Add JsonSerializerOptions.Default

src/libraries/System.Text.Json/System.Text.Json.sln

@@ -21,9 +21,9 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "System.Text.Encodings.Web",
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "System.Text.Encodings.Web", "..\System.Text.Encodings.Web\src\System.Text.Encodings.Web.csproj", "{9BCCDA15-8907-4AE3-8871-2F17775DDE4C}"
 EndProject
-Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "System.Text.Json.SourceGeneration", "gen\System.Text.Json.SourceGeneration.Roslyn3.11.csproj", "{04AEB008-EE4F-44DE-A361-2DBD2D0FD6A4}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "System.Text.Json.SourceGeneration.Roslyn3.11", "gen\System.Text.Json.SourceGeneration.Roslyn3.11.csproj", "{04AEB008-EE4F-44DE-A361-2DBD2D0FD6A4}"
 EndProject
-Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "System.Text.Json.SourceGeneration", "gen\System.Text.Json.SourceGeneration.Roslyn4.0.csproj", "{6485EED4-C313-4551-9865-8ADCED603629}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "System.Text.Json.SourceGeneration.Roslyn4.0", "gen\System.Text.Json.SourceGeneration.Roslyn4.0.csproj", "{6485EED4-C313-4551-9865-8ADCED603629}"
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "System.Text.Json", "ref\System.Text.Json.csproj", "{7015E94D-D20D-48C8-86D7-6A996BE99E0E}"
 EndProject

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

@@ -303,6 +303,7 @@ public JsonSerializerOptions(System.Text.Json.JsonSerializerDefaults defaults) {
         public JsonSerializerOptions(System.Text.Json.JsonSerializerOptions options) { }
         public bool AllowTrailingCommas { get { throw null; } set { } }
         public System.Collections.Generic.IList<System.Text.Json.Serialization.JsonConverter> Converters { get { throw null; } }
+        public static System.Text.Json.JsonSerializerOptions Default { get { throw null; } }
         public int DefaultBufferSize { get { throw null; } set { } }
         public System.Text.Json.Serialization.JsonIgnoreCondition DefaultIgnoreCondition { get { throw null; } set { } }
         public System.Text.Json.JsonNamingPolicy? DictionaryKeyPolicy { get { throw null; } set { } }

src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonArray.cs

@@ -168,7 +168,7 @@ public override void WriteTo(Utf8JsonWriter writer, JsonSerializerOptions? optio
                 CreateNodes();
                 Debug.Assert(_list != null);
 
-                options ??= JsonSerializerOptions.s_defaultOptions;
+                options ??= JsonSerializerOptions.Default;
 
                 writer.WriteStartArray();
 

src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonObject.cs

@@ -96,7 +96,7 @@ public override void WriteTo(Utf8JsonWriter writer, JsonSerializerOptions? optio
             }
             else
             {
-                options ??= JsonSerializerOptions.s_defaultOptions;
+                options ??= JsonSerializerOptions.Default;
 
                 writer.WriteStartObject();
 

src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonValueTrimmable.cs

@@ -34,7 +34,7 @@ public override void WriteTo(Utf8JsonWriter writer, JsonSerializerOptions? optio
 
             if (_converter != null)
             {
-                options ??= JsonSerializerOptions.s_defaultOptions;
+                options ??= JsonSerializerOptions.Default;
 
                 if (_converter.IsInternalConverterForNumberType)
                 {

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Helpers.cs

@@ -17,7 +17,7 @@ private static JsonTypeInfo GetTypeInfo(JsonSerializerOptions? options, Type run
         {
             Debug.Assert(runtimeType != null);
 
-            options ??= JsonSerializerOptions.s_defaultOptions;
+            options ??= JsonSerializerOptions.Default;
             if (!options.IsInitializedForReflectionSerializer)
             {
                 options.InitializeForReflectionSerializer();

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

@@ -371,7 +371,7 @@ public static partial class JsonSerializer
                 throw new ArgumentNullException(nameof(utf8Json));
             }
 
-            options ??= JsonSerializerOptions.s_defaultOptions;
+            options ??= JsonSerializerOptions.Default;
             if (!options.IsInitializedForReflectionSerializer)
             {
                 options.InitializeForReflectionSerializer();

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.cs

@@ -20,7 +20,15 @@ public sealed partial class JsonSerializerOptions
     {
         internal const int BufferSizeDefault = 16 * 1024;
 
-        internal static readonly JsonSerializerOptions s_defaultOptions = new JsonSerializerOptions();
+        /// <summary>
+        /// Gets a read-only, singleton instance of <see cref="JsonSerializerOptions" /> that uses the default configuration.
+        /// </summary>
+        /// <remarks>
+        /// Each <see cref="JsonSerializerOptions" /> instance encapsulates its own serialization metadata caches,
+        /// so using fresh default instances every time one is needed can result in redundant recomputation of converters.
+        /// This property provides a shared instance that can be consumed by any number of components without necessitating any converter recomputation.
+        /// </remarks>
+        public static JsonSerializerOptions Default { get; } = new JsonSerializerOptions { _haveTypesBeenCreated = true };
 
         private readonly ConcurrentDictionary<Type, JsonTypeInfo> _classes = new ConcurrentDictionary<Type, JsonTypeInfo>();
 
@@ -709,9 +717,6 @@ internal JsonWriterOptions GetWriterOptions()
 
         internal void VerifyMutable()
         {
-            // The default options are hidden and thus should be immutable.
-            Debug.Assert(this != s_defaultOptions);
-
             if (_haveTypesBeenCreated || _context != null)
             {
                 ThrowHelper.ThrowInvalidOperationException_SerializerOptionsImmutable(_context);

src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/OptionsTests.cs

@@ -512,6 +512,28 @@ public static void CopyConstructor_NullInput()
             Assert.Contains("options", ex.ToString());
         }
 
+        [Fact]
+        public static void JsonSerializerOptions_Default_MatchesDefaultConstructor()
+        {
+            var options = new JsonSerializerOptions();
+            JsonSerializerOptions optionsSingleton = JsonSerializerOptions.Default;
+            VerifyOptionsEqual(options, optionsSingleton);
+        }
+
+        [Fact]
+        public static void JsonSerializerOptions_Default_ReturnsSameInstance()
+        {
+            Assert.Same(JsonSerializerOptions.Default, JsonSerializerOptions.Default);
+        }
+
+        [Fact]
+        public static void JsonSerializerOptions_Default_IsReadOnly()
+        {
+            var optionsSingleton = JsonSerializerOptions.Default;
+            Assert.Throws<InvalidOperationException>(() => optionsSingleton.IncludeFields = true);
+            Assert.Throws<InvalidOperationException>(() => optionsSingleton.Converters.Add(new JsonStringEnumConverter()));
+        }
+
         [Fact]
         public static void DefaultSerializerOptions_General()
         {
@@ -542,7 +564,7 @@ private static JsonSerializerOptions GetFullyPopulatedOptionsInstance()
         {
             var options = new JsonSerializerOptions();
 
-            foreach (PropertyInfo property in typeof(JsonSerializerOptions).GetProperties())
+            foreach (PropertyInfo property in typeof(JsonSerializerOptions).GetProperties(BindingFlags.Public | BindingFlags.Instance))
             {
                 Type propertyType = property.PropertyType;
 
@@ -596,7 +618,7 @@ private static JsonSerializerOptions GetFullyPopulatedOptionsInstance()
 
         private static void VerifyOptionsEqual(JsonSerializerOptions options, JsonSerializerOptions newOptions)
         {
-            foreach (PropertyInfo property in typeof(JsonSerializerOptions).GetProperties())
+            foreach (PropertyInfo property in typeof(JsonSerializerOptions).GetProperties(BindingFlags.Public | BindingFlags.Instance))
             {
                 Type propertyType = property.PropertyType;