Stop modifying HttpClient.DefaultRequestHeaders (#2363)

* Stop modifying HttpClient.DefaultRequestHeaders (#2156)

Move Expect100Continue from HttpClient.DefaultRequestHeaders to
per-request HttpRequestMessage.Headers so that shared HttpClient
instances are not mutated by RestClient configuration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Set ExpectContinue on redirect messages too

Apply Options.Expect100Continue to HttpRequestMessages created during
redirect handling, matching the behavior of the initial request.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: alexeyzimarev

src/RestSharp.Extensions.DependencyInjection/ServiceCollectionExtensions.cs

@@ -26,7 +26,6 @@ public void AddRestClient(
 
             services
                 .AddHttpClient(name)
-                .ConfigureHttpClient(client => RestClient.ConfigureHttpClient(client, options))
                 .ConfigurePrimaryHttpMessageHandler(() => {
                         var handler = new HttpClientHandler();
                         RestClient.ConfigureHttpMessageHandler(handler, options);

src/RestSharp/RestClient.Async.cs

@@ -135,10 +135,11 @@ async Task<HttpResponse> ExecuteRequestAsync(RestRequest request, CancellationTo
             .AddCookieHeaders(url, Options.CookieContainer);
 
         var message = new HttpRequestMessage(httpMethod, url);
-        message.Content              = initialContent.BuildContent();
-        message.Headers.Host         = Options.BaseHost;
-        message.Headers.CacheControl = request.CachePolicy ?? Options.CachePolicy;
-        message.Version              = request.Version;
+        message.Content                = initialContent.BuildContent();
+        message.Headers.Host           = Options.BaseHost;
+        message.Headers.CacheControl   = request.CachePolicy ?? Options.CachePolicy;
+        message.Headers.ExpectContinue = Options.Expect100Continue;
+        message.Version                = request.Version;
         message.AddHeaders(headers);
 
 #pragma warning disable CS0618 // Type or member is obsolete
@@ -272,9 +273,10 @@ HttpRequestMessage CreateRedirectMessage(
         bool verbChangedToGet
     ) {
         var redirectMessage = new HttpRequestMessage(httpMethod, url);
-        redirectMessage.Version              = request.Version;
-        redirectMessage.Headers.Host         = Options.BaseHost;
-        redirectMessage.Headers.CacheControl = request.CachePolicy ?? Options.CachePolicy;
+        redirectMessage.Version                = request.Version;
+        redirectMessage.Headers.Host           = Options.BaseHost;
+        redirectMessage.Headers.CacheControl   = request.CachePolicy ?? Options.CachePolicy;
+        redirectMessage.Headers.ExpectContinue = Options.Expect100Continue;
 
         if (!verbChangedToGet && redirectOptions.ForwardBody) {
             var redirectContent = new RequestContent(this, request);

src/RestSharp/RestClient.cs

@@ -93,7 +93,6 @@ HttpClient GetClient() {
             ConfigureHttpMessageHandler(handler, options);
             var finalHandler = options.ConfigureMessageHandler?.Invoke(handler) ?? handler;
             var httpClient   = new HttpClient(finalHandler);
-            ConfigureHttpClient(httpClient, options);
 
             // We will use Options.Timeout in ExecuteAsInternalAsync method
             httpClient.Timeout = Timeout.InfiniteTimeSpan;
@@ -188,7 +187,6 @@ public RestClient(
         DefaultParameters = new(Options);
 
         if (options != null) {
-            ConfigureHttpClient(httpClient, options);
             ConfigureDefaultParameters(options);
         }
     }
@@ -224,10 +222,6 @@ public RestClient(
     )
         : this(new HttpClient(handler, disposeHandler), true, configureRestClient, configureSerialization) { }
 
-    internal static void ConfigureHttpClient(HttpClient httpClient, RestClientOptions options) {
-        if (options.Expect100Continue != null) httpClient.DefaultRequestHeaders.ExpectContinue = options.Expect100Continue;
-    }
-
     // ReSharper disable once CognitiveComplexity
     internal static void ConfigureHttpMessageHandler(HttpClientHandler handler, RestClientOptions options) {
 #if NET

test/RestSharp.Tests/RestClientTests.cs

@@ -123,4 +123,29 @@ public void ConfigureDefaultParameters_sets_user_agent_given_httpClient_instance
 
         Assert.Empty(httpClient.DefaultRequestHeaders.UserAgent);
     }
+
+    [Fact]
+    public void Should_not_set_expect_continue_on_shared_http_client_default_headers() {
+        // arrange
+        var httpClient = new HttpClient();
+        var options    = new RestClientOptions { Expect100Continue = true };
+
+        // act
+        using var restClient = new RestClient(httpClient, options);
+
+        // assert — the shared HttpClient's DefaultRequestHeaders must not be modified
+        httpClient.DefaultRequestHeaders.ExpectContinue.Should().BeNull();
+    }
+
+    [Fact]
+    public void Should_not_set_expect_continue_on_new_http_client_default_headers() {
+        // arrange
+        var options = new RestClientOptions { Expect100Continue = false };
+
+        // act
+        using var restClient = new RestClient(options);
+
+        // assert
+        restClient.HttpClient.DefaultRequestHeaders.ExpectContinue.Should().BeNull();
+    }
 }