summary>
+        /// <remarks>
+        /// If this is null, no attempt will be made to secure communication with the operations control
+        /// service. This may be appropriate for local development scenarios.
+        /// </remarks>
+        public string ResourceIdForMsiAuthentication { get; set; }
+    }
+}

Solutions/Marain.Operations.ControlClient/Microsoft/Extensions/DependencyInjection/OperationsControlClientServiceCollectionExtensions.cs

@@ -0,0 +1,81 @@
+// <copyright file="OperationsControlClientServiceCollectionExtensions.cs" company="Endjin Limited">
+// Copyright (c) Endjin Limited. All rights reserved.
+// </copyright>
+
+namespace Microsoft.Extensions.DependencyInjection
+{
+    using System;
+    using Corvus.Identity.ManagedServiceIdentity.ClientAuthentication;
+    using Marain.Operations.Client.OperationsControl;
+    using Microsoft.Extensions.DependencyInjection;
+    using Microsoft.Rest;
+
+    /// <summary>
+    /// DI initialization for clients of the Operations control service.
+    /// </summary>
+    public static class OperationsControlClientServiceCollectionExtensions
+    {
+        /// <summary>
+        /// Adds the Operations control client to a service collection.
+        /// </summary>
+        /// <param name="services">The service collection.</param>
+        /// <param name="getOptions">A callback that can be used to retrieve options for the client.</param>
+        /// <returns>The modified service collection.</returns>
+        public static IServiceCollection AddOperationsControlClient(
+            this IServiceCollection services,
+            Func<IServiceProvider, MarainOperationsControlClientOptions> getOptions)
+        {
+            services.AddSingleton(sp =>
+            {
+                MarainOperationsControlClientOptions options = getOptions(sp);
+
+                if (options is null || options.OperationsControlServiceBaseUri == null)
+                {
+                    throw new InvalidOperationException(
+                        $"Cannot instantiate the Operations Control client without supplying a value for {nameof(MarainOperationsControlClientOptions.OperationsControlServiceBaseUri)}.");
+                }
+
+                return BuildOperationsControlClient(sp, options.OperationsControlServiceBaseUri, options.ResourceIdForMsiAuthentication);
+            });
+
+            return services;
+        }
+
+        /// <summary>
+        /// Adds the Operations control client to a service collection.
+        /// </summary>
+        /// <param name="services">The service collection.</param>
+        /// <param name="options">Options for the client.</param>
+        /// <returns>The modified service collection.</returns>
+        public static IServiceCollection AddOperationsControlClient(
+            this IServiceCollection services,
+            MarainOperationsControlClientOptions options)
+        {
+            return services.AddOperationsControlClient(_ => options);
+        }
+
+        /// <summary>
+        /// Adds the Operations control client to a service collection.
+        /// </summary>
+        /// <param name="baseUri">The base URI of the Operations control service.</param>
+        /// <param name="resourceIdForMsiAuthentication">
+        /// The resource id to use when obtaining an authentication token representing the
+        /// hosting service's identity. Pass null to run without authentication.
+        /// </param>
+        /// <returns>The modified service collection.</returns>
+        private static IMarainOperationsControl BuildOperationsControlClient(
+            IServiceProvider serviceProvider,
+            Uri baseUri,
+            string resourceIdForMsiAuthentication = null)
+        {
+            return string.IsNullOrEmpty(resourceIdForMsiAuthentication)
+                ? new UnauthenticatedMarainOperationsControl(baseUri)
+                : new MarainOperationsControl(
+                        baseUri,
+                        new TokenCredentials(
+                            new ServiceIdentityTokenProvider(
+                                serviceProvider.GetRequiredService<IServiceIdentityTokenSource>(),
+                                resourceIdForMsiAuthentication)));
+        }
+    }
+}