Add write-raw APIs to Utf8JsonWriter

src/libraries/System.Text.Json/ref/System.Text.Json.cs

@@ -461,6 +461,9 @@ public void WritePropertyName(System.ReadOnlySpan<byte> utf8PropertyName) { }
         public void WritePropertyName(System.ReadOnlySpan<char> propertyName) { }
         public void WritePropertyName(string propertyName) { }
         public void WritePropertyName(System.Text.Json.JsonEncodedText propertyName) { }
+        public void WriteRawValue(string json, bool skipInputValidation = false) { }
+        public void WriteRawValue(System.ReadOnlySpan<byte> utf8Json, bool skipInputValidation = false) { }
+        public void WriteRawValue(System.ReadOnlySpan<char> json, bool skipInputValidation = false) { }
         public void WriteStartArray() { }
         public void WriteStartArray(System.ReadOnlySpan<byte> utf8PropertyName) { }
         public void WriteStartArray(System.ReadOnlySpan<char> propertyName) { }

src/libraries/System.Text.Json/src/System.Text.Json.csproj

@@ -257,6 +257,7 @@
     <Compile Include="System\Text\Json\Writer\Utf8JsonWriter.WriteValues.Guid.cs" />
     <Compile Include="System\Text\Json\Writer\Utf8JsonWriter.WriteValues.Helpers.cs" />
     <Compile Include="System\Text\Json\Writer\Utf8JsonWriter.WriteValues.Literal.cs" />
+    <Compile Include="System\Text\Json\Writer\Utf8JsonWriter.WriteValues.Raw.cs" />
     <Compile Include="System\Text\Json\Writer\Utf8JsonWriter.WriteValues.SignedNumber.cs" />
     <Compile Include="System\Text\Json\Writer\Utf8JsonWriter.WriteValues.String.cs" />
     <Compile Include="System\Text\Json\Writer\Utf8JsonWriter.WriteValues.UnsignedNumber.cs" />

src/libraries/System.Text.Json/src/System/Text/Json/JsonConstants.cs

@@ -64,6 +64,9 @@ internal static class JsonConstants
         // All other UTF-16 characters can be represented by either 1 or 2 UTF-8 bytes.
         public const int MaxExpansionFactorWhileTranscoding = 3;
 
+        // When transcoding from UTF8 -> UTF16, the byte count threshold where we rent from the array pool before performing a normal alloc.
+        public const long ArrayPoolMaxSizeBeforeUsingNormalAlloc = 1024 * 1024;
+
         public const int MaxEscapedTokenSize = 1_000_000_000;   // Max size for already escaped value.
         public const int MaxUnescapedTokenSize = MaxEscapedTokenSize / MaxExpansionFactorWhileEscaping;  // 166_666_666 bytes
         public const int MaxBase64ValueTokenSize = (MaxEscapedTokenSize >> 2) * 3 / MaxExpansionFactorWhileEscaping;  // 125_000_000 bytes

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Read.String.cs

@@ -355,12 +355,10 @@ public static partial class JsonSerializer
 
         private static TValue? ReadUsingMetadata<TValue>(ReadOnlySpan<char> json, JsonTypeInfo jsonTypeInfo)
         {
-            const long ArrayPoolMaxSizeBeforeUsingNormalAlloc = 1024 * 1024;
-
             byte[]? tempArray = null;
 
             // For performance, avoid obtaining actual byte count unless memory usage is higher than the threshold.
-            Span<byte> utf8 = json.Length <= (ArrayPoolMaxSizeBeforeUsingNormalAlloc / JsonConstants.MaxExpansionFactorWhileTranscoding) ?
+            Span<byte> utf8 = json.Length <= (JsonConstants.ArrayPoolMaxSizeBeforeUsingNormalAlloc / JsonConstants.MaxExpansionFactorWhileTranscoding) ?
                 // Use a pooled alloc.
                 tempArray = ArrayPool<byte>.Shared.Rent(json.Length * JsonConstants.MaxExpansionFactorWhileTranscoding) :
                 // Use a normal alloc since the pool would create a normal alloc anyway based on the threshold (per current implementation)

src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteValues.Raw.cs

@@ -0,0 +1,107 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Buffers;
+
+namespace System.Text.Json
+{
+    public sealed partial class Utf8JsonWriter
+    {
+        /// <summary>
+        /// Writes the input as JSON content.
+        /// </summary>
+        /// <param name="json">The raw JSON content to write.</param>
+        /// <param name="skipInputValidation">Whether to skip validation of the input JSON content.</param>
+        public void WriteRawValue(string json, bool skipInputValidation = false)
+        {
+            if (json == null)
+            {
+                throw new ArgumentNullException(nameof(json));
+            }
+
+            WriteRawValue(json.AsSpan(), skipInputValidation);
+        }
+
+        /// <summary>
+        /// Writes the input as JSON content.
+        /// </summary>
+        /// <param name="json">The raw JSON content to write.</param>
+        /// <param name="skipInputValidation">Whether to skip validation of the input JSON content.</param>
+        public void WriteRawValue(ReadOnlySpan<char> json, bool skipInputValidation = false)
+        {
+            byte[]? tempArray = null;
+
+            // For performance, avoid obtaining actual byte count unless memory usage is higher than the threshold.
+            Span<byte> utf8Json = json.Length <= (JsonConstants.ArrayPoolMaxSizeBeforeUsingNormalAlloc / JsonConstants.MaxExpansionFactorWhileTranscoding) ?
+                // Use a pooled alloc.
+                tempArray = ArrayPool<byte>.Shared.Rent(json.Length * JsonConstants.MaxExpansionFactorWhileTranscoding) :
+                // Use a normal alloc since the pool would create a normal alloc anyway based on the threshold (per current implementation)
+                // and by using a normal alloc we can avoid the Clear().
+                new byte[JsonReaderHelper.GetUtf8ByteCount(json)];
+
+            try
+            {
+                int actualByteCount = JsonReaderHelper.GetUtf8FromText(json, utf8Json);
+                utf8Json = utf8Json.Slice(0, actualByteCount);
+                WriteRawValue(utf8Json, skipInputValidation);
+            }
+            finally
+            {
+                if (tempArray != null)
+                {
+                    utf8Json.Clear();
+                    ArrayPool<byte>.Shared.Return(tempArray);
+                }
+            }
+        }
+
+        /// <summary>
+        /// Writes the input as JSON content.
+        /// </summary>
+        /// <param name="utf8Json">The raw JSON content to write.</param>
+        /// <param name="skipInputValidation">Whether to skip validation of the input JSON content.</param>
+        public void WriteRawValue(ReadOnlySpan<byte> utf8Json, bool skipInputValidation = false)
+        {
+            if (utf8Json.Length == 0)
+            {
+                ThrowHelper.ThrowArgumentException(SR.ExpectedJsonTokens);
+            }
+
+            if (!skipInputValidation)
+            {
+                Utf8JsonReader reader = new Utf8JsonReader(utf8Json);
+
+                try
+                {
+                    while (reader.Read());
+                }
+                catch (JsonReaderException ex)
+                {
+                    ThrowHelper.ThrowArgumentException(ex.Message);
+                }
+            }
+
+            int maxRequired = utf8Json.Length + 1; // Optionally, 1 list separator
+
+            if (_memory.Length - BytesPending < maxRequired)
+            {
+                Grow(maxRequired);
+            }
+
+            Span<byte> output = _memory.Span;
+
+            if (_currentDepth < 0)
+            {
+                output[BytesPending++] = JsonConstants.ListSeparator;
+            }
+
+            utf8Json.CopyTo(output.Slice(BytesPending));
+            BytesPending += utf8Json.Length;
+
+            SetFlagToAddListSeparatorBeforeNextItem();
+
+            // Treat all raw JSON value writes as string.
+            _tokenType = JsonTokenType.String;
+        }
+    }
+}

src/libraries/System.Text.Json/tests/System.Text.Json.Tests/System.Text.Json.Tests.csproj

@@ -189,6 +189,7 @@
   </ItemGroup>
   <ItemGroup>
     <Compile Include="..\..\src\System\Text\Json\BitStack.cs" Link="BitStack.cs" />
+    <Compile Include="Utf8JsonWriterTests.WriteRaw.cs" />
   </ItemGroup>
   <ItemGroup Condition="'$(TargetFramework)' != '$(NetCoreAppCurrent)'">
     <Compile Include="$(CommonPath)System\Runtime\CompilerServices\IsExternalInit.cs" Link="Common\System\Runtime\CompilerServices\IsExternalInit.cs" />