C# Array – Index was outside the bounds of array on line I declare it

Apologies in advance, I know this is probably a dumb question and I’m missing something simple.

I am just getting back into programming after a 3-year hiatus, and have hopped onto Leetcode to get myself back in the groove. My experience is limited to about a year of writing Unity components etc., as a hobby.

On the Two Sum problem ( https://leetcode.com/problems/two-sum/ ), I am using the following code (please don’t judge me, I want to improve just need to get myself back into it):

public class Solution {
    public int[] TwoSum(int[] nums, int target) {
        
        int[] solutionArray = new int[2];
        int arrayLength = nums.Length;
        
        for(int i = 0; i < arrayLength; i++){
            int testCase = nums[i] + nums[i+1];
            
            if(testCase == target){
                solutionArray[0] = i;
                solutionArray[1] = i+1;
                return solutionArray;
            }
        }
        
        return null;
        
    }
}

The code seems to work when I "run code", but when I submit, I receive the following Runtime Error:

Unhandled exception. System.IndexOutOfRangeException: Index was outside the bounds of the array.
Line 4: Solution.TwoSum(Int32[] nums, Int32 target) in Solution.cs
Line 35: __Driver__.Main(String[] args) in __Driver__.cs

Line 4 of course is:

int[] solutionArray = new int[2];

I have tried googling around for the answer, but it seems that most of the Q/A’s I can find are based on someone trying to read/write to a non-existent array index.

As far as I understand, I am receiving this error immediately after declaring the array, as not even the first test-case makes it through without this runtime error.

Please help me understand what I am missing here, and again, sorry for such a noob question.

Thank you for your time!

>Solution :

public class Solution {
    public int[] TwoSum(int[] nums, int target) {
        
        int[] solutionArray = new int[2];
        int arrayLength = nums.Length;
        
        for(int i = 1; i < arrayLength; i++){
            int testCase = nums[i] + nums[i-1]; // your line of code was wrong since your trying to access an array member out of its bound.
            
            if(testCase == target){
                solutionArray[0] = i-1;
                solutionArray[1] = i;
                return solutionArray;
            }
        }
        
        return null;
        
    }
}