From 881438d8353bd2072b3050fa84b5266c07c42932 Mon Sep 17 00:00:00 2001
From: MoonsvnLyn <287222957+FirmaSpring@users.noreply.github.com>
Date: Mon, 17 Aug 2026 18:25:28 +0800
Subject: [PATCH] Register default serializers lazily so custom serializers can
 replace them without loading System.Text.Json

Fixes #2402

Co-authored-by: FirmamentalSpring <287222957+FirmaSpring@users.noreply.github.com>
---
 src/RestSharp/Serializers/SerializerConfig.cs | 18 ++++++++++++-
 .../NewtonsoftJson/IntegratedSimpleTests.cs   |  8 ++++++
 test/RestSharp.Tests/RestClientTests.cs       | 25 +++++++++++++++++++
 3 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/src/RestSharp/Serializers/SerializerConfig.cs b/src/RestSharp/Serializers/SerializerConfig.cs
index 27055cf12..d8065ecaa 100644
--- a/src/RestSharp/Serializers/SerializerConfig.cs
+++ b/src/RestSharp/Serializers/SerializerConfig.cs
@@ -37,7 +37,23 @@ public SerializerConfig UseSerializer(Func<IRestSerializer> serializerFactory) {
         return this;
     }
 
-    public void UseDefaultSerializers() => UseSerializer<SystemTextJsonSerializer>().UseSerializer<XmlRestSerializer>();
+    public void UseDefaultSerializers() {
+        // Register factories without constructing the default serializers. Instantiating
+        // System.Text.Json here would pull it in even when the caller immediately replaces
+        // JSON with Newtonsoft.Json (or another custom serializer).
+        Serializers[DataFormat.Json] = new(
+            DataFormat.Json,
+            ContentType.JsonAccept,
+            contentType => contentType.Value.EndsWith("json", StringComparison.InvariantCultureIgnoreCase),
+            static () => new SystemTextJsonSerializer()
+        );
+        Serializers[DataFormat.Xml] = new(
+            DataFormat.Xml,
+            ContentType.XmlAccept,
+            contentType => contentType.Value.EndsWith("xml", StringComparison.InvariantCultureIgnoreCase),
+            static () => new XmlRestSerializer()
+        );
+    }
 
     /// <summary>
     /// Replace the default serializer with a custom one
diff --git a/test/RestSharp.Tests.Serializers.Json/NewtonsoftJson/IntegratedSimpleTests.cs b/test/RestSharp.Tests.Serializers.Json/NewtonsoftJson/IntegratedSimpleTests.cs
index 995834822..025b87c2d 100644
--- a/test/RestSharp.Tests.Serializers.Json/NewtonsoftJson/IntegratedSimpleTests.cs
+++ b/test/RestSharp.Tests.Serializers.Json/NewtonsoftJson/IntegratedSimpleTests.cs
@@ -24,6 +24,14 @@ public async Task Should_serialize_request() {
         actual.Should().BeEquivalentTo(testData);
     }
 
+    [Fact]
+    public void UseNewtonsoftJson_replaces_default_json_serializer() {
+        using var client = new RestClient(_server.Url!, configureSerialization: cfg => cfg.UseNewtonsoftJson());
+
+        client.Serializers.GetSerializer(DataFormat.Json).Should().BeOfType<JsonNetSerializer>();
+        client.Serializers.Serializers.Should().ContainKey(DataFormat.Xml);
+    }
+
     [Fact]
     public async Task Should_deserialize_response() {
         var expected = Fixture.Create<TestClass>();
diff --git a/test/RestSharp.Tests/RestClientTests.cs b/test/RestSharp.Tests/RestClientTests.cs
index 2c82278f1..f8c4067b6 100644
--- a/test/RestSharp.Tests/RestClientTests.cs
+++ b/test/RestSharp.Tests/RestClientTests.cs
@@ -71,6 +71,31 @@ public void UseOnlySerializer_leaves_only_custom_serializer() {
         client.Serializers.GetSerializer(DataFormat.Json).Should().NotBeNull();
     }
 
+    [Fact]
+    public void Default_serializers_are_registered_lazily_and_survive_replacement() {
+        // arrange
+        var baseUrl = new Uri(BaseUrl);
+
+        // act
+        using var client = new RestClient(baseUrl, configureSerialization: cfg => cfg.UseOnlySerializer(() => new JsonNetSerializerStub()));
+
+        // assert
+        client.Serializers.Serializers.Should().HaveCount(1);
+        client.Serializers.GetSerializer(DataFormat.Json).Should().BeOfType<JsonNetSerializerStub>();
+    }
+
+    sealed class JsonNetSerializerStub : IRestSerializer, ISerializer, IDeserializer {
+        public string? Serialize(object? obj)                                             => null;
+        public string? Serialize(Parameter bodyParameter)                                 => null;
+        public T? Deserialize<T>(RestResponse response)                                   => default;
+        public ContentType ContentType { get; set; }                                      = ContentType.Json;
+        public ISerializer Serializer                                                     => this;
+        public IDeserializer Deserializer                                                 => this;
+        public DataFormat DataFormat                                                      => DataFormat.Json;
+        public string[] AcceptedContentTypes                                              => ContentType.JsonAccept;
+        public SupportsContentType SupportsContentType { get; }                           = _ => false;
+    }
+
     [Fact]
     public void Should_reuse_httpClient_instance() {
         using var client1 = new RestClient(new Uri("https://fake.api"), useClientFactory: true);