C#使用HttpWebRequest、HttpClient发POST报400仅RestSharp正常问题排查

通过两份抓包结果对比，核心差异点是HttpClient和HttpWebRequest发送的请求默认携带了Expect: 100-continue请求头，你使用的目标服务端未兼容该HTTP 1.1特性，因此直接返回400错误；而RestSharp默认关闭了该特性，因此请求可以正常响应。

修复方案 #

1. HttpWebRequest实现修复 #

在初始化请求对象后添加禁用Expect 100-continue的配置即可：

var httpRequest = (HttpWebRequest) WebRequest.Create(uri);
httpRequest.Method = "POST";
httpRequest.Accept = "application/json";
httpRequest.Headers["Authorization"] = $"Bearer {token}";
httpRequest.Headers["Client_Id"] = clientId;
httpRequest.ContentType = "application/json";
// 新增以下行禁用Expect 100-continue
httpRequest.ServicePoint.Expect100Continue = false;

using (var streamWriter = new StreamWriter(httpRequest.GetRequestStream()))
{
    streamWriter.Write(json);
}

var httpResponse = (HttpWebResponse) httpRequest.GetResponse();

补充：如果仍报错可显式指定StreamWriter编码为无BOM的UTF8：new StreamWriter(httpRequest.GetRequestStream(), new UTF8Encoding(false))

2. HttpClient实现修复 #

建议将HttpClient定义为全局单例（不要每次请求新建），通过HttpClientHandler禁用Expect 100-continue特性：

// 全局单例声明
private static readonly HttpClient _httpClient = new HttpClient(new HttpClientHandler
{
    Expect100Continue = false
});

// 发送请求逻辑
_httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);
_httpClient.DefaultRequestHeaders.Add("Client_Id", clientId);
_httpClient.DefaultRequestHeaders.AcceptEncoding.Add(new StringWithQualityHeaderValue("gzip"));
_httpClient.DefaultRequestHeaders.AcceptEncoding.Add(new StringWithQualityHeaderValue("deflate"));
_httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
_httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("text/json"));
_httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("text/x-json"));
_httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("text/javascript"));
_httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/xml"));
_httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("text/xml"));

var content = new StringContent(json, Encoding.UTF8, "application/json");
// 建议用await异步调用，避免.Result可能引发的死锁问题
var result = await _httpClient.PostAsync(uri, content);

如果需要全局禁用所有HTTP请求的Expect 100-continue，可以在程序启动时添加配置：ServicePointManager.Expect100Continue = false;

内容的提问来源于stack exchange，提问作者Mihai Cranga