c＃System.NullReferenceException与锯齿状数组

Question

我有：

namespace CardGame
{
    class Dealer
    {
        static void Main(string[] args)
        {
            string[] suits = { "Clubs", "Spades", "Hearts", "Diamonds" };
            string[] specials = { "Jack", "Queen", "King", "Ace" };
            string[][] hand = new string[5][];

            Console.WriteLine("Please enter the number of players:");

            int playerCount = Int32.Parse(Console.ReadLine());

            for (int currentPlayer = 0; currentPlayer < playerCount; currentPlayer++)
            {
                Random rand = new Random();
                for (int cardNumber = 0; cardNumber < 5; cardNumber++)
                {
                    string card;

                    int mode = rand.Next(0, 2);
                    if (mode == 1) // Numeric card...
                    {
                        card = rand.Next(2, 10).ToString();
                    }
                    else // Face card or ace...
                    {
                        card = specials[rand.Next(0, 4)];
                    }
                    hand[currentPlayer][cardNumber] = card += " of " + suits[rand.Next(0, 3)];
                    Console.WriteLine(card += " of " + suits[rand.Next(0, 3)]);
                }
            }
            Console.ReadLine();
        }
    }
}

该行：hand [currentPlayer] [cardNumber] = card + =“ of” + suits [rand.Next（0，3）];

在标题中引发错误。 我不知道如何解决此问题，因为我是C＃的新手。

我需要做什么？

Answer 1

您创建了一个锯齿状的数组，但它不是数组的完整数组，而只是空引用的数组。 您必须为外部数组中的每个项目创建一个内部数组。

您已经创建了一个由五个项目组成的数组，但是它应该是一个具有玩家人数长度的数组。 因此，您必须在知道有多少玩家之后创建数组：

...
string[][] hand;

Console.WriteLine("Please enter the number of players:");
int playerCount = Int32.Parse(Console.ReadLine());

hand = new string[playerCount][];
...

现在，在循环内部，您应该为每个项目创建一个内部数组，该数组的长度应为5。 应该在外部循环外部创建Random实例，如果您创建的新实例在时间上过于接近，则它们会生成相同的数字序列。

...
Random rand = new Random();
for (int currentPlayer = 0; currentPlayer < playerCount; currentPlayer++)
{
  hand[currentPlayer] = new string[5];
  ...

---

由于所有内部数组都具有相同的长度，因此可以使用二维数组而不是锯齿状数组。 它以类似的方式声明：

string[,] hand;

它以类似的方式创建：

hand = new string[playerCount, 5];

由于它是单个数组，而不是数组数组，因此您无需在循环中创建任何内部数组。

项目的分配也略有不同：

hand[currentPlayer, cardNumber] = ...

Answer 2

我对您的代码进行了更改，它可以运行。 我相信这就是您想要实现的目标

using System.Linq;
namespace CardGame
{
    class Dealer
    {
        static void Main(string[] args)
        {
            string[] suits = { "Clubs", "Spades", "Hearts", "Diamonds" };
            string[] specials = { "Jack", "Queen", "King", "Ace" };

            // Two dimensional arrays - I believe this is what you want to achieve, run the application 
            string[,] hand = new string[5,5];

            Console.WriteLine("Please enter the number of players:");

            int playerCount = Int32.Parse(Console.ReadLine());

            for (int currentPlayer = 0; currentPlayer < playerCount; currentPlayer++)
            {
                Random rand = new Random();
                for (int cardNumber = 0; cardNumber < 5; cardNumber++)
                {
                    string card;

                    int mode = rand.Next(0, 2);
                    if (mode == 1) // Numeric card...
                    {
                        card = rand.Next(2, 10).ToString();
                    }
                    else // Face card or ace...
                    {
                        card = specials[rand.Next(0, 4)];
                    }

                    var temp = " of " + suits[rand.Next(0, 3)];

                    if (card != null && !card.Contains(temp))
                    {
                        hand[currentPlayer, cardNumber] = card += " of " + suits[rand.Next(0, 3)];


                       Console.WriteLine(card += " of " + suits[rand.Next(0, 3)]);

                        //Result
                        Console.WriteLine("Result: {0}", hand[currentPlayer, cardNumber]);
                    }




                }
            }
            Console.ReadLine();
        }
    }
}