在Kestrel与ASP.NET Core MVC中实现返回100 Continue的端点

我创建了一个仅包含如下代码的[HttpPut]控制器端点：

var bytes = System.Text.Encoding.ASCII.GetBytes("May your wisdom grace us until the stars rain down from the heavens.");

Response.ContentType = "text/plain";
Response.Body.Write(bytes, 0, bytes.Length);

向其发送如下请求：

PUT /api/vector HTTP/1.1
Host: localhost
Transfer-Encoding: chunked
Expect: 100-continue

12
0123456789ABCDEF

0

收到的响应为：

HTTP/1.1 200 OK
Transfer-Encoding: chunked
Content-Type: text/plain
Server: Microsoft-IIS/10.0
X-SourceFiles: =?UTF-8?B?RDpcZGV2XGZvcm1idWlsZGVyXHNyY1xDZWRhcm9uLkZvcm1CdWlsZGVyLldlYkFQSVxhcGlcdmVjdG9y?=
X-Powered-By: ASP.NET
Date: Mon, 14 Oct 2024 20:33:05 GMT

May your wisdom grace us until the stars rain down from the heavens.

未收到100 Continue响应。我尝试通过套接字分开发送请求，刚发送完请求头部分：

PUT /api/vector HTTP/1.1
Host: localhost
Transfer-Encoding: chunked
Expect: 100-continue

就收到了200响应，说明服务端并未等待后续请求内容。需要一个能生成100 Continue响应的端点来验证相关假设。

方式1：读取请求体触发自动处理 #

ASP.NET默认会在首次尝试读取请求体时，检查Expect: 100-continue头并返回100响应。只需在代码中读取请求体（哪怕是部分读取）即可触发该逻辑：

[HttpPut("api/vector")]
public async Task<IActionResult> Put()
{
    // 读取请求体片段，触发100 Continue响应
    await Request.Body.ReadAsync(new byte[1], 0, 1);

    var bytes = System.Text.Encoding.ASCII.GetBytes("May your wisdom grace us until the stars rain down from the heavens.");
    Response.ContentType = "text/plain";
    await Response.Body.WriteAsync(bytes, 0, bytes.Length);
    
    return Ok();
}

方式2：显式发送100 Continue响应 #

如果不需要读取请求体，也可以手动检查请求头并发送100响应：

[HttpPut("api/vector")]
public async Task<IActionResult> Put()
{
    // 检查请求头是否包含Expect:100-continue
    if (Request.Headers.TryGetValue("Expect", out var expectHeader) && 
        expectHeader.Equals("100-continue", StringComparison.OrdinalIgnoreCase))
    {
        // 发送100 Continue响应
        Response.StatusCode = StatusCodes.Status100Continue;
        await Response.StartAsync();
    }

    var bytes = System.Text.Encoding.ASCII.GetBytes("May your wisdom grace us until the stars rain down from the heavens.");
    Response.ContentType = "text/plain";
    await Response.Body.WriteAsync(bytes, 0, bytes.Length);
    
    return Ok();
}

ASP.NET的默认逻辑是：只有当代码尝试读取请求体时，才会处理Expect:100-continue头并返回100响应。如果端点直接返回响应而不触碰请求体，服务器会直接发送200响应，跳过100 Continue的步骤——这就是你遇到问题的核心原因。

内容的提问来源于stack exchange，提问作者Joshua