How to send a push notification from C# console to Firebase

Program.cs

/* Requires these packages to be included in the project (or whatever latest versions are available):
      <PackageReference Include="FirebaseAdmin" Version="2.2.0" />
      <PackageReference Include="System.Text.Json" Version="6.0.0" />
*/

using System;
using System.Collections.Generic;
using System.IO;
using System.Text.Json;
using System.Text.Json.Serialization;
using System.Threading.Tasks;
using FirebaseAdmin;
using FirebaseAdmin.Messaging;
using Google.Apis.Auth.OAuth2;

namespace FirebasePushTest
{
    class Program
    {
        static void Main(string[] args)
        {
            var configFilename = args.Length == 1 ? args[0] : "test-config.json";

            // parse json file containing configuration (not secrets)
            var config = JsonSerializer.Deserialize<TestConfiguration>(File.OpenRead(configFilename));
            if(config is null)
            {
                Fail("Couldn't parse config.");
                return;
            }

            // create firebase app, parse services json for secrets
            // you need to set the GOOGLE_APPLICATION_CREDENTIALS environment variable to point to the 
            // location of the JSON file on your computer for this to work (it must be a full path). 
            // if you don't have a JSON file, you need to get the private key from here:
            // https://console.firebase.google.com/u/0/project/api-XXXXXXXX-XXXXXXX/settings/serviceaccounts/adminsdk
            // and click "Generate new private key"... check with anyone on your team first if one has been created!
            
            FirebaseApp.Create(new AppOptions { Credential = GoogleCredential.GetApplicationDefault() });

            // send notification
            Console.WriteLine("Sending notification...");
            SendNotification(config).Wait();

            void Fail(string msg)
            {
                Console.WriteLine($"ERROR: {msg}");
                Environment.Exit(1);
            }
        }

        static async Task SendNotification(TestConfiguration configuration)
        {
            // this is how you send metadata in a push notification via Firebase
            // it is represented as a key/value string pair...
            // on Android, this will appear in the intent Extras (e.g. intent.GetStringExtra("go"))
            // on iOS, this will appear in the UserInfo NSDictionary in UNNotificationContent
            // in this example, we use this metadata to tell the app about a deep/shallow link to navigate to
            
            var data = new Dictionary<string, string>();
            if(!string.IsNullOrEmpty(configuration.DeepLink))
            {
                data.Add("go", configuration.DeepLink);
            }

            // multicast so it can be sent to more than one device
            var message = new MulticastMessage
            {
                Tokens = configuration.Tokens,
                Notification = new Notification
                {
                    Title = configuration.Title,
                    Body = configuration.Body
                },
                Data = data
            };

            var response = await FirebaseMessaging.DefaultInstance.SendMulticastAsync(message);
            Console.WriteLine("Notification sent.");
            Console.WriteLine($"Success count: {response.SuccessCount}, failures: {response.FailureCount}");
        }
    }

    // this matches the test-config.json file, deserialised from JSON
    class TestConfiguration
    {
        [JsonPropertyName("tokens")]
        public List<string> Tokens { get; set; }

        [JsonPropertyName("title")]
        public string Title { get; set; }

        [JsonPropertyName("body")]
        public string Body { get; set; }

        [JsonPropertyName("deepLink")]
        public string DeepLink { get; set; }
    }
}

test-config.json

{
	"tokens": [
    // you can include one or more tokens here. dump it to console/logs in your app when Firebase tells you.
    // on Android, this would be via the OnCompleteListener in FirebaseMessaging.Instance.GetToken().
		"<YOUR DEVICE TOKEN HERE>",
		"<YOUR DEVICE TOKEN HERE>",
		"<YOUR DEVICE TOKEN HERE>"   
	],
	"title": "Test message",
	"body": "This is a test message",
	"deepLink": "/a/route/to/go/to?param=abc&param2=123"
}