Graph_AppOnly_ClientSecret.cs

using Microsoft.IdentityModel.Clients.ActiveDirectory;
using System;
using System.Globalization;
using System.Net.Http;
using System.Net.Http.Headers;

namespace AppPermissionsDemo
{
    class Program
    {
        static void Main(string[] args)
        {

            string clientId = "<client-id-of-aad-app-registration>"; //e.g. 01e54f9a-81bc-4dee-b15d-e661ae13f382
            string clientSecret = "<client-secret-of-aad-app-registration>"; //e.g @FgQR{Q4I0.+^4v+/k{!.]&I/+j
            string tenantDomain = "yourtenant.onmicrosoft.com";

            string accessToken = GetMSGraphApplicationAccessToken(clientId, clientSecret, tenantDomain);

            HttpClient httpClient = new HttpClient();

            httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", accessToken);

            //Sample request to get all groups. The AAD App registration will need at least Group.Read.All application permission granted.
            var request = new HttpRequestMessage(HttpMethod.Get, $"https://graph.microsoft.com/v1.0/groups");

            var response = httpClient.SendAsync(request).Result;

            var content = response.Content.ReadAsStringAsync().Result;

            Console.WriteLine(content);
        }

        public static string GetMSGraphApplicationAccessToken(string clientId, string clientSecret, string tenantDomain)
        {
            string authority = string.Format(CultureInfo.InvariantCulture, "{0}/{1}/", "https://login.windows.net", tenantDomain);
            var authContext = new AuthenticationContext(authority);

            var clientCredential = new ClientCredential(clientId, clientSecret);

            var result = authContext.AcquireTokenAsync("https://graph.microsoft.com", clientCredential).Result;

            var accessToken = result.AccessToken;
            return accessToken;
        }

    }
}