如何将 HttpContext.Request/Response 的标头与 HttpRequestMessage.Request/Response 的标头进行比较？

Question

I am annoyed with the HttpContext class that uses different types for the request and response than HttpRequestMessage and HttpResponseMessage . The HttpContext class uses IHeaderDictionary for the headers while the other classes use HttpRequestHeaders and HttpResponseHeaders for the headers. (Both are derived from HttpHeaders .)
My problem is that I'm working on a Web API that needs to extract headers from the context and pass these headers onwards while using HttpClient.SendAsync(...) to call another site. (Yes, site! Not another service!)
What I want is a simple function that can copy the headers from the context request to the new request. And after the request is executed I want to use the same function to copy the headers from the response into my context response. This cannot be done because the headers are different types.

The types involved are:
interface IHeaderDictionary: IDictionary<string, StringValues>{}
class HttpHeaders: IEnumerable<KeyValuePair<string, IEnumerable<string>>>
So the challenge is that we have a dictionary with a StringValues value versus an enumerable with an enumerable as value. Comparing apples and pears, basically...
So how do I make a single function that can assign headers from one list to another?

Answer 1

The best thing that I could come up with is partially DRY, and is mostly typesafe.

void Copy<T, U>(IEnumerable<KeyValuePair<string, T>> from, IEnumerable<KeyValuePair<string, U>> to)
    where T : IEnumerable<string>
    where U : IEnumerable<string>
{
    if (to is IHeaderDictionary headerDictionary)
        foreach (var x in from)
            headerDictionary.Add(x.Key, new StringValues(x.Value.ToArray()));
    else
    if (to is HttpHeaders httpHeaders)
        foreach (var x in from)
            httpHeaders.Add(x.Key, new List<string?>(x.Value));
}

This method assumes both collections actually already exist. If you have to create the target collection and return it, the method would get way more complex, and you'd lose the ability to infer the type parameters at the call site.

To prove that the code can actually be compiled, I created a controller with a variation of Copy that takes nullable parameters. (This is just to keep all of it in a single place).

[ApiController]
public class WeatherForecastController : ControllerBase
{
    void Copy<T, U>(IEnumerable<KeyValuePair<string, T>>? from, IEnumerable<KeyValuePair<string, U>>? to)
        where T : IEnumerable<string>
        where U : IEnumerable<string>
    {
        if (from == null || to == null)
            return;

        if (to is IHeaderDictionary headerDictionary)
            foreach (var x in from)
                headerDictionary.Add(x.Key, new StringValues(x.Value.ToArray()));
        else
        if (to is HttpHeaders httpHeaders)
            foreach (var x in from)
                httpHeaders.Add(x.Key, new List<string?>(x.Value));
    }

    public void Test()
    {
        // in real life, these come from somewhere else and won't be null
        HttpRequestHeaders? requestHeaders = null;
        HttpRequestHeaders? responseHeaders = null;

        // actual types are inferred. yay!
        Copy(HttpContext.Request.Headers, requestHeaders);
        Copy(requestHeaders, HttpContext.Request.Headers);
        Copy(HttpContext.Response.Headers, responseHeaders);
        Copy(responseHeaders, HttpContext.Response.Headers);
        Copy(requestHeaders, responseHeaders);
        Copy(responseHeaders, requestHeaders);
    }
}