VSPackage.cs

using System;
using System.Runtime.InteropServices;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.VisualStudio;
using Microsoft.VisualStudio.Shell;
using Microsoft.VisualStudio.Shell.Events;
using Microsoft.VisualStudio.Shell.Interop;
using Task = System.Threading.Tasks.Task;

namespace SolutionLoadSample
{
    [Guid("61eadc52-5677-4548-b273-08f1e6574f71")]
    [PackageRegistration(UseManagedResourcesOnly = true, AllowsBackgroundLoading = true)]
    [InstalledProductRegistration("Solution Load Sample", "Demonstrates use of solution load events", "1.0")]
    // The following line will schedule the package to be initialized when a solution is being opened
    [ProvideAutoLoad(VSConstants.UICONTEXT.SolutionOpening_string, PackageAutoLoadFlags.BackgroundLoad)]
    public sealed class VSPackage : AsyncPackage
    {
        protected override async Task InitializeAsync(CancellationToken cancellationToken, IProgress<ServiceProgressData> progress)
        {
            // Since this package might not be initialized until after a solution has finished loading,
            // we need to check if a solution has already been loaded and then handle it.
            bool isSolutionLoaded = await IsSolutionLoadedAsync();

            if (isSolutionLoaded)
            {
                HandleOpenSolution();
            }

            // Listen for subsequent solution events
            SolutionEvents.OnAfterOpenSolution += HandleOpenSolution;
        }

        private async Task<bool> IsSolutionLoadedAsync()
        {
            await JoinableTaskFactory.SwitchToMainThreadAsync();
            var solService = await GetServiceAsync(typeof(SVsSolution)) as IVsSolution;

            ErrorHandler.ThrowOnFailure(solService.GetProperty((int)__VSPROPID.VSPROPID_IsSolutionOpen, out object value));

            return value is bool isSolOpen && isSolOpen;
        }

        private void HandleOpenSolution(object sender = null, EventArgs e = null)
        {
            // Handle the open solution and try to do as much work
            // on a background thread as possible
        }
    }
}