API Design Question

Question

I have an API defined as follows

bool IsValid()

Now, I want to modify the API so that if the return value is false, then I need additional
information why the validation failed.
I'm looking for an elegant want to solve the problem... Here are the options I have

Option 1:
bool IsValid(ref string errorMessage)

errorMessage is updated only if the result is false

option 2:

class Result<T>
{
   bool Succeeded;
   T Argument;
}

Result<string> IsValid()

Option 3:

void Validate();
//throw an exception if it invalid, just return if succeeded.

I'm not comfortable with any of the options listed above. So, wondering if there is any graceful solution that I might not be aware of

regards G

Answer 1

Are your possible error messages a fixed set? In that case, I would define them in an enumeration, then make a Validate() method that returns a value of the enum type. This has the added advantage that, if you need to show the message to the user, you can easily use localized messages.

Answer 2

If you were using a dynamically typed language you could do something like:

def IsValid():
   if condition:
       return (True, "")
   else:
       return (False, "error message")

However, I would really spring for the exception method. It's standard and can be handled at any level above the failing code.

Answer 3

I'd go for

int IsValid();

string GetErrorString(int id);

or if you can, better yet:

ValidCode IsValid();

string GetErrorString(ValidCode code);

where ValidCode would be an enum

Answer 4

If you must you could use "string isValid()" where the method would return null or empty string on success and some kind of an error message otherwise.

I would never do that personally though. Having APIs do more than one thing increases the complexity.