Function1.cs

using System;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Logging;
using Microsoft.Azure.Management.ResourceManager.Fluent.Authentication;
using Microsoft.Azure.Management.ResourceManager.Fluent;
using Microsoft.Azure.Management.Fluent;
using Microsoft.Azure.Services.AppAuthentication;
using Microsoft.Rest;
using Microsoft.Azure.Management.Dns;
using Microsoft.Azure.Management.ResourceManager.Fluent.Core;

namespace UpdateDNS
{
    public static class Function1
    {
        [FunctionName("Function1")]
        public static async Task<IActionResult> Run(
            [HttpTrigger(AuthorizationLevel.Function, "get", Route = "{subscription}/{rg_name}/{zone_name}/{lib}")] HttpRequest req,
            string subscription,
            string rg_name,
            string zone_name,
            string lib,
            ILogger log)
        {
            log.LogInformation("C# HTTP trigger function processed a request.");

            int count = 0;
            dynamic records;

            // determine the lib to use to get the dns data
            switch (lib)
            {
                case "fluent":

                    AzureCredentialsFactory factory = new AzureCredentialsFactory();
                    MSILoginInformation msi = new MSILoginInformation(MSIResourceType.AppService);
                    AzureCredentials msiCred = factory.FromMSI(msi, AzureEnvironment.AzureGlobalCloud);
                    var azureAuth = Azure.Configure().WithLogLevel(HttpLoggingDelegatingHandler.Level.BodyAndHeaders).Authenticate(msiCred);

                    // set the subscription to work with
                    var azure = azureAuth.WithSubscription(subscription);

                    var dnszone = azure.DnsZones.GetByResourceGroup(rg_name, zone_name);

                    records = dnszone.ListRecordSets();

                    break;

                default:

                    // get the token from the managed service identity
                    AzureServiceTokenProvider token_provider = new AzureServiceTokenProvider();
                    string token = await token_provider.GetAccessTokenAsync("https://management.azure.com");

                    TokenCredentials token_creds = new TokenCredentials(token);

                    // create the dns client
                    DnsManagementClient client = new DnsManagementClient(token_creds);
                    client.SubscriptionId = subscription;

                    records = client.RecordSets.ListAllByDnsZone(rg_name, zone_name);

                    break;
            }

            foreach (var record in records)
            {
                Console.WriteLine(record.Name);
                count++;
            }

            return new OkObjectResult($"Records: {count}");

        }
    }
}