发起HTTP POST请求时出现411(Length Required)错误求解决方案

遇到的错误 #

System.Net.Http.HttpRequestException: Response status code does not indicate success: 411 (Length Required).

相关代码 #

最初使用HttpRequestMessage的实现 #

public async Task<MyResponse?> MyMethodAsync(MyRequest request, string url, string accessToken)
{
    var client = new HttpClient();

    var requestJson = JsonConvert.SerializeObject(request);

    var httpRequest = new HttpRequestMessage(new HttpMethod("POST"), url);
    httpRequest.Content = new StringContent(requestJson, Encoding.UTF8, "application/json");
    httpRequest.Headers.TryAddWithoutValidation("Authorization", $"Bearer {accessToken}");
    //httpRequest.Headers.TryAddWithoutValidation("Content-Length", requestJson.Length.ToString()); //Adding this didn't help

    var response = await client.SendAsync(httpRequest);
    response.EnsureSuccessStatusCode();

    var result = await response.Content.ReadAsStringAsync();
    return JsonConvert.DeserializeObject<MyResponse>(result);
}

尝试改用PostAsync的实现 #

client.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "application/json");
client.DefaultRequestHeaders.TryAddWithoutValidation("Accept", "application/json");
client.DefaultRequestHeaders.TryAddWithoutValidation("Authorization", $"Bearer {accessToken}");
client.DefaultRequestHeaders.TryAddWithoutValidation("Content-Length", requestJson.Length.ToString());
var response = await client.PostAsync(url, content);

解决思路与建议 #

• 禁止手动在请求Headers中添加Content-Length：Content-Length属于请求内容（Content）的头字段，而非HttpRequestMessage的Headers。StringContent会自动根据内容计算并添加该头，手动添加到请求Headers会导致服务器无法识别正确的长度，反而引发问题。

• 检查序列化后的内容是否为空：如果requestJson是空字符串，StringContent可能不会生成Content-Length头，触发服务器的411校验。可以在序列化后增加空值检查：

  var requestJson = JsonConvert.SerializeObject(request);
  if (string.IsNullOrWhiteSpace(requestJson))
  {
      requestJson = "{}"; // 或者根据接口要求设置合法的空请求体
  }
  

• 修正PostAsync的写法：不要把Content-Type加到DefaultRequestHeaders中，该字段属于Content的属性，应通过StringContent的构造函数指定：

  var client = new HttpClient();
  // 只把非内容相关的头加到默认请求头
  client.DefaultRequestHeaders.TryAddWithoutValidation("Authorization", $"Bearer {accessToken}");
  client.DefaultRequestHeaders.TryAddWithoutValidation("Accept", "application/json");
  
  var requestJson = JsonConvert.SerializeObject(request);
  // 正确设置Content-Type
  var content = new StringContent(requestJson, Encoding.UTF8, "application/json");
  
  var response = await client.PostAsync(url, content);
  response.EnsureSuccessStatusCode();
  

• 排查自定义Handler影响：如果项目中使用了自定义的DelegatingHandler，检查是否有逻辑意外移除或修改了Content-Length头。

内容的提问来源于stack exchange，提问作者user007