public BusBuilder SubscribeToCommand <TCommand>() where TCommand : ICommand
{
var handler = (ICommandHandler <TCommand>)_webHost.Services
.GetService(typeof(ICommandHandler <TCommand>));
// we'll invoke a special extension method, and let's just say we'll create
// an extension method called WithCommandHandlerAsync, and provide our handler like this
_bus.WithCommandHanlderAsync(handler);
return(this);
}