What is the difference between request headers and content headers for HttpClient

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Can anybody explain what is the difference between the request header and content header?

In this particular case I'm talking about UWP HttpClient object. First you create HttpClient, then you create HttpRequestMessage and then you assign, in my case HttpStreamContent to the Content property of the HttpRequest message. There is Headers property on the HttpRequestMessage and there is Headers property on the HttpStreamContent.

When should I use one or another?
Where exactly the headers will appear in one or another case?

Here is a code snippet to explain what I mean

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;
        }
    }
}

Here I just add the same list of headers to HttpRequestMessage and to HttStreamContent. I guess it's wrong unless those objects are smart enough to apply only permitted headers in one or the other case. So, which headers should go where? Are they interchangeable?

dotnet-httpclient request-headers request c#
1 Answers

Theory

They serve different purpose:

Practice

Try to set Content-Type on the Request

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

This will produce the following exception:

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

Try to set Accept on Content

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

This will produce the following exception:

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

Try to set the same arbitrary header at both places:

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);

This will not produce any exception.

Which value is passed to the downstream?

In order to be able answer this question I will use the WireMock.Net nuget package to run a mock server.

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));
  • Here I have defined a mock server which listens on the 9000 port
  • It has a single endpoint on the / route and it anticipates a POST request
  • Depending on the headerKey value it may respond
    • either with From Request header
    • or with From Content header

If I send a request where the same header key is set on both objects then I will receive the following response:

From Request header

Does ordering matter?

What if I switch the order of the header key assignments?

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

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

The result will be the same: From Request header.

For the sake of completeness here is the full source code:

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);
}
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