Maximum number in Array of Random Numbers

private static Random random = new Random();
public static int MaxArrayRandomNumber(this int[] array)
{
    int maxnumber = array[0];
    if (array is null) { return  0; }
    else
    {
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = random.Next();
        }

        for (int i = 0; i < array.Length; i++)
        {
            maxnumber = Math.Max(maxnumber, array[i]);
        }
    }
    return maxnumber;
    
}

int[] arr = new int [5];
Console.WriteLine(arr.MaxArrayRandomNumber());