如何在 C# 的数组中找到某个 position 的值？

Question

I need to iterate through an Array and find the sum of the odd, and the even positioned values.

Here is what I have, but I'm getting the error: Cannot apply indexing with [] to an expression of type 'Array'

static void SumOfOddsAndEvens(Array arr)
        {
            int i = 0;
            bool isEven = true;
            int evenSum = 0;
            int oddSum = 0;

            while (i < arr.Length)
            {
                Console.WriteLine(arr[i]);

                if (isEven) evenSum += arr[i]; // Cannot apply indexing with [] to an expression of type 'Array'
                if (!isEven) oddSum += arr[i];
                isEven = !isEven;
                i++;
            }

            Console.WriteLine("Sum of even slots: " + evenSum);

            Console.WriteLine("Sum of odd slots: " + oddSum);
        }

Function is being called here:

 static void Main(string[] args)
        {
            SumOfOddsAndEvens(new int[] { 1, 2, 3, 4, 6, 7, 8 });
        }

Answer 1

Although all arrays in C# derive from thebase-class Array , the latter does not define an indexer. Thus you can´t use arr[i] when arr is just an Array .

However I wonder why you even use that base-class, instead of the actual one, which is int[] in your case:

static void SumOfOddsAndEvens(int[] arr)
{
    int i = 0;
    bool isEven = true;
    int evenSum = 0;
    int oddSum = 0;

    while (i < arr.Length)
    {
        Console.WriteLine(arr[i]);

        if (isEven) evenSum += arr[i];
        if (!isEven) oddSum += arr[i];
        isEven = !isEven;
        i++;
    }

    Console.WriteLine("Sum of even slots: " + evenSum);

    Console.WriteLine("Sum of odd slots: " + oddSum);
}

Using the base-class would also allow the following, which is pure non-sense:

SumOfOddAndEvens(new[] { "Hello",  "World" };

or even this:

SumOfOddAndEvens(new object[] { 1, "World" };

Answer 2

You can't use an indexer on an Array. The Array class is a base class for all array types, and arrays are implicitly inherit from Array. But, Array itself doesn't have an indexer. Here is a demonstration of your error:

int[] numbers = new[] {1, 2, 3, 4, 5};

numbers[2] = 11; // Okay

Array arr = numbers as Array;

arr[2] = 11; // ERROR!

You are creating your parameter right ( new int[] { 1, 2, 3, 4, 6, 7, 8 } ) but in the SumOfOddsAndEvens function you are actually using Array type. ( static void SumOfOddsAndEvens(Array arr) ). Just change this last one to int[] and you will be able to use an indexer with it.