Add a JsonNode roundtrip benchmark

src/benchmarks/micro/MicroBenchmarks.csproj

@@ -154,6 +154,7 @@
     <Compile Remove="libraries\Common\AlignedMemory.cs" />
     <Compile Remove="libraries\System.IO.FileSystem\Perf.RandomAccess.cs" />
     <Compile Remove="libraries\System.IO.FileSystem\Perf.RandomAccess.NoBuffering.cs" />
+    <Compile Remove="libraries\System.Text.Json\Node\*.cs" />
   </ItemGroup>
 
   <ItemGroup Condition=" '$(TargetFrameworkIdentifier)' == '.NETFramework' Or ('$(TargetFrameworkIdentifier)' == '.NETCoreApp' And '$(_TargetFrameworkVersionWithoutV)' &lt; '7.0')">

src/benchmarks/micro/libraries/System.Text.Json/Node/ParseThenWrite.cs

@@ -0,0 +1,89 @@
+using BenchmarkDotNet.Attributes;
+using MicroBenchmarks;
+using System.Buffers;
+using System.Collections.Generic;
+using System.Text.Json.Document.Tests;
+using System.Text.Json.Nodes;
+using System.Text.Json.Tests;
+
+namespace System.Text.Json.Node.Tests
+{
+    [BenchmarkCategory(Categories.Libraries, Categories.JSON)]
+    public class Perf_ParseThenWrite
+    {
+        public enum TestCaseType
+        {
+            HelloWorld,
+            DeepTree,
+            BroadTree,
+            LotsOfNumbers,
+            LotsOfStrings,
+            Json400B,
+            Json4KB,
+            Json400KB
+        }
+
+        private byte[] _dataUtf8;
+        private Utf8JsonWriter _writer;
+
+        [ParamsAllValues]
+        public TestCaseType TestCase;
+
+        [Params(true, false)]
+        public bool IsDataIndented;
+
+        [GlobalSetup]
+        public void Setup()
+        {
+            string jsonString = JsonStrings.ResourceManager.GetString(TestCase.ToString());
+
+            if (!IsDataIndented)
+            {
+                _dataUtf8 = DocumentHelpers.RemoveFormatting(jsonString);
+            }
+            else
+            {
+                _dataUtf8 = Encoding.UTF8.GetBytes(jsonString);
+            }
+
+            var abw = new ArrayBufferWriter<byte>();
+            _writer = new Utf8JsonWriter(abw, new JsonWriterOptions { Indented = IsDataIndented });
+        }
+
+        [GlobalCleanup]
+        public void CleanUp()
+        {
+            _writer.Dispose();
+        }
+
+        [Benchmark]
+        public void ParseThenWrite()
+        {
+            _writer.Reset();
+
+            JsonNode jsonNode = JsonNode.Parse(_dataUtf8);
+            WalkNode(jsonNode);
+            jsonNode.WriteTo(_writer);
+
+
+            static void WalkNode(JsonNode node)
+            {
+                // Forces conversion of lazy JsonElement representation of document into
+                // a materialized JsonNode tree so that we measure writing performance
+                // of the latter representation.
+
+                switch (node)
+                {
+                    case JsonObject obj:
+                        foreach (KeyValuePair<string, JsonNode> kvp in obj)
+                            WalkNode(kvp.Value);
+                        break;
+                    case JsonArray arr:
+                        foreach (JsonNode elem in arr)
+                            WalkNode(elem);
+                        break;
+                }
+            }
+        }
+    }
+}