public void Reverse(int index, int count) {
            if (index < 0) {
                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
            }

            if (count < 0) {
                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.count, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
            }

            if (_size - index < count)
                ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidOffLen);
            Contract.EndContractBlock();
            Array.Reverse(_items, index, count);
            _version++;
        }

内部调用的是array的Array.Reverse(_items, index, count);

https://referencesource.microsoft.com/#mscorlib/system/array.cs,f5edebc3412b666b

        public static void Reverse(Array array, int index, int length) {
            if (array==null) 
                throw new ArgumentNullException("array");
            if (index < array.GetLowerBound(0) || length < 0)
                throw new ArgumentOutOfRangeException((index<0 ? "index" : "length"), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
            if (array.Length - (index - array.GetLowerBound(0)) < length)
                throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
            if (array.Rank != 1)
                throw new RankException(Environment.GetResourceString("Rank_MultiDimNotSupported"));
            Contract.EndContractBlock();

            bool r = TrySZReverse(array, index, length);
            if (r)
                return;

            int i = index;
            int j = index + length - 1;
            Object[] objArray = array as Object[];
            if (objArray!=null) {
                while (i < j) {
                    Object temp = objArray[i];
                    objArray[i] = objArray[j];
                    objArray[j] = temp;
                    i++;
                    j--;
                }
            }
            else {
                while (i < j) {
                    Object temp = array.GetValue(i);
                    array.SetValue(array.GetValue(j), i);
                    array.SetValue(temp, j);
                    i++;
                    j--;
                }
            }
        }