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 +64,13 @@ 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;
+
+        // The maximum number of characters allowed when writing raw UTF-16 JSON. This is the maximum length that we can guarantee can
+        // be safely transcoded to UTF-8 and fit within an integer-length span, given the max expansion factor of a single character (3).
+        public const int MaxUtf16RawValueLength = int.MaxValue / MaxExpansionFactorWhileTranscoding;
+
         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 +1,201 @@
+// 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;
+using System.Diagnostics;
+
+namespace System.Text.Json
+{
+    public sealed partial class Utf8JsonWriter
+    {
+        /// <summary>
+        /// Writes the input as JSON content. It is expected that the input content is a single complete JSON value.
+        /// </summary>
+        /// <param name="json">The raw JSON content to write.</param>
+        /// <param name="skipInputValidation">Whether to validate if the input is an RFC 8259-compliant JSON payload.</param>
+        /// <exception cref="ArgumentNullException">Thrown if <paramref name="json"/> is <see langword="null"/>.</exception>
+        /// <exception cref="ArgumentException">Thrown if the length of the input is zero or greater than 715,827,882 (<see cref="int.MaxValue"/> / 3).</exception>
+        /// <exception cref="JsonException">
+        /// Thrown if <paramref name="skipInputValidation"/> is <see langword="false"/>, and the input
+        /// is not a valid, complete, single JSON value according to the JSON RFC (https://tools.ietf.org/html/rfc8259)
+        /// or the input JSON exceeds a recursive depth of 64.
+        /// </exception>
+        /// <remarks>
+        /// When writing untrused JSON values, do not set <paramref name="skipInputValidation"/> to <see langword="true"/> as this can result in invalid JSON
+        /// being written, and/or the overall payload being written to the writer instance being invalid.
+        ///
+        /// When using this method, the input content will be written to the writer destination as-is, unless validation fails (when it is enabled).
+        ///
+        /// The <see cref="JsonWriterOptions.SkipValidation"/> value for the writer instance is honored when using this method.
+        ///
+        /// The <see cref="JsonWriterOptions.Indented"/> and <see cref="JsonWriterOptions.Encoder"/> values for the writer instance are not applied when using this method.
+        /// </remarks>
+        public void WriteRawValue(string json, bool skipInputValidation = false)
+        {
+            if (!_options.SkipValidation)
+            {
+                ValidateWritingValue();
+            }
+
+            if (json == null)
+            {
+                throw new ArgumentNullException(nameof(json));
+            }
+
+            TranscodeAndWriteRawValue(json.AsSpan(), skipInputValidation);
+        }
+
+        /// <summary>
+        /// Writes the input as JSON content. It is expected that the input content is a single complete JSON value.
+        /// </summary>
+        /// <param name="json">The raw JSON content to write.</param>
+        /// <param name="skipInputValidation">Whether to validate if the input is an RFC 8259-compliant JSON payload.</param>
+        /// <exception cref="ArgumentException">Thrown if the length of the input is zero or greater than 715,827,882 (<see cref="int.MaxValue"/> / 3).</exception>
+        /// <exception cref="JsonException">
+        /// Thrown if <paramref name="skipInputValidation"/> is <see langword="false"/>, and the input
+        /// is not a valid, complete, single JSON value according to the JSON RFC (https://tools.ietf.org/html/rfc8259)
+        /// or the input JSON exceeds a recursive depth of 64.
+        /// </exception>
+        /// <remarks>
+        /// When writing untrused JSON values, do not set <paramref name="skipInputValidation"/> to <see langword="true"/> as this can result in invalid JSON
+        /// being written, and/or the overall payload being written to the writer instance being invalid.
+        ///
+        /// When using this method, the input content will be written to the writer destination as-is, unless validation fails (when it is enabled).
+        ///
+        /// The <see cref="JsonWriterOptions.SkipValidation"/> value for the writer instance is honored when using this method.
+        ///
+        /// The <see cref="JsonWriterOptions.Indented"/> and <see cref="JsonWriterOptions.Encoder"/> values for the writer instance are not applied when using this method.
+        /// </remarks>
+        public void WriteRawValue(ReadOnlySpan<char> json, bool skipInputValidation = false)
+        {
+            if (!_options.SkipValidation)
+            {
+                ValidateWritingValue();
+            }
+
+            TranscodeAndWriteRawValue(json, skipInputValidation);
+        }
+
+        /// <summary>
+        /// Writes the input as JSON content. It is expected that the input content is a single complete JSON value.
+        /// </summary>
+        /// <param name="utf8Json">The raw JSON content to write.</param>
+        /// <param name="skipInputValidation">Whether to validate if the input is an RFC 8259-compliant JSON payload.</param>
+        /// <exception cref="ArgumentException">Thrown if the length of the input is zero or equal to <see cref="int.MaxValue"/>.</exception>
+        /// <exception cref="JsonException">
+        /// Thrown if <paramref name="skipInputValidation"/> is <see langword="false"/>, and the input
+        /// is not a valid, complete, single JSON value according to the JSON RFC (https://tools.ietf.org/html/rfc8259)
+        /// or the input JSON exceeds a recursive depth of 64.
+        /// </exception>
+        /// <remarks>
+        /// When writing untrused JSON values, do not set <paramref name="skipInputValidation"/> to <see langword="true"/> as this can result in invalid JSON
+        /// being written, and/or the overall payload being written to the writer instance being invalid.
+        ///
+        /// When using this method, the input content will be written to the writer destination as-is, unless validation fails (when it is enabled).
+        ///
+        /// The <see cref="JsonWriterOptions.SkipValidation"/> value for the writer instance is honored when using this method.
+        ///
+        /// The <see cref="JsonWriterOptions.Indented"/> and <see cref="JsonWriterOptions.Encoder"/> values for the writer instance are not applied when using this method.
+        /// </remarks>
+        public void WriteRawValue(ReadOnlySpan<byte> utf8Json, bool skipInputValidation = false)
+        {
+            if (!_options.SkipValidation)
+            {
+                ValidateWritingValue();
+            }
+
+            if (utf8Json.Length == int.MaxValue)
+            {
+                ThrowHelper.ThrowArgumentException_ValueTooLarge(int.MaxValue);
+            }
+
+            WriteRawValueCore(utf8Json, skipInputValidation);
+        }
+
+        private void TranscodeAndWriteRawValue(ReadOnlySpan<char> json, bool skipInputValidation)
+        {
+            if (json.Length > JsonConstants.MaxUtf16RawValueLength)
+            {
+                ThrowHelper.ThrowArgumentException_ValueTooLarge(json.Length);
+            }
+
+            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);
+                WriteRawValueCore(utf8Json, skipInputValidation);
+            }
+            finally
+            {
+                if (tempArray != null)
+                {
+                    utf8Json.Clear();
+                    ArrayPool<byte>.Shared.Return(tempArray);
+                }
+            }
+        }
+
+        private void WriteRawValueCore(ReadOnlySpan<byte> utf8Json, bool skipInputValidation)
+        {
+            int len = utf8Json.Length;
+
+            if (len == 0)
+            {
+                ThrowHelper.ThrowArgumentException(SR.ExpectedJsonTokens);
+            }
+
+            // In the UTF-16-based entry point methods above, we validate that the payload length <= int.MaxValue /3.
+            // The result of this division will be rounded down, so even if every input character needs to be transcoded
+            // (with expansion factor of 3), the resulting payload would be less than int.MaxValue,
+            // as (int.MaxValue/3) * 3 is less than int.MaxValue.
+            Debug.Assert(len < int.MaxValue);
+
+            if (skipInputValidation)
+            {
+                // Treat all unvalidated raw JSON value writes as string. If the payload is valid, this approach does
+                // not affect structural validation since a string token is equivalent to a complete object, array,
+                // or other complete JSON tokens when considering structural validation on subsequent writer calls.
+                // If the payload is not valid, then we make no guarantees about the structural validation of the final payload.
+                _tokenType = JsonTokenType.String;
+            }
+            else
+            {
+                // Utilize reader validation.
+                Utf8JsonReader reader = new(utf8Json);
+                while (reader.Read());
+                _tokenType = reader.TokenType;
+            }
+
+            // TODO (https://github.com/dotnet/runtime/issues/29293):
+            // investigate writing this in chunks, rather than requesting one potentially long, contiguous buffer.
+            int maxRequired = len + 1; // Optionally, 1 list separator. We've guarded against integer overflow earlier in the call stack.
+
+            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 += len;
+
+            SetFlagToAddListSeparatorBeforeNextItem();
+        }
+    }
+}

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

@@ -186,6 +186,7 @@
     <Compile Include="Utf8JsonReaderTests.TryGet.Date.cs" />
     <Compile Include="Utf8JsonReaderTests.ValueTextEquals.cs" />
     <Compile Include="Utf8JsonWriterTests.cs" />
+    <Compile Include="Utf8JsonWriterTests.WriteRaw.cs" />
   </ItemGroup>
   <ItemGroup>
     <Compile Include="..\..\src\System\Text\Json\BitStack.cs" Link="BitStack.cs" />