dotnet new console -n SpConnectOnly
cd SpConnectOnly
dotnet add package Microsoft.Identity.Client

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

namespace SpConnectOnly
{
    internal class Program
    {
        // ========================= USER SETTINGS =========================
        // Entra ID (Azure AD)
        private static readonly string tenantId = "YOUR_TENANT_ID_GUID";
        private static readonly string clientId = "YOUR_PUBLIC_CLIENT_APP_ID_GUID";

        // SharePoint site to validate the connection against
        private static readonly string siteUrl = "https://contoso.sharepoint.com/sites/YourSite";
        // =================================================================

        static async Task<int> Main()
        {
            Console.WriteLine("=== SharePoint Interactive Connect (Base App) ===");

            try
            {
                // Build a tenant-agnostic scope from the siteUrl authority:
                // e.g., "https://contoso.sharepoint.com/AllSites.FullControl"
                var scopes = BuildScopesFromSite(siteUrl);

                // 1) Acquire token interactively
                var accessToken = await AcquireAccessTokenInteractiveAsync(tenantId, clientId, scopes);
                if (string.IsNullOrWhiteSpace(accessToken))
                {
                    Console.WriteLine("Failed to acquire token.");
                    return 1;
                }

                // 2) Call a tiny SharePoint endpoint to prove connectivity
                var siteTitle = await GetSiteTitleAsync(siteUrl, accessToken);
                if (string.IsNullOrWhiteSpace(siteTitle))
                {
                    Console.WriteLine("Connected, but could not read site title (check permissions/siteUrl).");
                    return 2;
                }

                Console.WriteLine($"Connected to: {siteTitle}");
                Console.WriteLine("Base connectivity successful. You’re ready for the next step.");
                return 0;
            }
            catch (Exception ex)
            {
                Console.WriteLine("Error: " + ex.Message);
                Console.WriteLine(ex);
                return 3;
            }
        }

        // ---------------------------- Auth ----------------------------
        private static async Task<string> AcquireAccessTokenInteractiveAsync(
            string tenantId,
            string clientId,
            string[] scopes)
        {
            var app = PublicClientApplicationBuilder.Create(clientId)
                .WithAuthority(AzureCloudInstance.AzurePublic, tenantId)
                .WithRedirectUri("http://localhost")
                .Build();

            try
            {
                var result = await app.AcquireTokenInteractive(scopes)
                                      .WithPrompt(Prompt.SelectAccount)
                                      .ExecuteAsync();
                return result.AccessToken;
            }
            catch (MsalException mex)
            {
                Console.WriteLine("MSAL error: " + mex.Message);
                return string.Empty;
            }
        }

        // ---------------------- Connectivity Probe ---------------------
        private static async Task<string?> GetSiteTitleAsync(string siteUrl, string accessToken)
        {
            using (var http = new HttpClient())
            {
                http.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", accessToken);
                http.DefaultRequestHeaders.Accept.Clear();
                http.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

                var endpoint = $"{TrimEndSlash(siteUrl)}/_api/web?$select=Title";
                using (var req = new HttpRequestMessage(HttpMethod.Get, endpoint))
                {
                    req.Headers.Add("Accept", "application/json;odata=nometadata");

                    using (var res = await http.SendAsync(req))
                    {
                        var body = await res.Content.ReadAsStringAsync();

                        if (!res.IsSuccessStatusCode)
                        {
                            throw new Exception($"SharePoint error {(int)res.StatusCode}: {res.ReasonPhrase}\n{body}");
                        }

                        using (var doc = JsonDocument.Parse(body))
                        {
                            if (doc.RootElement.TryGetProperty("Title", out var titleProp))
                                return titleProp.GetString();
                        }
                    }
                }
            }
            return null;
        }

        // --------------------------- Helpers --------------------------
        private static string[] BuildScopesFromSite(string siteUrl)
        {
            var resource = new Uri(siteUrl).GetLeftPart(UriPartial.Authority); // https://contoso.sharepoint.com
            return new[] { $"{resource}/AllSites.FullControl" };
        }

        private static string TrimEndSlash(string url)
        {
            if (string.IsNullOrWhiteSpace(url)) return url;
            return url.EndsWith("/") ? url.TrimEnd('/') : url;
        }
    }
}

dotnet run

Connected to: <Your Site Title>
Base connectivity successful. You’re ready for the next step.