Add a Language Server Protocol extension does not work

Question

I am trying to implement the example of a LSP client: https://learn.microsoft.com/en-us/visualstudio/extensibility/adding-an-lsp-extension?view=vs-2019#create-a-simple-language-client

When I use the code given on this page, I get an error.

1>A:\develop\Training\VSIXProject1\VSIXProject2\BarLanguageClient.cs(17,38,17,53): error CS0535: 'BarLanguageClient' does not implement interface member 'ILanguageClient.OnServerInitializeFailedAsync(ILanguageClientInitializationInfo)'
1>A:\develop\Training\VSIXProject1\VSIXProject2\BarLanguageClient.cs(17,38,17,53): error CS0535: 'BarLanguageClient' does not implement interface member 'ILanguageClient.ShowNotificationOnInitializeFailed'

It seems that ILanguageClient has changed. When I use the old version of Nuget package (https://www.nuget.org/packages/Microsoft.VisualStudio.LanguageServer.Client/16.10.1220/) it compiles. But when executing my ILanguageClient derived class is not created. Here is my Code:

using System;
using System.Collections.Generic;
using System.ComponentModel.Composition;
using System.Diagnostics;
using System.IO;
using System.Reflection;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.VisualStudio.LanguageServer.Client;
using Microsoft.VisualStudio.Threading;
using Microsoft.VisualStudio.Utilities;

namespace VSIXProject2
{
    [ContentType("reni")]
    [Export(typeof(ILanguageClient))]
    public class BarLanguageClient : ILanguageClient
    {
        public BarLanguageClient() { Debugger.Launch();}
        public string Name => "Bar Language Extension";

        public IEnumerable<string> ConfigurationSections => null;

        public object InitializationOptions => null;

        public IEnumerable<string> FilesToWatch => null;

        public event AsyncEventHandler<EventArgs> StartAsync;
        public event AsyncEventHandler<EventArgs> StopAsync;

        public async Task<Connection> ActivateAsync(CancellationToken token)
        {
            await Task.Yield();

            ProcessStartInfo info = new ProcessStartInfo();
            info.FileName = Path.Combine(Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location), "Server", @"MockLanguageServer.exe");
            info.Arguments = "bar";
            info.RedirectStandardInput = true;
            info.RedirectStandardOutput = true;
            info.UseShellExecute = false;
            info.CreateNoWindow = true;

            Process process = new Process();
            process.StartInfo = info;

            if (process.Start())
            {
                return new Connection(process.StandardOutput.BaseStream, process.StandardInput.BaseStream);
            }

            return null;
        }

        public async Task OnLoadedAsync()
        {
            await StartAsync.InvokeAsync(this, EventArgs.Empty);
        }

        public Task OnServerInitializeFailedAsync(Exception e)
        {
            return Task.CompletedTask;
        }

        public Task OnServerInitializedAsync()
        {
            return Task.CompletedTask;
        }
    }
}

This is my content type definition:

using System.ComponentModel.Composition;
using Microsoft.VisualStudio.Utilities;

namespace VSIXProject2
{
    public class ReniContentDefinition
    {
        [Export]
        [Name("reni")]
        [BaseDefinition("code")]
        internal static ContentTypeDefinition ReniContentTypeDefinition;

        [Export]
        [FileExtension(".reni")]
        [ContentType("reni")]
        internal static FileExtensionToContentTypeDefinition ReniFileExtensionDefinition;
    }
}

When I press F5 on the VSIX-project another Visual Studio is openend with commandline parameters "/rootsuffix Exp". I select a folder that contains some files with extension ".reni". I open such a file. I would expect that my ILanguageClient-derived class is instantiated then. But the "Debugger.Launch()" I put inside the ctor is obviously not reached.

After some investigation I wonder if it is a problem with different versions of ILanguageClient. My current version of VS2019 is Version 16.11.7. I guess the Microsoft.VisualStudio.LanguageServer.Client where ILanguageClient is contained in also has this version. The nuget packages for Microsoft.VisualStudio.LanguageServer.Client are available for 16.10 and for 17.x. Not for 16.11.7.

I can't imagine how that can work at all.