Address comments related to Transcoding classes.

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

@@ -31,7 +31,7 @@ public static partial class HttpContentJsonExtensions
         public static System.Threading.Tasks.Task<object?> ReadFromJsonAsync(this System.Net.Http.HttpContent content, System.Type type, System.Text.Json.JsonSerializerOptions? options = null, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) { throw null; }
         public static System.Threading.Tasks.Task<T> ReadFromJsonAsync<T>(this System.Net.Http.HttpContent content, System.Text.Json.JsonSerializerOptions? options = null, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) { throw null; }
     }
-    public partial class JsonContent : System.Net.Http.HttpContent
+    public sealed partial class JsonContent : System.Net.Http.HttpContent
     {
         internal JsonContent() { }
         public System.Type ObjectType { get { throw null; } }

src/libraries/System.Net.Http.Json/ref/System.Net.Http.Json.netcoreapp.cs

@@ -4,7 +4,7 @@
 
 namespace System.Net.Http.Json
 {
-    public partial class JsonContent : System.Net.Http.HttpContent
+    public sealed partial class JsonContent : System.Net.Http.HttpContent
     {
         protected override System.Threading.Tasks.Task SerializeToStreamAsync(System.IO.Stream stream, System.Net.TransportContext? context, System.Threading.CancellationToken cancellationToken) { throw null; }
     }

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

@@ -12,7 +12,7 @@
 
 namespace System.Net.Http.Json
 {
-    public partial class JsonContent : HttpContent
+    public sealed partial class JsonContent : HttpContent
     {
         internal const string JsonMediaType = "application/json";
         internal const string JsonType = "application";
@@ -70,6 +70,9 @@ private async Task SerializeToStreamAsyncCore(Stream targetStream, CancellationT
                 using (TranscodingWriteStream transcodingStream = new TranscodingWriteStream(targetStream, targetEncoding))
                 {
                     await JsonSerializer.SerializeAsync(transcodingStream, Value, ObjectType, _jsonSerializerOptions, cancellationToken).ConfigureAwait(false);
+                    // The transcoding streams use Encoders and Decoders that have internal buffers. We need to flush these
+                    // when there is no more data to be written. Stream.FlushAsync isn't suitable since it's
+                    // acceptable to Flush a Stream (multiple times) prior to completion.
                     await transcodingStream.FinalWriteAsync(cancellationToken).ConfigureAwait(false);
                 }
             }

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

@@ -8,7 +8,7 @@
 
 namespace System.Net.Http.Json
 {
-    public partial class JsonContent
+    public sealed partial class JsonContent
     {
         protected override Task SerializeToStreamAsync(Stream stream, TransportContext? context, CancellationToken cancellationToken)
             => SerializeToStreamAsyncCore(stream, cancellationToken);

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

@@ -18,7 +18,7 @@ internal sealed class TranscodingReadStream : Stream
         private static readonly int OverflowBufferSize = Encoding.UTF8.GetMaxByteCount(1); // The most number of bytes used to represent a single UTF char
 
         internal const int MaxByteBufferSize = 4096;
-        internal const int MaxCharBufferSize = 3 * MaxByteBufferSize;
+        internal const int MaxCharBufferSize = 4096;
 
         private readonly Stream _stream;
         private readonly Decoder _decoder;
@@ -118,7 +118,7 @@ private async Task<int> ReadAsyncCore(ArraySegment<byte> readBuffer, Cancellatio
             if (_charBuffer.Count == 0)
             {
                 int bytesRead = await ReadInputChars(cancellationToken).ConfigureAwait(false);
-                shouldFlushEncoder = bytesRead == 0;
+                shouldFlushEncoder = bytesRead == 0 && _byteBuffer.Count == 0;
             }
 
             bool completed = false;

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

@@ -101,11 +101,7 @@ private async Task WriteAsyncCore(ArraySegment<byte> bufferSegment, Cancellation
 
                 _charsDecoded += charsDecoded;
                 bufferSegment = bufferSegment.Slice(bytesDecoded);
-
-                if (!decoderCompleted)
-                {
-                    await WriteBufferAsync(cancellationToken).ConfigureAwait(false);
-                }
+                await WriteBufferAsync(cancellationToken).ConfigureAwait(false);
             }
         }
 
@@ -142,10 +138,7 @@ protected override void Dispose(bool disposing)
 
         public async Task FinalWriteAsync(CancellationToken cancellationToken)
         {
-            // First write any buffered content
-            await WriteBufferAsync(cancellationToken).ConfigureAwait(false);
-
-            // Now flush the encoder.
+            // Flush the encoder.
             byte[] byteBuffer = ArrayPool<byte>.Shared.Rent(_maxByteBufferSize);
             bool encoderCompleted = false;
 

src/libraries/System.Net.Http.Json/tests/FunctionalTests/JsonContentTests.cs

@@ -4,6 +4,8 @@
 
 using System.Net.Http.Headers;
 using System.Net.Test.Common;
+using System.Text;
+using System.Text.Json;
 using System.Threading.Tasks;
 using Xunit;
 
@@ -160,13 +162,16 @@ await LoopbackServer.CreateClientAndServerAsync(
                     {
                         var request = new HttpRequestMessage(HttpMethod.Post, uri);
                         MediaTypeHeaderValue mediaType = MediaTypeHeaderValue.Parse("application/json; charset=utf-16");
-                        request.Content = JsonContent.Create(Person.Create(), mediaType: mediaType);
+                        // Pass new options to avoid using the Default Web Options that use camelCase.
+                        request.Content = JsonContent.Create(Person.Create(), mediaType: mediaType, options: new JsonSerializerOptions());
                         await client.SendAsync(request);
                     }
                 },
                 async server => {
                     HttpRequestData req = await server.HandleRequestAsync();
                     Assert.Equal("application/json; charset=utf-16", req.GetSingleHeaderValue("Content-Type"));
+                    Person per = JsonSerializer.Deserialize<Person>(Encoding.Unicode.GetString(req.Body));
+                    per.Validate();
                 });
         }
 

src/libraries/System.Net.Http.Json/tests/UnitTests/TranscodingWriteStreamTests.cs

@@ -78,6 +78,9 @@ private static async Task WriteAsyncTest(Encoding targetEncoding, string message
 
             var transcodingStream = new TranscodingWriteStream(stream, targetEncoding);
             await JsonSerializer.SerializeAsync(transcodingStream, model, model.GetType());
+            // The transcoding streams use Encoders and Decoders that have internal buffers. We need to flush these
+            // when there is no more data to be written. Stream.FlushAsync isn't suitable since it's
+            // acceptable to Flush a Stream (multiple times) prior to completion.
             await transcodingStream.FinalWriteAsync(default);
             await transcodingStream.FlushAsync();