public class CacheService 
{
    public Dictionary<int, string> Pages = new Dictionary<int, string>();
    public int CurrentPage = 1;

    public CacheService()
    {
        for (int i = 1; i < 53; i++)
        {
            Pages.Add(i, $"https://linedd19f3.blob.core.windows.net/slides/linedd19-{i.ToString().PadLeft(2,'0')}.png");
        }
    }
}

services.AddSingleton<CacheService, CacheService>();

public class EchoBot : ActivityHandler
{
    CacheService cache;
    public EchoBot(ConversationState conversationState, UserState userState, CacheService cache)
    {
        this.cache = cache;
    }

    protected override async Task OnMessageActivityAsync(ITurnContext<IMessageActivity> turnContext, CancellationToken cancellationToken)
    {
        var page = cache.Pages[cache.CurrentPage];
        await turnContext.SendActivityAsync(
            MessageFactory.Attachment(new Attachment("image/png", page)),
            cancellationToken);
    } 
}

using Microsoft.AspNetCore.Mvc;
using Microsoft.BotBuilderSamples.Services;
using System.Threading.Tasks;

namespace Microsoft.BotBuilderSamples.Controllers
{
    [Route("api/pages")]
    [ApiController]
    public class PagesController : ControllerBase
    {
        private CacheService cacheService;

        public PagesController(CacheService cacheService)
        {
            this.cacheService = cacheService;
        }

        [HttpGet]
        public async Task SetPageAsync(int id)
        {
            cacheService.CurrentPage = id;
        }
    }
}

using System.Net.Http;
using PowerPoint = Microsoft.Office.Interop.PowerPoint;

namespace LDD19
{
    public partial class ThisAddIn
    {
        private HttpClient client;
        private void ThisAddIn_Startup(object sender, System.EventArgs e)
        {
            client = new HttpClient();
            this.Application.SlideShowNextSlide += Application_SlideShowNextSlide;
        }

        private void Application_SlideShowNextSlide(PowerPoint.SlideShowWindow Wn)
        {
            var r = client.GetAsync($"http://<サーバー名>/api/pages?id={Wn.View.Slide.SlideNumber}").Result;
        }
        private void ThisAddIn_Shutdown(object sender, System.EventArgs e)
        {
        }

        #region VSTO で生成されたコード

        /// <summary>
        /// デザイナーのサポートに必要なメソッドです。
        /// このメソッドの内容をコード エディターで変更しないでください。
        /// </summary>
        private void InternalStartup()
        {
            this.Startup += new System.EventHandler(ThisAddIn_Startup);
            this.Shutdown += new System.EventHandler(ThisAddIn_Shutdown);
        }

        #endregion
    }
}