I am wanting to use the ISingleClientProxy.InvokeAsync send a message SignalR client and wait for a response. I can make this work until very easily using the following:

Server sends:

return await Clients.Client(connectionInfo.ConnectionId).InvokeAsync<string>("Test", default);

Client Returns:

this.hubConnection.On<string>("Test", () => { return "Successful"; });

My problem is I want to be able to send a parameter to the client and have it do something with it.

Something like this
Server sends:

return await Clients.Client(connectionInfo.ConnectionId).InvokeAsync<string>("Test", "Prefix", default);

Client Returns:

this.hubConnection.On<string>("Test", (prefix) => { return $"{prefix}Successful"; });

However I hit issues as soon as I send the parameters. In this example I get:

Anonymous function converted to a void returning delegate cannot return a value; 

Can anyone assist?

Thanks in advance