Refactor JsonSerializerOptions resolution

- Move resolution to own private class.
- Only resolve the value once.
- Add comments about resolution.
- Make `ISerializerDataContractResolver` a singleton by default.

src/Swashbuckle.AspNetCore.SwaggerGen/DependencyInjection/SwaggerGenServiceCollectionExtensions.cs

@@ -29,23 +29,10 @@ public static class SwaggerGenServiceCollectionExtensions
             services.TryAddTransient(s => s.GetRequiredService<IOptions<SwaggerGeneratorOptions>>().Value);
             services.TryAddTransient<ISchemaGenerator, SchemaGenerator>();
             services.TryAddTransient(s => s.GetRequiredService<IOptions<SchemaGeneratorOptions>>().Value);
-            services.TryAddTransient<ISerializerDataContractResolver>(s =>
+            services.AddSingleton<JsonSerializerOptionsProvider>();
+            services.TryAddSingleton<ISerializerDataContractResolver>(s =>
             {
-#if (!NETSTANDARD2_0)
-                var serializerOptions =
-                    s.GetService<IOptions<AspNetCore.Mvc.JsonOptions>>()?.Value?.JsonSerializerOptions
-#if NET8_0_OR_GREATER
-                    ?? s.GetService<IOptions<AspNetCore.Http.Json.JsonOptions>>()?.Value?.SerializerOptions
-#endif
-#if NET7_0_OR_GREATER
-                    ?? JsonSerializerOptions.Default;
-#else
-                    ?? new JsonSerializerOptions();
-#endif
-#else
-                var serializerOptions = new JsonSerializerOptions();
-#endif
-
+                var serializerOptions = s.GetRequiredService<JsonSerializerOptionsProvider>().Options;
                 return new JsonSerializerDataContractResolver(serializerOptions);
             });
 
@@ -63,5 +50,46 @@ public static class SwaggerGenServiceCollectionExtensions
         {
             services.Configure(setupAction);
         }
+
+        private sealed class JsonSerializerOptionsProvider
+        {
+            private readonly IServiceProvider _serviceProvider;
+            private JsonSerializerOptions _options;
+
+            public JsonSerializerOptionsProvider(IServiceProvider serviceProvider)
+            {
+                _serviceProvider = serviceProvider;
+            }
+
+            public JsonSerializerOptions Options => _options ??= ResolveOptions();
+
+            private JsonSerializerOptions ResolveOptions()
+            {
+                JsonSerializerOptions serializerOptions;
+
+                /*
+                 * First try to get the options configured for MVC,
+                 * then try to get the options configured for Minimal APIs if available,
+                 * then try the default JsonSerializerOptions if available,
+                 * otherwise create a new instance as a last resort as this is an expensive operation.
+                 */
+#if !NETSTANDARD2_0
+                serializerOptions =
+                    _serviceProvider.GetService<IOptions<AspNetCore.Mvc.JsonOptions>>()?.Value?.JsonSerializerOptions
+#if NET8_0_OR_GREATER
+                    ?? _serviceProvider.GetService<IOptions<AspNetCore.Http.Json.JsonOptions>>()?.Value?.SerializerOptions
+#endif
+#if NET7_0_OR_GREATER
+                    ?? JsonSerializerOptions.Default;
+#else
+                    ?? new JsonSerializerOptions();
+#endif
+#else
+                serializerOptions = new JsonSerializerOptions();
+#endif
+
+                return serializerOptions;
+            }
+        }
     }
 }