Finding Sum of Neighbors in 2D or Multidimensional Array (Grid)

This code snippet for Finding Sum of Neighbors in 2D or Multidimensional Array (Grid).

Good way of finding the sum of the neighboring values in a 2d array. I need to find the sum of all neighboring values at an index position.

E.g. int[,] array = new int[3, 4] { { 0, 1, 2, 3 }, { 4, 5, 6, 7 }, { 8, 9, 10, 11 } };

Here neighbors of 5.

I.e. [1.1] will be [0,0] + [0,1] + [0,2] + [1,0] + [1,2] + [2,0] + [2,1] + [2,2]

And Sum will be 40.

public class SumOfNeibours
{
int[, ] array = new int[3, 4]
{
{
0, 1, 2, 3
}, {
4, 5, 6, 7
}, {
8, 9, 10, 11
}
};
public void GetSumOfNeibours()
{
int newval = 0;
for (int i = 0; i < array.Length; i++)
{
var x = i / 4;
var y = i % 4;
var startX = x - 1 < 0 ? 0 : x - 1;
var endX = x + 1 > 2 ? 2 : x + 1;
var startY = y - 1 < 0 ? 0 : y - 1;
var endY = y + 1 > array.GetLength(0) ? array.GetLength(0) : y + 1;
//array.GetLength(0) ? array.GetLength(0) : y + 1;
for (int z = startX; z <= endX; z++)
{
for (int j = startY; j <= endY; j++)
{
if (x == z && y == j)
{
continue;
}
var v = array[z, j];
Console.WriteLine(array[z, j] + " ");
newval += array[z, j];
}
}
Console.WriteLine("\n ");
Console.WriteLine("Sum is" + newval);
newval = 0;
}
}
}