C＃对象引用未设置为对象？

Question

public static void duel(String user, String user1, String user2)
        {
            int[] value = null;
            String winner;
            for (int i = 0; i < 2; i++)
            {
                Random rand = new Random((int)DateTime.Now.Ticks);
                int numIterations = 0;
                numIterations = rand.Next(2, 12);
                value[i] = numIterations;//This is line 286
            }
            if (value[0] > value[1])
            {
                winner = user1;
            }
            else
            {
                winner = user2;
            }
            Console.WriteLine(user + " Started a duel against " + user1 + " and " + user2 + "!");
            Console.WriteLine(user1 + " rolled " + value[0] + " and " + user2 + " rolled " + value[1] + "!");
            Console.WriteLine(winner + " is the winner!");
        }

What exactly is wrong here? When I set int[] value = null it compiles but if I remove the null it says Use of unassigned local variable 'value' Line 286

Answer 1

Well yes, you have to assign a value to the value variable before you use it - but when you've set it to null, you will get a NullReferenceException when you try to use it. It looks like you want a two element array, so use

int[] value = new int[2];

However, with the code you've written you'll almost certainly end up with the two values being the same, as you're creating two instances of Random at almost exactly the same time . See my article on Random for more information.

I would pass a Random instance into the method, and rewrite it like this - without the somewhat-pointless loop:

public static void Duel(Random rng, String user, String user1, String user2)
{
    // Using Next(2, 12) doesn't really mimic 2 dice of 1-6; it actually mimics
    // a single 10-sided die with values 2 to 11 inclusive.
    int score1 = rng.Next(1, 7) + rng.Next(1, 7);
    int score2 = rng.Next(1, 7) + rng.Next(1, 7);

    // Note: this is biased in favour of user2 in case of equality
    string winner = score1 > score2 ? user1 : user2;

    Console.WriteLine("{0} started a duel between {0} and {1}!",
                      user, user1, user2);
    Console.WriteLine("{0} rolled {1} and {2} rolled {3}!", user1, score1,
                      user2, score2);
    Console.WriteLine("{0} is the winner!", winner);
}

Things to note here:

• Our method depends on something which it can't necessarily create correctly itself (the Random ) so that is injected.
• Simulate rolling two dice (which is what I assume the aim is) by rolling two single dice, not picking a single number in the range [2, 12] uniformly.
• The second parameter to Random.Next is an exclusive upper bound (I got that wrong until I was checking it over)
• Use of the conditional operator when you want to pick one expression or another based on a condition, and do the same thing with the result either way
• Use of composite format strings to make it easier to format strings than simple concatenation
• Following .NET naming conventions ( Duel , not duel )

Answer 2

Change the line to:

int[] value = new int[2];

---

You need to create an empty array so that you can use it later.

If you don't set it to anything you get the Use of unassigned local variable , because you've declared the variable, but haven't given it a value.

If you set it to null then that's giving it a value, but you're also not putting anything in that variable. Your code further down expects to be able to use elements 0 and 1, however although you declared that value is an array you've not created an array to put into the variable, so you get the error when trying to access the elements of a non-existent array.

The code above fixes this by setting the variable to an array with 2 element, which would contain the default for the type in the array ( int ), which in this case would be 0 until you set them equal to something.

---

Bad analogy time:

Imagine i'm planning on putting a bookshelf in my bedroom. I make space for it (declare the variable), but i don't put a bookshelf in that space (set the variable it to null, or don't set it at all).

If i go to get something on the 2nd shelf i'm obviously going to have a problem as although i've made space (a variable) in my room (my program) i've not put a bookshelf there (declared an array to go in the variable).

---

For more info, see the Arrays Tutorial on MSDN .

Answer 3

You are referencing value[i], but you haven't actually intialized value[] yet. You need to do something like

value = new int[2];

Answer 4

You haven't assigned your int[] an instance. If you know there will always be 2 values, you can use:

int[] value = new int[2];

If you don't know that you'll only ever have 2 values, consider using a List:

List<int> value = new List<int>();

Answer 5

In c# you need to initialize your variables to some value or it will not compile. To fix your runtime error, you need to initialize value into an array (instead of null). Since you are looping through value twice I assume you need to do something like this:

int[] value = new int[2];

Answer 6

你需要在某处创建你的数组：

int[] value = new int[2];

Answer 7

如果您只是在开头声明数组，那么您应该是安全的。

int[] value = new int[2];