Q: SignalR call to specific client from controller ASP.NET Core 2.1 I need to send an instant message from the server to the client after the user has submitted a form in a browser.
I followed the Microsoft steps here to set up a signalR connection, created a Hub class, signalr.js etc. 
The problem is that I can only invoke a message to all clients, but I need to invoke the message to the specific caller who initiated the request (otherwise everyone will get the message).
This is my POST Action in the HomeController.cs:
[HttpPost]
    [ValidateAntiForgeryToken]
    public async Task<IActionResult> Submit(string signalRconnectionId, Dictionary<string, string> inputs)
    {
        //Invoke signal to all clients sending a message to initSignal WORKS FINE
        await _signalHubContext.Clients.All.SendAsync("initSignal", "This is a message from the server!");

        //Invoke signal to specified client where signalRConnectionId = connection.id DOES NOT WORK
        await _signalHubContext.Clients.Client(signalRconnectionId).SendAsync("initSignal", "This is a message from server to this client: " + signalRconnectionId);

        return RedirectToAction("Success", inputs);
    }

my client javascript file:
    //Create connection and start it
const connection = new signalR.HubConnectionBuilder()
    .withUrl("/signalHub")
    .configureLogging(signalR.LogLevel.Information)
    .build();
connection.start().catch(err => console.error(err.toString()));

console.log("connectionID: " + connection.id);
$("#signalRconnectionId").attr("value", connection.id);

//Signal method invoked from server
connection.on("initSignal", (message) => {

    console.log("We got signal! and the message is: " + message);


});

I have tried debugging the action method and I get correctly passed in the connectionId which is "0" (incrementing by 1 per connection)

A: So here's the final solution I came up with inspired by the answer from this thread
I got the connectionId by calling the Hub class from client and then from client calling the controller passing in the connectionId.
Hub class:
public class SignalHub : Hub
{
    

    public string GetConnectionId()
    {
        return Context.ConnectionId;
    }
}

client-side javascript code executed at startup:
connection.start().catch(err => console.error(err.toString())).then(function(){
connection.invoke('getConnectionId')
    .then(function (connectionId) {
        // Send the connectionId to controller
        console.log("connectionID: " + connectionId);
        $("#signalRconnectionId").attr("value", connectionId);
    });
});

HomeController.cs:
public async Task<IActionResult> Submit(string signalRconnectionId, Dictionary<string, string> inputs)
    {
        
        //Invoke signal to specified client WORKS NOW
        await _signalHubContext.Clients.Client(signalRconnectionId).SendAsync("initSignal", "This is a message from server to this client: " + signalRconnectionId);

        return RedirectToAction("Success", inputs);
    }

It works fine, but still feels a little like a roundtrip, it would have been easier if we didn't have to go through the hub class to make this happen. Maybe just having the connectionId from the client-side to begin with, but maybe there is a good reason for the design :)

A: According to Microsoft, you can not access to the ConnectionId and Caller from outside a hub
https://learn.microsoft.com/en-us/aspnet/core/signalr/hubcontext?view=aspnetcore-2.1

When hub methods are called from outside of the Hub class, there's no
  caller associated with the invocation. Therefore, there's no access to
  the ConnectionId, Caller, and Others properties.

