SubViewModelBase.cs

public abstract class SubViewModelBase<TResult> : ViewModelBase
{
    protected string MessageId { get; private set; }

    protected SubViewModelBase(string messageId)
    {
        MessageId = messageId;
    }

    protected void Cancel()
    {
        ReturnResult(default(TResult));
    }

    protected void ReturnResult(TResult result)
    {
        var message = new SubNavigationResultMessage<TResult>(this, MessageId, result);
        MessengerHub.Publish(message);

        Close();
    }
}