Push Notifications in Android using C#

You can use firebase cloud messaging.

Following method, you can use.

static void SendMessage()
{
    string serverKey = "Your server key";

    try
    {
        var result = "-1";
        var webAddr = "https://fcm.googleapis.com/fcm/send";

        var httpWebRequest = (HttpWebRequest)WebRequest.Create(webAddr);
        httpWebRequest.ContentType = "application/json";
        httpWebRequest.Headers.Add("Authorization:key=" + serverKey);
        httpWebRequest.Method = "POST";

        using (var streamWriter = new StreamWriter(httpWebRequest.GetRequestStream()))
        {
            string json = "{\"to\": \"client Device token\",\"data\": {\"message\": \"This is a Firebase Cloud Messaging Topic Message!\",}}";
            streamWriter.Write(json);
            streamWriter.Flush();
        }

        var httpResponse = (HttpWebResponse)httpWebRequest.GetResponse();
        using (var streamReader = new StreamReader(httpResponse.GetResponseStream()))
        {
            result = streamReader.ReadToEnd();
        }

       // return result;
    }
    catch (Exception ex)
    {
      //  Response.Write(ex.Message);
    }
}

using FCM.Net;
using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Text;
using System.Threading.Tasks;


using System.Net;
using System.Net.Http;
using System.Net.Http.Headers;
using System.IO;


Dear All thisis working code using fcm.net core and asp.net core web api

public async Task<bool> NotifyAsync(string deviceToken, string serverKey, string body )
{

string json = "{\"to\": \"" + deviceToken + "\",\"data\": {\"message\": \"" + body + "\",}}";
try
{
//var result = "-1";
//var webAddr = "https://fcm.googleapis.com/fcm/send";

//var httpWebRequest = (HttpWebRequest)WebRequest.Create(webAddr);
//httpWebRequest.ContentType = "application/json";
//httpWebRequest.Headers.Add("Authorization:key=" + serverKey);
//httpWebRequest.Method = "POST";

//using (var streamWriter = new StreamWriter(httpWebRequest.GetRequestStream()))
//{
// string json = "{\"to\": \"" + deviceToken + "\",\"data\": {\"message\": \"This is a Firebase Cloud Messaging Topic Message!\",}}";
// streamWriter.Write(json);
// streamWriter.Flush();
//}

//var httpResponse = await httpWebRequest.GetResponseAsync();
//using (var streamReader = new StreamReader(httpResponse.GetResponseStream()))
//{
// result = streamReader.ReadToEnd();
//}

string Baseurl = "https://fcm.googleapis.com/fcm/send";
var client = new HttpClient();
client.BaseAddress = new Uri(Baseurl);
client.DefaultRequestHeaders.Clear();

//var param = JsonConvert.SerializeObject(Myproduct);
//string postBody = JsonConvert.SerializeObject(Myproduct);
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
client.DefaultRequestHeaders.TryAddWithoutValidation("Authorization", "key="+ serverKey);

HttpResponseMessage Res = await client.PostAsync(Baseurl, new StringContent(json, Encoding.UTF8, "application/json"));

if (Res.IsSuccessStatusCode)
{
string returndata = await Res.Content.ReadAsStringAsync();
//Myproduct = JsonConvert.DeserializeObject<product>(returndata);
// System.Windows.Forms.MessageBox.Show("record shaved ");
return true;
}

}
catch (Exception ex)
{
// _logger.LogError($"Exception thrown in Notify Service: {ex}");
throw;
}

return false;
}
}