int num1, num2; //前后两个数
num1 = num2 = 0;
int c = 101;

int[] i = { 1, 8, 9, 11, 12, 19, 33, 100, 101, 200 };
for (int k = 0; k < i.Length; k++)
{
    if (i[k] == c)
    {
        num1 = i[k - 1 < 0 ? k : k - 1];
        num2 = i[k + 1 == i.Length ? k : k + 1];
        break;
    }
}

------解决方案--------------------

int num1=0;
int num2=0;
int c = 100;

int[] i = { 1, 8, 9, 11, 12, 19, 33, 100, 101, 200 };
int len=i.Length;//我都叫我手下这么定义一个值，不然循环里面如果数量大的情况会变慢
for (int k = 0; k < len; k++)
{
   if (i[k] == c)
   {
       num1 = i[k - 1];
      if((k + 1)>len)
      {
        num2 =0;
      }else{
       num2=i[k +1];
      }

       break;
   }
}