Graph.Net.SDK.SPO.Thumbnail.cs

using Microsoft.Graph;
using Microsoft.Identity.Client;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http.Headers;
using System.Threading.Tasks;

namespace Graph.SPO.Thumbnails
{
    class Program
    {
        public static async Task Main(string[] args)
        {
            GraphServiceClient graphClient = await GetGraphClient();

            //Get the site object. You can use other ways to get the site object as well:
            //https://docs.microsoft.com/en-us/graph/api/site-get?view=graph-rest-1.0&tabs=http

            Site site = await graphClient.Groups["<your-office365-group-id>"].Sites["root"].Request().GetAsync();

            //Get the modern page thumbnails
            var queryOptions = new List<QueryOption>
                        {
                                new QueryOption("expand", "thumbnails"),
                                new QueryOption("select", "thumbnails/large") //large, medium, small
                        };

            var driveItem = await graphClient.Sites[site.Id].Lists["Site Pages"].Drive.Root.ItemWithPath($"/MySPOModernPage.aspx").Request(queryOptions).GetAsync();

            var thumbnail = driveItem.Thumbnails.First();

            Console.WriteLine(thumbnail?.Large?.Url);
        }

        private static async Task<GraphServiceClient> GetGraphClient()
        {
            var scopes = new string[] { "https://graph.microsoft.com/.default" };
            var clientID = "<client-id>";
            var clientSecret = "<client-secret>";
            var tenantID = "<tenant-id>";

            IConfidentialClientApplication clientApp = ConfidentialClientApplicationBuilder
                                 .Create(clientID)
                                 .WithClientSecret(clientSecret)
                                 .WithTenantId(tenantID)
                                 .Build();

            AuthenticationResult authResult = await clientApp.AcquireTokenForClient(scopes).ExecuteAsync();
            string accessToken = authResult.AccessToken;

            var graphClient = new GraphServiceClient(
                new DelegateAuthenticationProvider(
                    requestMessage =>
                    {
                        requestMessage.Headers.Authorization = new AuthenticationHeaderValue("bearer", accessToken);
                        return Task.CompletedTask;
                    }));
            return graphClient;
        }
    }
}