[IronPython] Expose a C# function as a Python function taking a default argument

Dino Viehland dinov at exchange.microsoft.com
Mon Nov 19 18:37:20 CET 2007

You can use either the [Optional] attribute or the [DefaultParameterValue(null)] attributes on the parameter in the C# signature to mark the parameter as being optional.  And if you're using a language that's more friendly to default/optional attributes (e.g. VB.NET) you could just use their native syntax which will generate one of those attributes.

The interesting difference between these is I believe if you're typed to object and do [Optional] you get Missing.Value instead of null so you can disambiguate between the two.

-----Original Message-----
From: users-bounces at lists.ironpython.com [mailto:users-bounces at lists.ironpython.com] On Behalf Of Jeff Hardy
Sent: Sunday, November 18, 2007 12:31 PM
To: Discussion of IronPython
Subject: [IronPython] Expose a C# function as a Python function taking a default argument

Hi all,
I have a function declared in C#:
void StartResponse(string status, List response_headers, Tuple exc_info);

I need to pass this to a Python function, and the Python function will
call it. Currently, the code looks like this:
object responsesObj = engine.CallObject(callable, module, environ, new

This works fine if all three arguments are passed. However, from the
Python side start_response has this signature:
def start_response(status, response_headers, exc_info=None)

So my question is: How do I expose the C# function as a Python
function with a default argument? It looks like
Runtime.Calls.PythonFunction is what I want, but I couldn't figure out
how to call it - I tried tracing the code and ended up in the bowels
of the DLR.

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