EchoGrpcMagicOnion.Shared.csproj

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

  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MagicOnion" Version="4.0.0-preview.1" />
    <PackageReference Include="MagicOnion.Abstractions" Version="4.0.0-preview.1" />
  </ItemGroup>

</Project>

IEchoService.cs

using ConsoleAppFramework;
using EchoGrpcMagicOnion.Shared;
using Grpc.Core;
using Grpc.Net.Client;
using MagicOnion.Client;
using Microsoft.Extensions.Hosting;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace EchoGrpcMagicOnion.Client
{
    class Program : ConsoleAppBase, IMyHubReceiver
    {
        static async Task Main(string[] args)
        {
            await Host.CreateDefaultBuilder()
                .RunConsoleAppFrameworkAsync<Program>(args);
        }

        [Command("Echo")]
        public async Task Echo(string hostPort, string message, [Option("H")]string headers, [Option("v")]bool verbose = false)
        {
            // under .NET Core 3.x: https://docs.microsoft.com/ja-jp/aspnet/core/grpc/troubleshoot?view=aspnetcore-3.1#call-insecure-grpc-services-with-net-core-client
            AppContext.SetSwitch("System.Net.Http.SocketsHttpHandler.Http2UnencryptedSupport", true);

            await Task.Delay(TimeSpan.FromSeconds(3));

            var channel = GrpcChannel.ForAddress(hostPort);
            var client = MagicOnionClient.Create<IEchoService>(channel, new IClientFilter[]
            {
                new AppendHeaderFilter(headers),
                new ResponseHandlingFilter(verbose),
            });

            try
            {
                var res = await client.EchoAsync(message);
                Console.WriteLine(res);
            }
            catch (Exception ex)
            {
                Console.WriteLine($"{ex.Message} {ex.StackTrace}");
            }
        }
    }
}