Enable to pass an existing IServiceCollection (#1060).

src/Microsoft.AspNetCore.Hosting/WebHostBuilder.cs

@@ -32,6 +32,7 @@ public class WebHostBuilder : IWebHostBuilder
         private WebHostBuilderContext _context;
         private bool _webHostBuilt;
         private List<Action<WebHostBuilderContext, IConfigurationBuilder>> _configureAppConfigurationBuilderDelegates;
+        private Func<IServiceCollection> _serviceCollectionDelegate;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="WebHostBuilder"/> class.
@@ -87,6 +88,12 @@ public IWebHostBuilder UseSetting(string key, string value)
             return this;
         }
 
+        public IWebHostBuilder UseExternalServiceCollection(Func<IServiceCollection> serviceCollectionDelegate)
+        {
+            _serviceCollectionDelegate = serviceCollectionDelegate;
+            return this;
+        }
+
         /// <summary>
         /// Adds a delegate for configuring additional services for the host or web application. This may be called
         /// multiple times.
@@ -237,7 +244,16 @@ private IServiceCollection BuildCommonServices(out AggregateException hostingSta
             _hostingEnvironment.Initialize(contentRootPath, _options);
             _context.HostingEnvironment = _hostingEnvironment;
 
-            var services = new ServiceCollection();
+            IServiceCollection services;
+            if (_serviceCollectionDelegate == null)
+            {
+                services = new ServiceCollection();
+            }
+            else
+            {
+                services = _serviceCollectionDelegate();
+            }
+
             services.AddSingleton(_options);
             services.AddSingleton<IHostingEnvironment>(_hostingEnvironment);
             services.AddSingleton<Extensions.Hosting.IHostingEnvironment>(_hostingEnvironment);