What exceptions should be thrown for invalid or unexpected parameters in NET

Question

What types of exceptions should be thrown for invalid or unexpected parameters in  NET  When would I choose one instead of another    Follow-up   Which exception would you use if you have a function expecting an integer corresponding to a month and you passed in  42   Would this fall into the  out of range  category even though it s not a collection

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argument exception    System ArgumentException System ArgumentNullException System ArgumentOutOfRangeException

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I like to use  ArgumentException  ArgumentNullException  and ArgumentOutOfRangeException    ArgumentException     Something is wrong with the argument  ArgumentNullException     Argument is null  ArgumentOutOfRangeException     I don   t use this one much  but a common use is indexing into a collection  and giving an index which is to large    There are other options  too  that do not focus so much on the argument itself  but rather judge the call as a whole    InvalidOperationException     The argument might be OK  but not in the current state of the object  Credit goes to STW  previously Yoooder   Vote his answer up as well  NotSupportedException     The arguments passed in are valid  but just not supported in this implementation  Imagine an FTP client  and you pass a command in that the client doesn   t support    The trick is to throw the exception that best expresses why the method cannot be called the way it is  Ideally  the exception should be detailed about what went wrong  why it is wrong  and how to fix it   I love when error messages point to help  documentation  or other resources  For example  Microsoft did a good first step with their KB articles  e g     Why do I receive an  Operation aborted  error message when I visit a Web page in Internet Explorer      When you encounter the error  they point you to the KB article in the error message  What they don   t do well is that they don   t tell you  why specifically it failed   Thanks to STW  ex Yoooder  again for the comments     In response to your followup  I would throw an ArgumentOutOfRangeException  Look at what MSDN says about this exception      ArgumentOutOfRangeException is thrown   when a method is invoked and at least   one of the arguments passed to the   method is not null   reference  Nothing in Visual Basic    and does not contain a valid value    So  in this case  you are passing a value  but that is not a valid value  since your range is 1   12  However  the way you document it makes it clear  what your API throws  Because although I might say ArgumentOutOfRangeException  another developer might say ArgumentException  Make it easy and document the behavior

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There is a standard ArgumentException that you could use  or you could subclass and make your own  There are several specific ArgumentException classes   http   msdn microsoft com en-us library system argumentexception VS 71  aspx  Whichever one works best

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ArgumentException      ArgumentException is thrown when a   method is invoked and at least one of   the passed arguments does not meet the   parameter specification of the called   method  All instances of   ArgumentException should carry a   meaningful error message describing   the invalid argument  as well as the   expected range of values for the   argument    A few subclasses also exist for specific types of invalidity  The link has summaries of the subtypes and when they should apply

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Depending on the actual value and what exception fits best    ArgumentException  something is wrong with the value  ArgumentNullException  the argument is null while this is not allowed  ArgumentOutOfRangeException  the argument has a value outside of the valid range     If this is not precise enough  just derive your own exception class from ArgumentException   Yoooder s answer enlightened me  An input is invalid if it is not valid at any time  while an input is unexpected if it is not valid for the current state of the system  So in the later case an InvalidOperationException is a reasonable choice

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Short answer  Neither  Longer answer  using Argument Exception  except in a library that is a product on its on  such as component library  is a smell  Exceptions are to handle exceptional situation  not bugs  and not user s  i e  API consumer  shortfalls   Longest answer  Throwing exceptions for invalid arguments is rude  unless you write a library  I prefer using assertions  for two  or more  reasons      Assertions don t need to be tested  while throw assertions do  and test against ArgumentNullException looks ridiculous  try it     Assertions better communicate the intended use of the unit  and is closer to being executable documentation than a class behavior specification    You can change behavior of assertion violation  For example in debug compilation a message box is fine  so that your QA will hit you with it right away  you also get your IDE breaking on the line where it happens   while in unit test you can indicate assertion failure as a test failure    Here is what handling of null exception looks like  being sarcastic  obviously    try       library Method null     catch  ArgumentNullException e           retry with real argument this time     library Method realArgument       Exceptions shall be used when situation is expected but exceptional  things happen that are outside of consumer s control  such as IO failure   Argument Exception is an indication of a bug and shall be  my opinion  handled with tests and assisted with Debug Assert  BTW  In this particular case  you could have used Month type  instead of int  C  falls short when it comes to type safety  Aspect  rulez   but sometimes you can prevent  or catch at compile time  those bugs all together   And yes  MicroSoft is wrong about that

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I voted for Josh s answer  but would like to add one more to the list   System InvalidOperationException should be thrown if the argument is valid  but the object is in a state where the argument shouldn t be used   Update Taken from MSDN       InvalidOperationException is used in   cases when the failure to invoke a   method is caused by reasons other than   invalid arguments    Let s say that your object has a PerformAction enmSomeAction action  method  valid enmSomeActions are Open and Close   If you call PerformAction enmSomeAction Open  twice in a row then the second call should throw the InvalidOperationException  since the arugment was valid  but not for the current state of the control   Since you re already doing the right thing by programming defensively I have one other exception to mention is ObjectDisposedException   If your object implements IDisposable then you should always have a class variable tracking the disposed state  if your object has been disposed and a method gets called on it you should raise the ObjectDisposedException   public void SomeMethod         If  m Disposed              throw new ObjectDisposedException  Object has been disposed                     Normal execution code     Update  To answer your follow-up  It is a bit of an ambiguous situation  and is made a little more complicated by a generic  not in the  NET Generics sense  data type being used to represent a specific set of data  an enum or other strongly typed object would be a more ideal fit--but we don t always have that control   I would personally lean towards the ArgumentOutOfRangeException and provide a message that indicates the valid values are 1-12   My reasoning is that when you talk about months  assuming all integer representations of months are valid  then you are expecting a value in the range of 1-12   If only certain months  like months that had 31 days  were valid then you would not be dealing with a Range per-se and I would throw a generic ArgumentException that indicated the valid values  and I would also document them in the method s comments

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