Work around bugs in HttpClient and long form POST data.

src/RestSharp/Request/RequestContent.cs

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+using System.Net;
 using System.Net.Http.Headers;
 using System.Runtime.Serialization;
 using RestSharp.Extensions;
@@ -157,13 +158,16 @@ class RequestContent : IDisposable {
             }
         }
         else {
-            // we should not have anything else except the parameters, so we send them as form URL encoded
-            var formContent = new FormUrlEncodedContent(
-                _request.Parameters
-                    .Where(x => x.Type == ParameterType.GetOrPost)
-                    .Select(x => new KeyValuePair<string, string>(x.Name!, x.Value!.ToString()!))!
-            );
-            Content = formContent;
+            // we should not have anything else except the parameters, so we send them as form URL encoded. However due
+            // to bugs in HttpClient FormUrlEncodedContent (see https://github.com/restsharp/RestSharp/issues/1814) we
+            // do the encoding ourselves using WebUtility.UrlEncode instead.
+            var formData = _request.Parameters
+                .Where(x => x.Type == ParameterType.GetOrPost)
+                .Select(x => new KeyValuePair<string, string>(x.Name!, x.Value!.ToString()!))!;
+            var encodedItems   = formData.Select(i => $"{WebUtility.UrlEncode(i.Key)}={WebUtility.UrlEncode(i.Value)}"/*.Replace("%20", "+")*/);
+            var encodedContent = new StringContent(string.Join("&", encodedItems), null, "application/x-www-form-urlencoded");
+
+            Content = encodedContent;
         }
     }
 

test/RestSharp.Tests.Integrated/PostTests.cs

@@ -1,3 +1,4 @@
+using System.Net;
 using RestSharp.Tests.Integrated.Server;
 
 namespace RestSharp.Tests.Integrated;
@@ -33,4 +34,19 @@ public class PostTests {
 
         response.Message.Should().Be(body.Data);
     }
+
+    class Response {
+        public string Message { get; set; }
+    }
+
+    [Fact]
+    public async Task Should_post_large_form_data() {
+        const int length          = 1024 * 1024;
+        var       superLongString = new string('?', length);
+        var       request         = new RestRequest("post/form", Method.Post).AddParameter("big_string", superLongString);
+        var       response        = await _client.ExecuteAsync<Response>(request);
+
+        response.StatusCode.Should().Be(HttpStatusCode.OK);
+        response.Data!.Message.Should().Be($"Works! Length: {length}");
+    }
 }

test/RestSharp.Tests.Integrated/Server/TestServer.cs

@@ -53,6 +53,7 @@ public sealed class HttpServer {
 
         // POST
         _app.MapPost("/post/json", (TestRequest request) => new TestResponse { Message = request.Data });
+        _app.MapPost("/post/form", (HttpContext context) => new TestResponse { Message               = $"Works! Length: {context.Request.Form["big_string"].ToString().Length}" });
 
         IResult HandleHeaders(HttpContext ctx) {
             var response = ctx.Request.Headers.Select(x => new TestServerResponse(x.Key, x.Value));