Program.cs

using System;
using System.Text;
using Consul;
using System.Threading.Tasks;
using System.Threading.Tasks.Dataflow;
using System.Threading;




namespace TestApp
{
    public static class Program
    {

        public static async void startWatchService(ConsulClient client, string key, BufferBlock<object> queue, CancellationToken cancellationToken)
        {

            var lastIndex = (ulong)0;

            while (!cancellationToken.IsCancellationRequested)
            {
                try
                {
                    var res = await client.KV.List(key, new QueryOptions() { WaitIndex = lastIndex }, cancellationToken);
                    lastIndex = res.LastIndex;
                    queue.Post("new services info");
                }
                catch (Exception)
                {
                    Console.WriteLine($"get key failed when watch key {key}");
                    return;
                }

            }
        }

        private static void Main(string[] args)
        {
            var consulAddress = "http://127.0.0.1:8500";
            if (args.Length > 0)
            {
                consulAddress = args[0];
            }

            var cancelSrc = new System.Threading.CancellationTokenSource();
            var queue = new BufferBlock<object>();
            var client = new ConsulClient((c) =>
            {
                c.Address = new System.Uri(consulAddress);
                c.WaitTime = new TimeSpan(0, 0, 100);
                c.Token = "";
            });
            startWatchService(client, "SomeServiceName", queue, cancelSrc.Token);

            Task.Run(() =>
            {
                try
                {
                    while (!cancelSrc.IsCancellationRequested)
                    {
                        var objs = queue.Receive(cancelSrc.Token);
                    }
                }
                catch (OperationCanceledException)
                {
                    return;
                }
                catch (Exception e)
                {
                    Console.Write($"{e.ToString()}");
                }

            });

			Task.Delay(100000000).Wait();
        }
    }
}

TestApp.csproj

<Project Sdk="Microsoft.NET.Sdk">       

  <PropertyGroup>
    <OutputType>Exe</OutputType>        
    <TargetFramework>netcoreapp2.1</TargetFramework>
  </PropertyGroup>

    <PropertyGroup>                     
    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
  </PropertyGroup>                      

  <ItemGroup>
    <PackageReference Include="Consul" Version="0.7.2.6" />
    <PackageReference Include="System.Threading.Tasks.Dataflow" Version="4.9.0" />
  </ItemGroup>                          

</Project>