ViewModelBase.cs

protected bool RequestSubNavigate<TViewModel, TResult>(IDictionary<string, string> parameterValues, Action<TResult> onResult) 
    where TViewModel : SubViewModelBase<TResult>
{
    parameterValues = parameterValues ?? new Dictionary<string, string>();

    if (parameterValues.ContainsKey("messageId"))
        throw new ArgumentException("parameterValues cannot contain an item with the key 'messageId'");

    string messageId = Guid.NewGuid().ToString();

    parameterValues["messageId"] = messageId;

    TinyMessageSubscriptionToken token = null;
    token = MessengerHub.Subscribe<SubNavigationResultMessage<TResult>>(msg =>
	{
	    if (token != null)
	        MessengerHub.Unsubscribe<SubNavigationResultMessage<TResult>>(token);

	    onResult(msg.Result);
	},
	msg => msg.MessageId == messageId);

    return RequestNavigate<TViewModel>(parameterValues);
}