[C#] 用 checked 來避免溢位

當進行強制轉換時，容易因為長短不同而造成溢位（overflow）

// Bad 
short shortNum;
int i = 32768;
shortNum = (short) i; 
// problem after statment excution
// the shortNum variable has an uninitialized value,


// Good
try {
  shortNum = checked((short)i); // solution 
}
catch(OverflowException efx) {}