Delay resolution until after application services are built (#1553)

- Resolve the logger from the right service provider to log duplicate hosting startup assemblies. - Don't create a 3rd IServiceProvider if we resolved the default implementation.

Author: davidfowl

src/Microsoft.AspNetCore.Hosting/WebHostBuilder.cs

@@ -174,13 +174,6 @@ public IWebHost Build()
  }
  }
 
- var logger = hostingServiceProvider.GetRequiredService<ILogger<WebHost>>();
- // Warn about duplicate HostingStartupAssemblies
- foreach (var assemblyName in _options.GetFinalHostingStartupAssemblies().GroupBy(a => a, StringComparer.OrdinalIgnoreCase).Where(g => g.Count() > 1))
- {
- logger.LogWarning($"The assembly {assemblyName} was specified multiple times. Hosting startup assemblies should only be specified once.");
- }
-
  AddApplicationServices(applicationServices, hostingServiceProvider);
 
  var host = new WebHost(
@@ -193,6 +186,14 @@ public IWebHost Build()
  {
  host.Initialize();
 
+ var logger = host.Services.GetRequiredService<ILogger<WebHost>>();
+
+ // Warn about duplicate HostingStartupAssemblies
+ foreach (var assemblyName in _options.GetFinalHostingStartupAssemblies().GroupBy(a => a, StringComparer.OrdinalIgnoreCase).Where(g => g.Count() > 1))
+ {
+ logger.LogWarning($"The assembly {assemblyName} was specified multiple times. Hosting startup assemblies should only be specified once.");
+ }
+
  return host;
  }
  catch
@@ -208,7 +209,7 @@ IServiceProvider GetProviderFromFactory(IServiceCollection collection)
  var provider = collection.BuildServiceProvider();
  var factory = provider.GetService<IServiceProviderFactory<IServiceCollection>>();
 
- if (factory != null)
+ if (factory != null && !(factory is DefaultServiceProviderFactory))
  {
  using (provider)
  {