🔥 Send Push Notifications via Google Firebase v1 API using .NET Core

Push notifications are a critical part of modern apps — they keep users informed and engaged in real-time.
In this article, we’ll learn how to send push notifications from a .NET Core Web API using the Firebase Cloud Messaging (FCM) v1 API — the latest and most secure way to communicate with client apps.

🧠 What is Firebase Cloud Messaging (FCM)?

Firebase Cloud Messaging (FCM) is a cross-platform messaging solution that lets you send notifications and data messages reliably to Android, iOS, and web clients.

With the v1 HTTP API, Google moved from legacy key-based authentication to OAuth2-based authentication using service accounts, providing better security and scalability.

👉 Official Documentation:
Firebase Cloud Messaging HTTP v1 API (Google)

🎯 Why Use the v1 API?

Feature	Legacy API	v1 API
Authentication	Server Key	OAuth2 (Service Account)
Message Format	JSON (limited)	Structured JSON (with advanced options)
Targeting	Tokens / Topics	Tokens / Topics / Conditions
Platform Support	Basic	Full (Android, iOS, Web)
Future Support	Deprecated	✅ Active

🧱 Step-by-Step Implementation in .NET Core

Let’s implement a .NET Core Web API that can send notifications using Firebase Cloud Messaging v1 API.

⚙️ Step 1. Create a .NET Core Web API Project

dotnet new webapi -n FirebasePushAPI
cd FirebasePushAPI

📦 Step 2. Install Required NuGet Packages

Install packages for Google Auth and HTTP communication:

dotnet add package Google.Apis.Auth
dotnet add package Newtonsoft.Json

🔑 Step 3. Create a Firebase Service Account Key

Go to Firebase Console.
Navigate to Project Settings → Service Accounts → Generate new private key.
Download the JSON file and save it securely (e.g., /App_Data/firebase-key.json).

⚠️ Never commit this file to source control.

🧩 Step 4. Create a Model for Notification Request

namespace FirebasePushAPI.Models
{
    public class FcmNotificationRequest
    {
        public string Token { get; set; }  // FCM device token
        public string Title { get; set; }
        public string Body { get; set; }
        public string ImageUrl { get; set; }
    }
}

🪄 Step 5. Create a Firebase Notification Service

Create a new file Services/FirebaseService.cs:

using Google.Apis.Auth.OAuth2;
using Newtonsoft.Json;
using System.Net.Http.Headers;

namespace FirebasePushAPI.Services
{
    public class FirebaseService
    {
        private readonly string _projectId = "your-firebase-project-id";
        private readonly string _firebaseKeyPath = "App_Data/firebase-key.json";

        public async Task<bool> SendNotificationAsync(string token, string title, string body, string imageUrl = null)
        {
            var message = new
            {
                message = new
                {
                    token = token,
                    notification = new
                    {
                        title = title,
                        body = body,
                        image = imageUrl
                    }
                }
            };

            var accessToken = await GetAccessTokenAsync();

            using (var httpClient = new HttpClient())
            {
                httpClient.DefaultRequestHeaders.Authorization =
                    new AuthenticationHeaderValue("Bearer", accessToken);

                var json = JsonConvert.SerializeObject(message);
                var content = new StringContent(json, System.Text.Encoding.UTF8, "application/json");

                var response = await httpClient.PostAsync(
                    $"https://fcm.googleapis.com/v1/projects/{_projectId}/messages:send",
                    content);

                return response.IsSuccessStatusCode;
            }
        }

        private async Task<string> GetAccessTokenAsync()
        {
            GoogleCredential credential;

            using (var stream = new FileStream(_firebaseKeyPath, FileMode.Open, FileAccess.Read))
            {
                credential = GoogleCredential.FromStream(stream)
                    .CreateScoped("https://www.googleapis.com/auth/firebase.messaging");
            }

            var token = await credential.UnderlyingCredential.GetAccessTokenForRequestAsync();
            return token;
        }
    }
}

🧠 Explanation

Generates an OAuth2 access token using the service account JSON.
Sends a POST request to https://fcm.googleapis.com/v1/projects/{project_id}/messages:send.
The token, title, and body are passed dynamically.

🧮 Step 6. Create an API Controller

Create Controllers/NotificationController.cs:

using Microsoft.AspNetCore.Mvc;
using FirebasePushAPI.Models;
using FirebasePushAPI.Services;

namespace FirebasePushAPI.Controllers
{
    [ApiController]
    [Route("api/[controller]")]
    public class NotificationController : ControllerBase
    {
        private readonly FirebaseService _firebaseService;

        public NotificationController()
        {
            _firebaseService = new FirebaseService();
        }

        [HttpPost("send")]
        public async Task<IActionResult> SendNotification([FromBody] FcmNotificationRequest request)
        {
            var result = await _firebaseService.SendNotificationAsync(request.Token, request.Title, request.Body, request.ImageUrl);

            if (result)
                return Ok(new { success = true, message = "Notification sent successfully" });
            else
                return BadRequest(new { success = false, message = "Failed to send notification" });
        }
    }
}

🧪 Step 7. Test API in Postman

POST URL

https://localhost:5001/api/notification/send

Request Body (JSON)

{"token": "DEVICE_FCM_TOKEN","title": "Welcome!","body": "Your registration was successful 🎉","imageUrl": "https://yourdomain.com/welcome.png"}

Response

{"success": true,"message": "Notification sent successfully"}

✅ The notification should instantly appear on your mobile device or web app registered with Firebase.

⚡ Bonus: Sending to a Topic Instead of a Single Token

If you want to send to a topic (e.g., “news”):

Replace this block:

token = token,

with:

topic = "news",

Then, your Firebase clients should subscribe to the topic using the FCM SDK.

📚 Subscribe to FCM Topics (Docs)

🧩 Common Errors & Fixes

Error	Cause	Solution
`401 Unauthorized`	Invalid service account or scope	Ensure correct JSON key and project ID
`404 Not Found`	Wrong endpoint	Check FCM v1 API URL
`InvalidRegistration`	Invalid device token	Ensure valid token from FCM SDK
`Permission denied`	Missing Firebase Messaging permission	Check IAM roles in GCP Console

🔗 External References

Firebase Cloud Messaging v1 API Docs
Google OAuth2 Authentication for Service Accounts
Firebase Admin SDK Setup Guide
Send Messages with FCM REST API

🏁 Conclusion

By integrating Firebase v1 API with your .NET Core Web API, you can deliver real-time push notifications to any platform — securely and efficiently.

This modern approach replaces the old FCM server key method with OAuth2 tokens, offering better security and control for enterprise-grade applications.

Whether it’s order updates, chat alerts, or system notifications, Firebase + .NET Core makes it fast and reliable.