RhSolutions-AddIn/src/AddIn/AddIn.cs

using ExcelDna.Integration;
using ExcelDna.IntelliSense;
using Microsoft.Office.Interop.Excel;
using RhSolutions.Services;
using System.Net.Http;

namespace RhSolutions.AddIn
{
    class RhSolutionsAddIn : IExcelAddIn
    {
        public static Application Excel;
        public static HttpClient httpClient;

        public void AutoOpen()
        {
            Excel = (Application)ExcelDnaUtil.Application;
            httpClient = new HttpClient();
            IntelliSenseServer.Install();
            RegistryUtil.Initialize();
            EventsUtil.Initialize();
        }

        public void AutoClose()
        {
            IntelliSenseServer.Uninstall();
            RegistryUtil.Uninitialize();
            EventsUtil.Uninitialize();
        }
    }
}