Allows using UnityWebRequest with an IHttpClient interface, for example with nswag

UnityWebRequestHttpClient.cs

// Depends on UniTask to support cancellation token and GetAwaiter: https://github.com/Cysharp/UniTask
// Otherwise, the code can be adapted using https://gist.github.com/krzys-h/9062552e33dd7bd7fe4a6c12db109a1a

using System;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Threading;
using System.Threading.Tasks;
using Cysharp.Threading.Tasks;
using UnityEngine.Networking;

public interface IHttpClient {
	public Uri BaseAddress { get; set; }
	public HttpRequestHeaders DefaultRequestHeaders { get; }

	public Task<HttpResponseMessage> SendAsync(HttpRequestMessage message, HttpCompletionOption option,
		CancellationToken token);

	public void Dispose();
}

public class UnityWebRequestHttpClient : IHttpClient {
	public UnityWebRequestHttpClient() { }

	public UnityWebRequestHttpClient(string baseUri) {
		BaseAddress = new Uri(baseUri);
	}

	public UnityWebRequestHttpClient(Uri baseUri) {
		BaseAddress = baseUri;
	}

	public Uri BaseAddress { get; set; }
	public HttpRequestHeaders DefaultRequestHeaders => _httpClient.DefaultRequestHeaders;

	private readonly HttpClient _httpClient = new HttpClient();

	public async Task<HttpResponseMessage> SendAsync(HttpRequestMessage message, HttpCompletionOption option,
		CancellationToken token) {
		var content = await (message.Content?.ReadAsStringAsync() ?? Task.FromResult(""));
		var webRequest = GetUnityWebRequest(message.Method.Method, message.RequestUri, content);

		AppendHeaders(webRequest);

		try {
			await webRequest
				.SendWebRequest()
				.WithCancellation(cancellationToken: token);
		}
		catch (Exception) {
			webRequest.Dispose();
			throw;
		}

		var responseMessage = CreateHttpResponseMessage(webRequest);
		webRequest.Dispose();
		return responseMessage;
	}

	public void Dispose() {
		_httpClient.Dispose();
		DefaultRequestHeaders.Clear();
		BaseAddress = null;
	}

	private UnityWebRequest GetUnityWebRequest(string method, Uri endpoint, string content = "") {
		var requestUri = BaseAddress.AbsoluteUri + endpoint;
		var webRequest = UnityWebRequest.Get(requestUri);
		webRequest.method = method;

		webRequest.disposeUploadHandlerOnDispose = true;
		webRequest.disposeDownloadHandlerOnDispose = true;

		if (!string.IsNullOrEmpty(content)) {
			var data = new System.Text.UTF8Encoding().GetBytes(content);
			webRequest.uploadHandler = new UploadHandlerRaw(data);
			webRequest.SetRequestHeader("Content-Type", "application/json");
		}

		return webRequest;
	}

	private void AppendHeaders(UnityWebRequest webRequest) {
		using var enumerator = DefaultRequestHeaders.GetEnumerator();

		while (enumerator.MoveNext()) {
			var (key, value) = enumerator.Current;
			webRequest.SetRequestHeader(key, value.First());
		}
	}

	private HttpResponseMessage CreateHttpResponseMessage(UnityWebRequest webRequest) {
		var response = new HttpResponseMessage();
		var responseContent = webRequest.downloadHandler?.text;

		response.Content = new StringContent(responseContent);
		response.StatusCode = (HttpStatusCode) webRequest.responseCode;

		foreach (var (key, value) in webRequest.GetResponseHeaders()) {
			switch (key.ToLower().Trim()) {
				case "content-type": {
					var trimmed = value.ToLower().Split(";").FirstOrDefault();
					response.Content.Headers.ContentType = new MediaTypeHeaderValue(trimmed);
					break;
				}
				case "content-length":
					response.Content.Headers.ContentLength = long.Parse(value);
					break;

				default:
					response.Headers.Add(key, value);
					break;
			}
		}

		return response;
	}
}

It is possible to remove the Cysharp dependency by replacing await webRequest.SendWebRequest().WithCancellation(cancellationToken: token); with following:

var task = webRequest.SendWebRequest();
while (!task.isDone)
{
	if (token.IsCancellationRequested)
	{
		token.ThrowIfCancellationRequested();
	}
	
	await Task.Yield();
}