Derived exceptions not caught by C#

[slozier]

For example:

class Test(SystemExit): pass

raise Test

does not terminate the process. The C# code expects a SystemExitException but the exception ends up being of type Exception.

Also, an IndexError (which becomes an IndexOutOfRangeException) would not be caught by LookupException handler in C# even though LookupError is a base of IndexError. For example:

try {
    PythonCalls.Call(context, somethingthatthrowsindexerror);
} catch (LookupException e) {
    // do stuff
}

would not catch the exception.

This came up in https://github.com/IronLanguages/ironpython3/pull/1011.

[orent]

Probably related: raising a subclass of SystemExit displays a stack trace rather than exiting silently.

Found this in docopt module. It raises DocoptExit which is a subclasss of SystemExit.

[BCSharp]

Apparently, by design, any user-defined exception is a subclass of System.Exception, as per documentation. My guess is that this is to allow multiple inheritance.

A working way of catching Python exceptions in C# including their subclasses is with a when clause:

try {
    PythonCalls.Call(context, somethingthatthrowsindexerror);
} catch (Exception ex) when (PythonOps.IsInstance(ex.GetPythonException(), PythonExceptions.LookupError)) {
    // do stuff
}

Note: the methods used are currently internal. An idea: define a public helper method checking exception subclasses.

[slozier]

Apparently, by design, any user-defined exception is a subclass of System.Exception, as per documentation. My guess is that this is to allow multiple inheritance.

Hmm, still seems like it would be cleaner to have it be a subclass of the more specific .NET class (e.g. SystemExitException).

Note: the methods used are currently internal. An idea: define a public helper method checking exception subclasses.

As you know my preference is to have everything be internal unless there's a specific reason (or request) for it to be public. A helper method could be interesting (and possibly more efficient than an IsInstance call).

Anyway, at the very least we should special case SystemExit since we know it's an issue...

[slozier]

I guess multiple inheritance on user types makes it complicated to have something other than System.Exception as the .NET exception type:

class MyError(ValueError, AttributeError): pass