如果无法发生异常，是否应该引发并处理异常？

Question

I have a function that takes three parameters (day, month & year) and creates a new dateTime. It's a public function that gets called when any of three comboboxes are dropped down.

When unit testing I accidentally entered an invalid value and it threw a invalid date time exception, but this won't be possible in the application as the comboboxes are pre-populated with valid values only.

So question is should I still check and handle this exception in the function?

Answer 1

In general, yes, any public function could be called from anywhere and it is good practice to defend your code from invalid inputs also if, at certain point in time, you are sure about who feeds the inputs to the function.

However, this supposed function could handle the impossible situations by itself without triggering an exception if the inputs are not good.

It is relatively easy to check the inputs and follow the well known pattern of TryParse

 public bool TryMakeDateTime(int year, int month, int day, out DateTime date)
 {
      date = DateTime.MinValue;

      if(!IsValidDay(year, month, day))
        return false;

      date = new DateTime(year, month, day);
      return true;
 }

 public bool IsValidDay(int year, int month, int day)
 {
     if(day < 1 || day > 31)
        return false;

     if(month < 1 || month > 12)
        return false;

     if(day > 30 && (month == 2 ||
                     month == 4 || 
                     month == 6 || 
                     month == 9 || 
                     month == 11))
        return false;

     // This is arbitrary, adjust the check to your constraints
     if(year < 1900 || year > 2099)
         return false;

     if(month == 2)
     {
         // IsLeapYear cannot handle values below 1 or higher than 9999
         // but we have already checked the year with more retrictive
         // constraints.
         int extraDay = (DateTime.IsLeapYear(year) ? 1 : 0);
         if(day > (28 + extraDay))
             return false;
     }
     return true;
 }

Answer 2

Yes, the function should throw an exception for invalid inputs, if the function, in isolation, allows invalid inputs to be submitted. You don't know how, or from where, a future developer might call this function. But another, better option is to code the function so that only valid inputs are allowed.

You can do this by changing the type of the inputs from integer values to Enums. Create a Month Enum

public enum CalendarMonth {
  NotSet = 0, January = 1, February = 2,
  March = 3, April = 4, May = 5, June = 6,
  July = 7, August = 8, September = 9,
  October = 10, November = 11, December = 12}

and a DayOfMonth Enum

public enum DayOfMonth {
  NotSet = 0, dom1 = 1, dom2 = 2, ....etc., ... dom31 = 31 }

You could code the function to treat the 31st of months with only 30 days in them as the first of the next month, and Feb 29, 30 and 31 as March 1,2,3, etc., to avoid treating this as invalid. Then your function's signature would be

public DateTime NewDate(DayOfMonth dom, CalendarMonth month, int year);

and it would not be possible to pass it invalid values. (except I guess for year values outside the DateIme.MinDate to DateTime.MaxDate range)

Answer 3

You should not prevent or catch the exception. But you should make sure an exception will really happen in that "impossible" case.

Exceptions are meant for "impossible" situations that are thought not to occur "normally".

If for example you call a DateTime constructor overload, that constructor will already throw an exception if the input is invalid. If you reason that will not happen in your situation, do not handle that case. The exception message generated by the framework is just fine.