AspNetCore.Docs/aspnetcore/signalr/hubs/sample/ChatHub.cs

using Microsoft.AspNetCore.SignalR;
using System;
using System.Collections.Generic;
using System.Threading.Tasks;

namespace SignalRChat
{
    public class ChatHub : Hub
    {
        public Task SendMessageToAll(string message)
        {
            return Clients.All.SendAsync("ReceiveMessage", message);    
        }

        public Task SendMessageToCaller(string message)
        {
            return Clients.Caller.SendAsync("ReceiveMessage", message);
        }

        public Task SendMessageToGroups(string user, string message)
        {
            List<string> groups = new List<string>() { "SignalR Users" };
            return Clients.Groups(groups).SendAsync("ReceiveMessage", message);
        }

        public override async Task OnConnectedAsync()
        {
            await Groups.AddAsync(Context.User.Identity.Name, "SignalR Users");
            await base.OnConnectedAsync();
        }
    }
}