Added unit test. Test works with the old code in .NET 6, but cannot test .NET 4.8 as the test framework needs all the .NET 6 stuff.

Author: kendallb

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 async Task Should_post_json_with_PostJsonAsync() {
 
         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 HttpServer(ITestOutputHelper output = null) {
 
         // 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));