如何使用标准C#.NET发送带有apns-expiration头部的推送通知？

using System;
using System.IO;
using System.Net;
using System.Net.Http;
using System.Security.Cryptography;
using System.Text;

class Program
{
    static void Main()
    {
        string deviceToken = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
        string notificationPayload = "{\"aps\":{\"alert\":\"Hello World!\"}}";
        string privateKeyFilePath = "your_private_key.p8";
        string teamId = "your_team_id";
        string keyId = "your_key_id";
        string bundleId = "your_bundle_id";
        int apnsExpiration = (int)(DateTime.UtcNow.AddMinutes(5) - new DateTime(1970, 1, 1)).TotalSeconds;

        byte[] key = File.ReadAllBytes(privateKeyFilePath);

        using (HttpClient httpClient = new HttpClient())
        {
            httpClient.DefaultRequestHeaders.TryAddWithoutValidation("apns-expiration", apnsExpiration.ToString());
            httpClient.DefaultRequestHeaders.TryAddWithoutValidation("apns-topic", bundleId);
            httpClient.DefaultRequestHeaders.TryAddWithoutValidation("apns-push-type", "alert");

            string jwtToken = GenerateJwtToken(teamId, keyId, key);
            httpClient.DefaultRequestHeaders.TryAddWithoutValidation("authorization", $"bearer {jwtToken}");

            var requestContent = new StringContent(notificationPayload, Encoding.UTF8, "application/json");
            HttpResponseMessage response = httpClient.PostAsync($"https://api.development.push.apple.com/3/device/{deviceToken}", requestContent).Result;

            if (response.StatusCode == HttpStatusCode.OK)
            {
                Console.WriteLine("Push notification sent successfully.");
            }
            else
            {
                Console.WriteLine($"Failed to send push notification. Error: {response.StatusCode}");
            }
        }
    }

    static string GenerateJwtToken(string teamId, string keyId, byte[] privateKey)
    {
        var header = new { alg = "ES256", kid = keyId };
        var payload = new { iss = teamId, iat = ToUnixTime(DateTime.Now) };

        string headerString = SerializeToJson(header);
        string payloadString = SerializeToJson(payload);
        string jwtData = Base64UrlEncode(headerString) + '.' + Base64UrlEncode(payloadString);

        byte[] keyBytes = privateKey;
        using (CngKey cngKey = CngKey.Import(keyBytes, CngKeyBlobFormat.Pkcs8PrivateBlob))
        using (ECDsaCng ecdsa = new ECDsaCng(cngKey))
        {
            byte[] signature = ecdsa.SignData(Encoding.UTF8.GetBytes(jwtData));
            string encodedSignature = Base64UrlEncode(signature);

            return jwtData + '.' + encodedSignature;
        }
    }

    static string SerializeToJson(object obj)
    {
        return Newtonsoft.Json.JsonConvert.SerializeObject(obj, Newtonsoft.Json.Formatting.None);
    }

    static string Base64UrlEncode(byte[] input)
    {
        string base64 = Convert.ToBase64String(input);
        return base64.TrimEnd('=').Replace('+', '-').Replace('/', '_');
    }

    static int ToUnixTime(DateTime dateTime)
    {
        return (int)(dateTime.ToUniversalTime() - new DateTime(1970, 1, 1)).TotalSeconds;
    }
}