C#: Bulk Send Loops on an Android SMS Gateway API

C# bulk send loops against an android sms gateway api fail when someone puts a foreach in a controller. The phone sends one PDU at a time. Samples: C# HTTPS. Docs: Developer Center.

Priced by devices and SMS send volume. You use your own phone and operator SMS credit.

Parallel.ForEach on a single SIM is how you discover rate ceilings and a surprise airtime bill in the same afternoon. One worker. One pace. One promo device.

Not a sync foreach in a web request

Hangfire, Channel, Azure Queue. Idempotency per logical send. ASP.NET notifications cover inbound; this spoke is the blast worker.

Several numbers in to[]

Documented bulk shape (docs win):

using System;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;

public static class BulkSmsGateway
{
    private const string SendUrl = "https://app.sms-gateway.app/api/v1/messages";

    public static async Task SendManyAsync(string apiKey)
    {
        using var client = new HttpClient();
        client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", apiKey);

        var json = "{\"to\":[\"+14155552671\",\"+14155552672\"],\"text\":\"Hello from SMS Gateway!\",\"type\":\"sms\",\"dedupe\":true}";
        using var content = new StringContent(json, Encoding.UTF8, "application/json");
        var body = await (await client.PostAsync(SendUrl, content)).Content.ReadAsStringAsync();
        Console.WriteLine(body);
    }
}

Lists and panel CSV: Excel/CSV. Do not invent /api/send-bulk-sms.

Loop vs campaign vs radio

Pace like a modem, not a thread pool

Back off on 429-class responses. Watch DLR age. Carrier rejected. This is not unlimited send.

OTP stays off this worker

Pin promo with deviceIds after docs confirm the field. OTP.

HttpClient sample, not NuGet

No Complete C# SDK. Keys in env. Campaign gates still apply if the loop is marketing.

Checklist

• Worker, not MVC action.
• E.164 to[]; chunk size written.
• Backoff + airtime budget.
• OTP device cannot be targeted.
• Webhook handler idempotent.
• Halt switch for on-call.

Next steps

Canary 10 numbers, then scale. Device count is how throughput grows — not a tighter loop.