NSwag生成C#客户端调用POST文件上传API时连接被强制关闭问题

错误详情 #

Exception occured on line : (after PrepareRequest)

var response_ = await client_.SendAsync(request_,

System.Net.Http.HttpRequestException: 'Error while copying content to a stream.'
IOException: Unable to write data to the transport connection: An existing connection was forcibly closed by the remote host
SocketException: An existing connection was forcibly closed by the remote host

本地Swagger UI的JavaScript调用该接口可正常工作。

生成的C#客户端代码 #

public virtual async System.Threading.Tasks.Task<string> UploadFileAsync(System.IO.Stream body,
            System.Threading.CancellationToken cancellationToken)
        {
            if (body == null)
                throw new System.ArgumentNullException("body");

            var urlBuilder_ = new System.Text.StringBuilder();
            urlBuilder_.Append(BaseUrl != null ? BaseUrl.TrimEnd('/') : "").Append("/api/uploadFile");

            var client_ = _httpClient;
            var disposeClient_ = false;
            try
            {
                using (var request_ = new System.Net.Http.HttpRequestMessage())
                {
                    var content_ = new System.Net.Http.StreamContent(body);
                    content_.Headers.ContentType =
                        System.Net.Http.Headers.MediaTypeHeaderValue.Parse("multipart/form-data");
                    request_.Content = content_;
                    request_.Method = new System.Net.Http.HttpMethod("POST");
                    request_.Headers.Accept.Add(
                        System.Net.Http.Headers.MediaTypeWithQualityHeaderValue.Parse("application/json"));

                    PrepareRequest(client_, request_, urlBuilder_);

                    var url_ = urlBuilder_.ToString();
                    request_.RequestUri = new System.Uri(url_, System.UriKind.RelativeOrAbsolute);

                    PrepareRequest(client_, request_, url_);

                    var response_ = await client_.SendAsync(request_,
                            System.Net.Http.HttpCompletionOption.ResponseHeadersRead, cancellationToken)
                        .ConfigureAwait(false);
                    var disposeResponse_ = true;
                    try
                    {
                        var headers_ =
                            System.Linq.Enumerable.ToDictionary(response_.Headers, h_ => h_.Key, h_ => h_.Value);
                        if (response_.Content != null && response_.Content.Headers != null)
                        {
                            foreach (var item_ in response_.Content.Headers)
                                headers_[item_.Key] = item_.Value;
                        }

                        ProcessResponse(client_, response_);

                        var status_ = (int)response_.StatusCode;
                        {
                            var objectResponse_ =
                                await ReadObjectResponseAsync<string>(response_, headers_, cancellationToken)
                                    .ConfigureAwait(false);
                            if (objectResponse_.Object == null)
                            {
                                throw new ApiException("Response was null which was not expected.", status_,
                                    objectResponse_.Text, headers_, null);
                            }

                            return objectResponse_.Object;
                        }
                    }
                    finally
                    {
                        if (disposeResponse_)
                            response_.Dispose();
                    }
                }
            }
            finally
            {
                if (disposeClient_)
                    client_.Dispose();
            }
        }

客户端调用代码 #

var httpClient = new HttpClient();
httpClient.BaseAddress = new System.Uri(@"https://server/api");

var c = new ControllerClient(httpClient);

FileInfo fi = new FileInfo(txtFilePath.Text);
var stream = File.OpenRead(fi.FullName);

var res = await c.UploadFileAsync(stream);
txtAskResult.Text = res;

问题分析与解决方法 #

核心问题 #

生成的代码错误地使用StreamContent并手动设置Content-Type为multipart/form-data。multipart/form-data请求需要包含边界分隔符、表单字段名等结构，直接给StreamContent设置该类型会导致请求格式不符合服务端预期，触发连接强制关闭。

解决步骤 #

1. 修改客户端代码的请求内容构建逻辑
   替换原有的StreamContent为MultipartFormDataContent，正确构建多部分表单请求：

   // 替换原StreamContent相关代码
   var content_ = new MultipartFormDataContent();
   // 注意：第二个参数"file"要和服务端接口接收文件的参数名完全一致
   content_.Add(new StreamContent(body), "file", fi.Name);
   // 无需手动设置ContentType，MultipartFormDataContent会自动生成带边界的正确ContentType
   request_.Content = content_;
   

2. 调整NSwag生成配置避免重复修改
   如果不想每次生成代码都手动修改，可在NSwag配置文件（如nswag.json）中：

   • 确保GenerateClient.OperationGeneratorSettings.GenerateMultipartFormDataMethods设为true
   • 验证Swagger文档中该接口的请求体是否正确定义为multipart/form-data类型，并包含文件参数

3. 额外检查项

   • 调用接口前重置流位置：如果文件流被读过，需添加stream.Position = 0;
   • 检查HTTPS证书：若服务端用自签名证书，可临时添加证书验证跳过逻辑测试：

     var handler = new HttpClientHandler();
     handler.ServerCertificateCustomValidationCallback = (sender, cert, chain, sslPolicyErrors) => true;
     var httpClient = new HttpClient(handler);
     

内容的提问来源于stack exchange，提问作者Julien Duprat