DEV-2959: Fix HttpClient socket exhaustion in .NET SDK

mustache-templates/csharp/ApiClient.mustache

@@ -170,6 +170,8 @@ namespace {{packageName}}.Client
     {{>visibility}} partial class ApiClient : ISynchronousClient{{#supportsAsync}}, IAsynchronousClient{{/supportsAsync}}
     {
         private readonly string _baseUrl;
+        private RestClient _restClient;
+        private readonly object _restClientLock = new object();
 
         /// <summary>
         /// Specifies the settings on a <see cref="JsonSerializer" /> object.
@@ -436,6 +438,14 @@ namespace {{packageName}}.Client
                 }
             }
 
+            if (options.Cookies != null && options.Cookies.Count > 0)
+            {
+                foreach (var cookie in options.Cookies)
+                {
+                    request.AddCookie(cookie.Name, cookie.Value, cookie.Path ?? "/", cookie.Domain ?? "");
+                }
+            }
+
             StopTiming(debugModeEnabled, stopwatch, "Request construction", method.ToString(), path);
 
             return request;
@@ -492,6 +502,24 @@ namespace {{packageName}}.Client
             return transformed;
         }
 
+        /// <summary>
+        /// Gets or creates a shared RestClient for the given options (lazy init, thread-safe).
+        /// Used to avoid socket exhaustion by reusing one client per ApiClient instance.
+        /// </summary>
+        private RestClient GetOrCreateRestClient(RestClientOptions clientOptions, IReadableConfiguration configuration)
+        {
+            if (_restClient != null)
+                return _restClient;
+            lock (_restClientLock)
+            {
+                if (_restClient != null)
+                    return _restClient;
+                _restClient = new RestClient(clientOptions,
+                    configureSerialization: serializerConfig => serializerConfig.UseSerializer(() => new CustomJsonCodec(SerializerSettings, configuration)));
+                return _restClient;
+            }
+        }
+
         /// <summary>
         /// Executes the HTTP request for the current service.
         /// Based on functions received it can be async or sync.
@@ -534,8 +562,19 @@ namespace {{packageName}}.Client
             }
 
             {{/hasOAuthMethods}}
-            using (RestClient client = new RestClient(clientOptions,
-                configureSerialization: serializerConfig => serializerConfig.UseSerializer(() => new CustomJsonCodec(SerializerSettings, configuration))))
+            RestClient client;
+            bool useSharedClient = (baseUrl == _baseUrl);
+            if (useSharedClient)
+            {
+                client = GetOrCreateRestClient(clientOptions, configuration);
+            }
+            else
+            {
+                client = new RestClient(clientOptions,
+                    configureSerialization: serializerConfig => serializerConfig.UseSerializer(() => new CustomJsonCodec(SerializerSettings, configuration)));
+            }
+
+            try
             {
                 InterceptRequest(request);
 
@@ -602,6 +641,13 @@ namespace {{packageName}}.Client
                 }
                 return result;
             }
+            finally
+            {
+                if (!useSharedClient)
+                {
+                    client?.Dispose();
+                }
+            }
         }
 
         private async Task<RestResponse<T>> DeserializeRestResponseFromPolicyAsync<T>(RestClient client, RestRequest request, PolicyResult<RestResponse> policyResult, bool debugModeEnabled = false, CancellationToken cancellationToken = default)
@@ -629,16 +675,7 @@ namespace {{packageName}}.Client
         {
             Action<RestClientOptions> setOptions = (clientOptions) =>
             {
-                var cookies = new CookieContainer();
-
-                if (options.Cookies != null && options.Cookies.Count > 0)
-                {
-                    foreach (var cookie in options.Cookies)
-                    {
-                        cookies.Add(new Cookie(cookie.Name, cookie.Value));
-                    }
-                }
-                clientOptions.CookieContainer = cookies;
+                // Cookies are added per-request in NewRequest to avoid mutating shared RestClient
             };
 
             Func<RestClient, Task<RestResponse<T>>> getResponse = (client) =>

mustache-templates/csharp/README.mustache

@@ -269,7 +269,8 @@ This SDK is auto-generated (except for tests), so changes made here will be over
 {{/authMethods}}
 
 ## 📅 Changelog
-
+- **2026-03-16** - `9.0.2`
+    - Fixed: HttpClient/RestClient is no longer disposed after every request; a single shared instance per ApiClient is used to prevent socket exhaustion.
 - **2026-03-05** - `9.0.1`
     - Added debug mode to measure request timings
 - **2025-12-14** - `9.0.0`