Add the ConfigurationManager to the IServiceCollection (#36832)

src/DefaultBuilder/src/WebApplicationBuilder.cs

@@ -99,6 +99,8 @@ internal WebApplicationBuilder(WebApplicationOptions options, Action<IHostBuilde
             Logging = new LoggingBuilder(Services);
             Host = new ConfigureHostBuilder(hostContext, Configuration, Services);
             WebHost = new ConfigureWebHostBuilder(webHostContext, Configuration, Services);
+
+            Services.AddSingleton<IConfiguration>(Configuration);
         }
 
         /// <summary>
@@ -171,6 +173,17 @@ public WebApplication Build()
                 // we called ConfigureWebHostDefaults on both the _deferredHostBuilder and _hostBuilder.
                 foreach (var s in _services)
                 {
+                    // Skip the configuration manager instance we added earlier
+                    // we're already going to wire it up to this new configuration source
+                    // after we've built the application. There's a chance the user manually added
+                    // this as well but we still need to remove it from the final configuration
+                    // to avoid cycles in the configuration graph
+                    if (s.ServiceType == typeof(IConfiguration) &&
+                        s.ImplementationInstance == Configuration)
+                    {
+                        continue;
+                    }
+
                     services.Add(s);
                 }
 

src/DefaultBuilder/test/Microsoft.AspNetCore.Tests/WebApplicationTests.cs

@@ -693,6 +693,39 @@ public async Task WebApplicationConfiguration_HostFilterOptionsAreReloadable()
             Assert.Contains("NewHost", options.AllowedHosts);
         }
 
+        [Fact]
+        public void CanResolveIConfigurationBeforeBuildingApplication()
+        {
+            var builder = WebApplication.CreateBuilder();
+            var sp = builder.Services.BuildServiceProvider();
+
+            var config = sp.GetService<IConfiguration>();
+            Assert.NotNull(config);
+            Assert.Same(config, builder.Configuration);
+
+            var app = builder.Build();
+
+            // These are different
+            Assert.NotSame(app.Configuration, builder.Configuration);
+        }
+
+        [Fact]
+        public void ManuallyAddingConfigurationAsServiceWorks()
+        {
+            var builder = WebApplication.CreateBuilder();
+            builder.Services.AddSingleton<IConfiguration>(builder.Configuration);
+            var sp = builder.Services.BuildServiceProvider();
+
+            var config = sp.GetService<IConfiguration>();
+            Assert.NotNull(config);
+            Assert.Same(config, builder.Configuration);
+
+            var app = builder.Build();
+
+            // These are different
+            Assert.NotSame(app.Configuration, builder.Configuration);
+        }
+
         [Fact]
         public async Task WebApplicationConfiguration_EnablesForwardedHeadersFromConfig()
         {