SharePoint App Only Helper

SharePointAppOnlyHelper.ts

/* 
If you have a .Net Core/Standard solution or you don't want to use PnP Authentication Manager, this is the best approach to get SharePoint App Only Permission.

1- Create an App Registration in Azure
2- Navigate to http://<SharePointWebSite>/_layouts/15/AppInv.aspx and follow the steps from the link below to give your application access:
  https://docs.microsoft.com/en-us/sharepoint/dev/sp-add-ins/add-in-permissions-in-sharepoint
3- Add this class to your solution and install Microsoft.SharePointOnline.CSOM NuGet Package.
4- Update Client Id, Client Secret and Tenant Id in this class
5- Get the SharePoint context and do what your application needs to do:
 
  var token = await SharePointAppOnlyHelper.AcquireTokenAsync();
  string webUrl = Environment.GetEnvironmentVariable("WebURL");
  var clientContext = SharePointAppOnlyHelper.GetClientContext(webUrl, token);
  
*/  

using System;
using System.Threading.Tasks;
using System.Net.Http;
using System.Text;
using System.Text.Json;
using Microsoft.SharePoint.Client;

namespace Contoso.AuthenticationHelpers
{
    public static class SharePointAppOnlyHelper
    {
        private static readonly string ClientId = Environment.GetEnvironmentVariable("CLIENTID");
        private static readonly string ClientSecret = Environment.GetEnvironmentVariable("CLIENTSECRET");
        private static readonly string targetHost = Environment.GetEnvironmentVariable("TARGETHOST"); //e.x. contoso.sharepoint.com
        private static readonly string TenantId = Environment.GetEnvironmentVariable("TENANTID");
        private static readonly string principal = "00000003-0000-0ff1-ce00-000000000000";
        private static readonly string tokenEndpoint = $"https://accounts.accesscontrol.windows.net/{TenantId}/tokens/OAuth/2";

        public static async Task<string> AcquireTokenAsync()
        {
            string resource = $"{principal}/{targetHost}@{TenantId}";

            var body = $"resource={resource}" +
                $"&client_id={ClientId}@{TenantId}" +
                $"&grant_type=client_credentials" +
                $"&client_secret={ClientSecret}";

            using (var stringContent = new StringContent(body, Encoding.UTF8, "application/x-www-form-urlencoded"))
            {
                using (HttpClient httpClient = new HttpClient())
                {
                    var result = await httpClient.PostAsync(tokenEndpoint, stringContent).ContinueWith((response) =>
                    {
                        return response.Result.Content.ReadAsStringAsync().Result;
                    }).ConfigureAwait(false);

                    var tokenResult = System.Text.Json.JsonSerializer.Deserialize<JsonElement>(result);
                    var token = tokenResult.GetProperty("access_token").GetString();
                    return token;
                }
                    
            }
        }

        public static ClientContext GetClientContext(string targetUrl, string accessToken)
        {
            ClientContext clientContext = new ClientContext(targetUrl);
            clientContext.ExecutingWebRequest +=
                delegate (object oSender, WebRequestEventArgs webRequestEventArgs)
                {
                    webRequestEventArgs.WebRequestExecutor.RequestHeaders["Authorization"] =
                        "Bearer " + accessToken;
                };
            return clientContext;
        }


    }
}