FinalAddCommand.cs

private IDisableableCommand _addCommand;
public IDisableableCommand AddCommand
{
    get
    {
        if (_addCommand == null)
            _addCommand = new DisableableCommandAsync(
            AddCommandHandler,
            obj => true,
            new List<IDisableableCommand>() { RemoveCommand, UpdateCommand });
        return _addCommand;
    }
}

private async Task AddCommandHandler(object arg)
{
    // do time consuming job
}