Add Parse for MutableJsonDocument from Utf8JsonReader (Azure#38188)

* Add parse for MutableJsonDocument from Utf8JsonReader

* pr fb

Author: annelo-msft

sdk/core/Azure.Core/src/DynamicData/DynamicData.cs

@@ -348,8 +348,8 @@ private class DynamicDataJsonConverter : JsonConverter<DynamicData>
         {
             public override DynamicData Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
             {
-                JsonDocument document = JsonDocument.ParseValue(ref reader);
-                return new DynamicData(new MutableJsonDocument(document, options).RootElement, DynamicDataOptions.FromSerializerOptions(options));
+                MutableJsonDocument mdoc = MutableJsonDocument.Parse(ref reader);
+                return new DynamicData(mdoc.RootElement, DynamicDataOptions.FromSerializerOptions(options));
             }
 
             public override void Write(Utf8JsonWriter writer, DynamicData value, JsonSerializerOptions options)

sdk/core/Azure.Core/src/DynamicData/MutableJsonDocument.cs

@@ -76,14 +76,26 @@ private void WriteJson(Stream stream)
         {
             if (!Changes.HasChanges)
             {
-                Write(stream, _original.Span);
+                WriteOriginal(stream);
                 return;
             }
 
             using Utf8JsonWriter writer = new(stream);
             RootElement.WriteTo(writer);
         }
 
+        private void WriteOriginal(Stream stream)
+        {
+            if (_original.Length == 0)
+            {
+                using Utf8JsonWriter writer = new(stream);
+                _originalDocument.WriteTo(writer);
+                return;
+            }
+
+            Write(stream, _original.Span);
+        }
+
         private void WritePatch(Stream stream)
         {
             if (!Changes.HasChanges)
@@ -142,6 +154,18 @@ public static MutableJsonDocument Parse(ReadOnlyMemory<byte> utf8Json, JsonSeria
             return new MutableJsonDocument(doc, utf8Json, serializerOptions);
         }
 
+        /// <summary>
+        /// Parses JSON into a <see cref="MutableJsonDocument"/>.
+        /// </summary>
+        /// <param name="reader">Reader holding the JSON value.</param>
+        /// <param name="serializerOptions">Serializer options used to serialize and deserialize any changes to the JSON.</param>
+        /// <returns>A <see cref="MutableJsonDocument"/> representation of the value.</returns>
+        public static MutableJsonDocument Parse(ref Utf8JsonReader reader, JsonSerializerOptions? serializerOptions = default)
+        {
+            JsonDocument doc = JsonDocument.ParseValue(ref reader);
+            return new MutableJsonDocument(doc, default, serializerOptions);
+        }
+
         /// <summary>
         /// Parses a UTF-8 encoded string representing a single JSON value into a <see cref="MutableJsonDocument"/>.
         /// </summary>
@@ -175,34 +199,18 @@ public void Dispose()
             _originalDocument.Dispose();
         }
 
-        internal MutableJsonDocument(JsonDocument document, JsonSerializerOptions? serializerOptions) : this(document, GetBytesFromDocument(document), serializerOptions)
-        {
-        }
-
-        internal MutableJsonDocument(JsonDocument document, ReadOnlyMemory<byte> utf8Json, JsonSerializerOptions? serializerOptions)
+        private MutableJsonDocument(JsonDocument document, ReadOnlyMemory<byte> utf8Json, JsonSerializerOptions? serializerOptions)
         {
-            _original = utf8Json;
             _originalDocument = document;
+            _original = utf8Json;
             _serializerOptions = serializerOptions ?? new JsonSerializerOptions();
         }
 
-        private static ReadOnlyMemory<byte> GetBytesFromDocument(JsonDocument document)
-        {
-            using MemoryStream stream = new();
-            using (Utf8JsonWriter writer = new(stream))
-            {
-                document.WriteTo(writer);
-            }
-
-            return new ReadOnlyMemory<byte>(stream.GetBuffer(), 0, (int)stream.Position);
-        }
-
         private class MutableJsonDocumentConverter : JsonConverter<MutableJsonDocument>
         {
             public override MutableJsonDocument Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
             {
-                JsonDocument document = JsonDocument.ParseValue(ref reader);
-                return new MutableJsonDocument(document, options);
+                return Parse(ref reader);
             }
 
             public override void Write(Utf8JsonWriter writer, MutableJsonDocument value, JsonSerializerOptions options)

sdk/core/Azure.Core/src/DynamicData/MutableJsonElement.cs

@@ -1208,8 +1208,8 @@ private class MutableJsonElementConverter : JsonConverter<MutableJsonElement>
         {
             public override MutableJsonElement Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
             {
-                JsonDocument document = JsonDocument.ParseValue(ref reader);
-                return new MutableJsonDocument(document, options).RootElement;
+                MutableJsonDocument mdoc = MutableJsonDocument.Parse(ref reader);
+                return mdoc.RootElement;
             }
 
             public override void Write(Utf8JsonWriter writer, MutableJsonElement value, JsonSerializerOptions options)

sdk/core/Azure.Core/tests/DynamicData/MutableJsonDocumentTests.cs

@@ -1106,6 +1106,28 @@ public void CanChangeRootElement()
             Assert.AreEqual(2, mdoc.RootElement.GetInt32());
         }
 
+        [Test]
+        public void CanParseFromUtf8JsonReader()
+        {
+            ReadOnlySpan<byte> utf8Json = """
+                {
+                    "foo": 1
+                }
+                """u8;
+            Utf8JsonReader reader = new(utf8Json);
+
+            MutableJsonDocument mdoc = MutableJsonDocument.Parse(ref reader);
+
+            Assert.AreEqual(1, mdoc.RootElement.GetProperty("foo").GetInt32());
+
+            using MemoryStream stream = new();
+            mdoc.WriteTo(stream, "J");
+            stream.Flush();
+            stream.Position = 0;
+
+            Assert.AreEqual("""{"foo":1}""", BinaryData.FromStream(stream).ToString());
+        }
+
         #region Helpers
 
         internal static void ValidateWriteTo(BinaryData json, MutableJsonDocument mdoc)