各位大神，目前刚自学c#，但是在第十章扑克牌程序卡住了，遇到了个问题，如下：
using System;
using System.Collections.Generic;
using System.Text;
namespace Ch10CardLib
{
    public enum Suit
    {
        Club,
        Diamond,
        Heart,
        Spade,
    }
    public enum Rank
    {
        Ace = 1,
        Deuce,
        Three,
        Four,
        Five,
        Six,
        Seven,
        Eight,
        Nine,
        Ten,
        Jack,
        Queen,
        King,
    }
    public class Card
    {
        public readonly Suit suit;
        public readonly Rank rank;
        public  Card(Suit newSuit,Rank newRank)
        {
            this.suit = newSuit;
            this.rank = newRank;
        }
        private Card() { }
        public override string ToString()
        {
            return "The " + rank + " of " + suit + "s";
        }
    }
  public class Deck
  {
    private Card[] cards;
    public Deck()
    {
      cards = new Card[52];
      for (int suitVal = 0; suitVal < 4; suitVal++)
      {
        for (int rankVal = 1; rankVal < 14; rankVal++)
        {
          cards[suitVal * 13 + rankVal - 1] = new Card((Suit)suitVal,(Rank)rankVal);
        }
      }
    }
    [color=#FF0000]public Card GetCard(int cardNum)
    {
      if (cardNum >= 0 && cardNum <= 51)
        return cards[cardNum ];
      else
        throw (new System.ArgumentOutOfRangeException("cardNum", cardNum, "数字在0-51间"));
    }[/color]
    public void Shuffle()
    {
      Card[] newDeck = new Card[52];
      bool[] assigned=new bool[52];
      Random sourceGen = new Random();
      for (int i = 0; i < 52; i++)
      {
        int destCard = 0;
        bool foundCard = false;
        while (foundCard == false)
        {
          destCard = sourceGen.Next(52);
          if (assigned[destCard] == false)
            foundCard = true;
        }
        assigned[destCard] = true;
        newDeck[destCard] = cards[i];
      }
      newDeck.CopyTo(cards ,0);
    }
  }
}
创建GetCard方法时，VS2010显示错误“.Deck.GetCard(int)”: 并非所有的代码路径都返回值，求解，输入几次都显示这种错误？