Fix NRE in KestrelServerOptions.Listen*()

Author: halter73

src/Servers/Kestrel/Core/src/Internal/ConfigurationReader.cs

@@ -19,19 +19,20 @@ internal class ConfigurationReader
 
         private readonly IConfiguration _configuration;
 
+        private IDictionary<string, CertificateConfig> _certificates;
+        private EndpointDefaults _endpointDefaults;
+        private IEnumerable<EndpointConfig> _endpoints;
+        private bool? _latin1RequestHeaders;
+
         public ConfigurationReader(IConfiguration configuration)
         {
             _configuration = configuration ?? throw new ArgumentNullException(nameof(configuration));
-            Certificates = ReadCertificates();
-            EndpointDefaults = ReadEndpointDefaults();
-            Endpoints = ReadEndpoints();
-            Latin1RequestHeaders = _configuration.GetValue<bool>(Latin1RequestHeadersKey);
         }
 
-        public IDictionary<string, CertificateConfig> Certificates { get; }
-        public EndpointDefaults EndpointDefaults { get; }
-        public IEnumerable<EndpointConfig> Endpoints { get; }
-        public bool Latin1RequestHeaders  { get; }
+        public IDictionary<string, CertificateConfig> Certificates => _certificates ??= ReadCertificates();
+        public EndpointDefaults EndpointDefaults => _endpointDefaults ??= ReadEndpointDefaults();
+        public IEnumerable<EndpointConfig> Endpoints => _endpoints ??= ReadEndpoints();
+        public bool Latin1RequestHeaders => _latin1RequestHeaders ??= _configuration.GetValue<bool>(Latin1RequestHeadersKey);
 
         private IDictionary<string, CertificateConfig> ReadCertificates()
         {

src/Servers/Kestrel/Core/src/KestrelConfigurationLoader.cs

@@ -29,6 +29,8 @@ internal KestrelConfigurationLoader(KestrelServerOptions options, IConfiguration
             Options = options ?? throw new ArgumentNullException(nameof(options));
             Configuration = configuration ?? throw new ArgumentNullException(nameof(configuration));
             ReloadOnChange = reloadOnChange;
+
+            ConfigurationReader = new ConfigurationReader(configuration);
         }
 
         public KestrelServerOptions Options { get; }

src/Servers/Kestrel/Core/test/KestrelServerOptionsTests.cs

@@ -49,5 +49,16 @@ public void ConfigureEndpointDefaultsAppliesToNewEndpoints()
             });
             Assert.Equal(HttpProtocols.Http2, options.CodeBackedListenOptions[3].Protocols);
         }
+
+        [Fact]
+        public void CanCallListenAfterConfigure()
+        {
+            var options = new KestrelServerOptions();
+            options.Configure();
+
+            // This is a regression test to verify the Listen* methods don't throw a NullReferenceException if called after Configure().
+            // https://github.com/dotnet/aspnetcore/issues/21423
+            options.ListenLocalhost(5000);
+        }
     }
 }