HttpClient 的请求 headers 和内容 headers 有什么区别

Question

任何人都可以解释请求 header 和内容 header 之间的区别吗？

在这种特殊情况下，我说的是 UWP HttpClient object。首先创建 HttpClient，然后创建 HttpRequestMessage，然后将 HttpStreamContent 分配给 HttpRequest 消息的 Content 属性。 HttpRequestMessage上有Headers属性，HttpStreamContent上有Headers属性。

我什么时候应该使用一个或另一个？
在一种或另一种情况下，header 究竟会出现在哪里？

这里有一个代码片段来解释我的意思

using(var objProtocolFilter = new HttpBaseProtocolFilter()) {
    objProtocolFilter.AllowUI = false;
    objProtocolFilter.CacheControl.ReadBehavior = HttpCacheReadBehavior.NoCache;
    objProtocolFilter.CacheControl.WriteBehavior = HttpCacheWriteBehavior.NoCache;
    
    using(var objClient = new HttpClient(objProtocolFilter)) {
        HttpMethod eMethod = Method switch {
            HttpUploadMethod.Post => HttpMethod.Post,
            HttpUploadMethod.Put => HttpMethod.Put,
            _ => throw new ValueOutOfRangeException(nameof(Method))
        };
        
        using(var objRequest = new HttpRequestMessage(eMethod, RemoteUri)) {
            _Headers.Cast<string>().Execute(item => objRequest.Headers.TryAppendWithoutValidation(item, _Headers[item]));

            objRequest.Content = new HttpStreamContent(objInputStream.AsInputStream());
            _Headers.Cast<string>().Execute(item => objRequest.Content.Headers.TryAppendWithoutValidation(item, _Headers[item]));
            objRequest.Content.Headers.ContentLength = (ulong)objInputStream.Length;
        }
    }
}

这里我只是将相同的 header 列表添加到 HttpRequestMessage 和 HttStreamContent。我想这是错误的，除非那些 objects 足够聪明以在一种或另一种情况下只应用允许的 headers。那么，哪个 header 应该去哪里？它们可以互换吗？

Answer 1

理论

它们有不同的用途：

• HttpStreamContent.Headers returns an HttpContentHeaderCollection
  • 您可以设置如下内容：Content-Disposition、Content-Range、Content-Length、Content-Type 等
  • 换句话说，这些与请求的body
  有关
• HttpRequestMessage.Headers returns an HttpRequestHeaderCollection
  • 您可以设置如下内容：Accept、Accept-Encoding、Authorization、Cookie 等
  • 换句话说，这些与请求本身有关。
• +1: (HttpResponseMessage.Headers returns an HttpResponseHeaderCollection)

练习

尝试在请求

上设置Content-Type

HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, address);
request.Headers.Add("Content-Type", "application/json");

这将产生以下异常：

System.InvalidOperationException: 'Misused header name. Make sure request headers are used with HttpRequestMessage, response headers with HttpResponseMessage, and content headers with HttpContent objects.'

尝试在内容上设置接受

var content = new StringContent("Test", Encoding.UTF8);
content.Headers.Add("Accept","application/json");

这将产生以下异常：

System.InvalidOperationException: 'Misused header name. Make sure request headers are used with HttpRequestMessage, response headers with HttpResponseMessage, and content headers with HttpContent objects.'

尝试在两个地方设置相同的任意header:

const string headerKey = "A", requestHeaderValue = "B", contentHeaderValue = "C";

var content = new StringContent("Test", Encoding.UTF8);
content.Headers.Add(headerKey, requestHeaderValue);

HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, address);
request.Headers.Add(headerKey, contentHeaderValue);

这不会产生任何异常。

哪个值传递给下游？

为了能够回答这个问题，我将使用 WireMock.Net nuget 包来 运行 模拟服务器。

const string address = "http://localhost:9000", route = "/";
var server = WireMockServer.Start(new WireMockServerSettings { Urls = new[] { address } });

server
    .Given(Request.Create()
        .WithPath(route)
        .WithHeader(headerKey, new ExactMatcher(requestHeaderValue))
        .UsingPost())
    .RespondWith(Response.Create()
        .WithBody("From Request header")
        .WithStatusCode(200));

server
    .Given(Request.Create()
        .WithPath(route)
        .WithHeader(headerKey, new ExactMatcher(contentHeaderValue))
        .UsingPost())
    .RespondWith(Response
        .Create()
        .WithBody("From Content header")
        .WithStatusCode(200));

• 这里我定义了一个监听9000端口的模拟服务器
• 它在 / 路由上有一个端点，它预期 POST 请求
• 根据 headerKey 值它可能会响应
  • 或者 From Request header
  • 或 From Content header

如果我发送的请求在两个 objects 上设置了相同的 header 键，那么我将收到以下响应：

From Request header

顺序重要吗？

如果我调换 header 按键分配的顺序会怎样？

HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, address);
request.Headers.Add(headerKey, contentHeaderValue);

var content = new StringContent("Test", Encoding.UTF8);
content.Headers.Add(headerKey, requestHeaderValue);

结果将是相同的：From Request header。

---

为了完整起见，这里是完整的源代码：

static readonly HttpClient httpClient = new HttpClient();
static async Task Main(string[] args)
{
    const string headerKey = "A", requestHeaderValue = "B", contentHeaderValue = "C";
    const string address = "http://localhost:9000", route = "/";
    var server = WireMockServer.Start(new WireMockServerSettings { Urls = new[] { address } });

    server
        .Given(Request.Create()
            .WithPath(route)
            .WithHeader(headerKey, new ExactMatcher(requestHeaderValue))
            .UsingPost())
        .RespondWith(Response.Create()
            .WithBody("From Request header")
            .WithStatusCode(200));

    server
        .Given(Request.Create()
            .WithPath(route)
            .WithHeader(headerKey, new ExactMatcher(contentHeaderValue))
            .UsingPost())
        .RespondWith(Response
            .Create()
            .WithBody("From Content header")
            .WithStatusCode(200));

    var request = new HttpRequestMessage(HttpMethod.Post, address);
    request.Headers.Add(headerKey, contentHeaderValue);

    var content = new StringContent("Test", Encoding.UTF8);
    content.Headers.Add(headerKey, requestHeaderValue);

    var response = await httpClient.SendAsync(request);
    var headerSource = await response.Content.ReadAsStringAsync();
    Console.WriteLine(headerSource);
}