Fix Http.Json serialization performance by using static options (#35040)

Author: jozkee

src/libraries/System.Net.Http.Json/src/System/Net/Http/Json/HttpContentJsonExtensions.cs

@@ -19,7 +19,7 @@ public static class HttpContentJsonExtensions
             Debug.Assert(content.Headers.ContentType != null);
             Encoding? sourceEncoding = JsonContent.GetEncoding(content.Headers.ContentType.CharSet);
 
-            return ReadFromJsonAsyncCore(content, type, sourceEncoding, options ?? JsonContent.DefaultSerializerOptions, cancellationToken);
+            return ReadFromJsonAsyncCore(content, type, sourceEncoding, options, cancellationToken);
         }
 
         public static Task<T> ReadFromJsonAsync<T>(this HttpContent content, JsonSerializerOptions? options = null, CancellationToken cancellationToken = default)
@@ -28,7 +28,7 @@ public static Task<T> ReadFromJsonAsync<T>(this HttpContent content, JsonSeriali
             Debug.Assert(content.Headers.ContentType != null);
             Encoding? sourceEncoding = JsonContent.GetEncoding(content.Headers.ContentType.CharSet);
 
-            return ReadFromJsonAsyncCore<T>(content, sourceEncoding, options ?? JsonContent.DefaultSerializerOptions, cancellationToken);
+            return ReadFromJsonAsyncCore<T>(content, sourceEncoding, options, cancellationToken);
         }
 
         private static async Task<object?> ReadFromJsonAsyncCore(HttpContent content, Type type, Encoding? sourceEncoding, JsonSerializerOptions? options, CancellationToken cancellationToken)
@@ -43,7 +43,7 @@ public static Task<T> ReadFromJsonAsync<T>(this HttpContent content, JsonSeriali
 
             using (contentStream)
             {
-                return await JsonSerializer.DeserializeAsync(contentStream, type, options, cancellationToken).ConfigureAwait(false);
+                return await JsonSerializer.DeserializeAsync(contentStream, type, options ?? JsonContent.s_defaultSerializerOptions, cancellationToken).ConfigureAwait(false);
             }
         }
 
@@ -59,7 +59,7 @@ private static async Task<T> ReadFromJsonAsyncCore<T>(HttpContent content, Encod
 
             using (contentStream)
             {
-                return await JsonSerializer.DeserializeAsync<T>(contentStream, options, cancellationToken).ConfigureAwait(false);
+                return await JsonSerializer.DeserializeAsync<T>(contentStream, options ?? JsonContent.s_defaultSerializerOptions, cancellationToken).ConfigureAwait(false);
             }
         }
 

src/libraries/System.Net.Http.Json/src/System/Net/Http/Json/JsonContent.cs

@@ -20,8 +20,8 @@ public sealed partial class JsonContent : HttpContent
         private static MediaTypeHeaderValue DefaultMediaType
             => new MediaTypeHeaderValue(JsonMediaType) { CharSet = "utf-8" };
 
-        internal static JsonSerializerOptions DefaultSerializerOptions
-            => new JsonSerializerOptions { PropertyNameCaseInsensitive = true, PropertyNamingPolicy = JsonNamingPolicy.CamelCase };
+        internal static readonly JsonSerializerOptions s_defaultSerializerOptions
+            = new JsonSerializerOptions { PropertyNameCaseInsensitive = true, PropertyNamingPolicy = JsonNamingPolicy.CamelCase };
 
         private readonly JsonSerializerOptions? _jsonSerializerOptions;
         public Type ObjectType { get; }
@@ -42,7 +42,7 @@ private JsonContent(object? inputValue, Type inputType, MediaTypeHeaderValue? me
             Value = inputValue;
             ObjectType = inputType;
             Headers.ContentType = mediaType ?? DefaultMediaType;
-            _jsonSerializerOptions = options ?? DefaultSerializerOptions;
+            _jsonSerializerOptions = options ?? s_defaultSerializerOptions;
         }
 
         public static JsonContent Create<T>(T inputValue, MediaTypeHeaderValue? mediaType = null, JsonSerializerOptions? options = null)