Program.cs

using System;
using System.Diagnostics;
using System.Threading.Tasks;
using Microsoft.Extensions.Logging;
using OmniSharp.Extensions.LanguageServer;
using OmniSharp.Extensions.LanguageServer.Abstractions;
using OmniSharp.Extensions.LanguageServer.Capabilities.Client;
using OmniSharp.Extensions.LanguageServer.Capabilities.Server;
using OmniSharp.Extensions.LanguageServer.Models;
using OmniSharp.Extensions.LanguageServer.Protocol;
using OmniSharp.Extensions.LanguageServer.Protocol.Document;

namespace SampleServer
{
    class Program
    {
        static void Main(string[] args)
        {
            MainAsync(args).Wait();
        }

        static async Task MainAsync(string[] args)
        {
            //while (!System.Diagnostics.Debugger.IsAttached)
            //{
            //    await Task.Delay(100);
            //}

            var server = new LanguageServer(Console.OpenStandardInput(), Console.OpenStandardOutput(), new LoggerFactory());

            server.AddHandler(new TextDocumentHandler(server));

            await server.Initialize();
            await server.WasShutDown;
        }
    }

    class TextDocumentHandler : ITextDocumentSyncHandler
    {
        private readonly ILanguageServer _router;

        private readonly DocumentSelector _documentSelector = new DocumentSelector(
            new DocumentFilter()
            {
                Pattern = "**/*.csproj",
                Language = "xml"
            }
        );

        private SynchronizationCapability _capability;

        public TextDocumentHandler(ILanguageServer router)
        {
            _router = router;
        }

        public TextDocumentSyncOptions Options { get; } = new TextDocumentSyncOptions()
        {
            WillSaveWaitUntil = false,
            WillSave = true,
            Change = TextDocumentSyncKind.Full,
            Save = new SaveOptions()
            {
                IncludeText = true
            },
            OpenClose = true
        };

        public Task Handle(DidChangeTextDocumentParams notification)
        {
            _router.LogMessage(new LogMessageParams()
            {
                Type = MessageType.Log,
                Message = "Hello World!!!!"
            });
            return Task.CompletedTask;
        }

        TextDocumentChangeRegistrationOptions IRegistration<TextDocumentChangeRegistrationOptions>.GetRegistrationOptions()
        {
            return new TextDocumentChangeRegistrationOptions()
            {
                DocumentSelector = _documentSelector,
                SyncKind = Options.Change
            };
        }

        public void SetCapability(SynchronizationCapability capability)
        {
            _capability = capability;
        }

        public async Task Handle(DidOpenTextDocumentParams notification)
        {
            _router.LogMessage(new LogMessageParams()
            {
                Type = MessageType.Log,
                Message = "Hello World!!!!"
            });
        }

        TextDocumentRegistrationOptions IRegistration<TextDocumentRegistrationOptions>.GetRegistrationOptions()
        {
            return new TextDocumentRegistrationOptions()
            {
                DocumentSelector = _documentSelector,
            };
        }

        public Task Handle(DidCloseTextDocumentParams notification)
        {
            return Task.CompletedTask;
        }

        public Task Handle(DidSaveTextDocumentParams notification)
        {
            return Task.CompletedTask;
        }

        TextDocumentSaveRegistrationOptions IRegistration<TextDocumentSaveRegistrationOptions>.GetRegistrationOptions()
        {
            return new TextDocumentSaveRegistrationOptions()
            {
                DocumentSelector = _documentSelector,
                IncludeText = Options.Save.IncludeText
            };
        }
        public TextDocumentAttributes GetTextDocumentAttributes(Uri uri)
        {
            return new TextDocumentAttributes(uri, "csharp");
        }
    }
}