Show InfoBar: visual studio extension sample described at https://www.visualstudiogeeks.com/extensions/how-to-show-notifications-inside-visual-studio-2017

VSIXInfoBarPackage.cs

using EnvDTE;
using Microsoft.VisualStudio.Imaging;
using Microsoft.VisualStudio.Shell;
using Microsoft.VisualStudio.Shell.Interop;
using System;
using System.Collections.Generic;
using System.Runtime.InteropServices;
using System.Threading;
using Task = System.Threading.Tasks.Task;

namespace VSIXInfoBarProject
{
    [PackageRegistration(UseManagedResourcesOnly = true, AllowsBackgroundLoading = true)]
    [Guid(VSIXInfoBarProjectPackage.PackageGuidString)]
    [ProvideAutoLoad(UIContextGuids80.SolutionHasSingleProject, PackageAutoLoadFlags.BackgroundLoad)]
    [ProvideAutoLoad(UIContextGuids80.SolutionHasMultipleProjects, PackageAutoLoadFlags.BackgroundLoad)]
    public sealed class VSIXInfoBarProjectPackage : AsyncPackage, IVsInfoBarUIEvents
    {
        private class FileOpenContext
        {
            public string FilePath { get; set; }
        }

        public const string PackageGuidString = "27482cd9-ac27-4bb8-ac49-46b72c00ddfb";

        private DTE _dte;
        private IVsShell _shell;
        private IVsInfoBarUIFactory _infoBarFactory;
        private IVsInfoBarHost _infoBarHost;

        protected override async Task InitializeAsync(CancellationToken cancellationToken, IProgress<ServiceProgressData> progress)
        {
            await this.JoinableTaskFactory.SwitchToMainThreadAsync(cancellationToken);

            _dte = GetGlobalService(typeof(DTE)) as DTE;
            _shell = GetGlobalService(typeof(SVsShell)) as IVsShell;
            _infoBarFactory = GetGlobalService(typeof(SVsInfoBarUIFactory)) as IVsInfoBarUIFactory;

            if (_shell != null)
            {
                _shell.GetProperty((int)__VSSPROPID7.VSSPROPID_MainWindowInfoBarHost, out var obj);
                _infoBarHost = obj as IVsInfoBarHost;
            }

            var element = ShowInfoBar("title", "message");
        }

        public IVsInfoBarUIElement ShowInfoBar(string title, string message, string path = null)
        {
            ThreadHelper.ThrowIfNotOnUIThread();

            if (_dte == null || _shell == null || _infoBarHost == null || _infoBarFactory == null)
            {
                return null;
            }

            var textSpan = new InfoBarTextSpan($"{title}: {message}");

            var actions = new List<IVsInfoBarActionItem>
            {
                new InfoBarHyperlink("Yes", "yes"),
                new InfoBarHyperlink("No", "no")
            };

            if (path != null)
            {
                actions.Add(new InfoBarHyperlink("Open", new FileOpenContext { FilePath = path }));
            }

            var model = new InfoBarModel(new[] { textSpan }, actions, KnownMonikers.StatusInformation, isCloseButtonVisible: true);

            IVsInfoBarUIElement element = _infoBarFactory.CreateInfoBar(model);

            element.Advise(this, out var _ /* cookie */);

            _infoBarHost.AddInfoBar(element);

            return element;
        }

        void IVsInfoBarUIEvents.OnClosed(IVsInfoBarUIElement infoBarUIElement)
        {
        }

        void IVsInfoBarUIEvents.OnActionItemClicked(IVsInfoBarUIElement infoBarUIElement, IVsInfoBarActionItem actionItem)
        {
            ThreadHelper.ThrowIfNotOnUIThread();
            if (actionItem.ActionContext is FileOpenContext context)
            {
                if (System.IO.File.Exists(context.FilePath))
                {
                    _dte?.ItemOperations.OpenFile(context.FilePath);
                }
            }
        }
    }
}