Show item in ErrorList: visual studio extension sample described at https://vsxexperience.net/2010/03/23/writing-to-the-vs-errorlist/

VSIXErrorListPackage.cs

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

namespace VSIXProject1
{
    [PackageRegistration(UseManagedResourcesOnly = true, AllowsBackgroundLoading = true)]
    [Guid(VSIXProject1Package.PackageGuidString)]
    [ProvideAutoLoad(UIContextGuids80.SolutionHasSingleProject, PackageAutoLoadFlags.BackgroundLoad)]
    [ProvideAutoLoad(UIContextGuids80.SolutionHasMultipleProjects, PackageAutoLoadFlags.BackgroundLoad)]
    public sealed partial class VSIXProject1Package : AsyncPackage
    {
        public const string PackageGuidString = "2f65f260-e507-48c5-8a72-11a14d2578a2";

        private static readonly string ProviderName = typeof(VSIXProject1Package).FullName;

        private static readonly Guid ProviderGuid = Guid.NewGuid();

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

            AddErrorTask(TaskCategory.User, TaskErrorCategory.Error, "Context", "Error Message", "README.txt", 1, 1);

            await ThreadHelper.JoinableTaskFactory.RunAsync(() => Tasks.Task.Delay(TimeSpan.FromSeconds(10)));
        }

        internal ErrorTask AddErrorTask(
            TaskCategory category,
            TaskErrorCategory errorCategory,
            string context,
            string text,
            string document,
            int line,
            int column)
        {
            var errorTask = new ErrorTask()
            {
                Text = text,
                ErrorCategory = errorCategory,
                Line = line - 1,
                Column = column - 1,
                Document = document,
                Category = category,
            };

            if (!string.IsNullOrEmpty(document))
            {
                errorTask.Navigate += NavigateDocument;
            }

            GetErrorListProvider().Tasks.Add(errorTask);

            return errorTask;
        }

        private ErrorListProvider GetErrorListProvider()
        {
            return new ErrorListProvider(this)
            {
                ProviderName = ProviderName,
                ProviderGuid = ProviderGuid,
            };
        }

        private void NavigateDocument(object sender, EventArgs e)
        {
            ThreadHelper.ThrowIfNotOnUIThread();
            var task = sender as Microsoft.VisualStudio.Shell.Task ?? throw new ArgumentException(nameof(sender));
            OpenDocumentAndNavigateTo(task.Document, task.Line, task.Column);
        }

        private void OpenDocumentAndNavigateTo(string documentPath, int line, int column)
        {
            ThreadHelper.ThrowIfNotOnUIThread();
            if (!(GetGlobalService(typeof(IVsUIShellOpenDocument)) is IVsUIShellOpenDocument openDoc))
            {
                return;
            }

            Guid logicalView = VSConstants.LOGVIEWID.Code_guid;
            if (ErrorHandler.Failed(openDoc.OpenDocumentViaProject(documentPath, ref logicalView, out _, out _, out _, out var frame)) || frame == null)
            {
                return;
            }

            if (frame.GetProperty((int)__VSFPROPID.VSFPROPID_DocData, out var docData) != VSConstants.S_OK || docData == null)
            {
                return;
            }

            var textBuffer = docData is IVsTextBuffer buffer ? buffer
                : (docData is IVsTextBufferProvider provider && ErrorHandler.ThrowOnFailure(provider.GetTextBuffer(out var lines)) == VSConstants.S_OK) ? (lines as IVsTextBuffer)
                : null;
            if (textBuffer == null)
            {
                return;
            }

            if (!(GetGlobalService(typeof(VsTextManagerClass)) is IVsTextManager manager))
            {
                return;
            }

            manager.NavigateToLineAndColumn(textBuffer, ref logicalView, line, column, line, column);
        }
    }
}