Daniel Torramilans Daniel Torramilans - 1 month ago 9
Python Question

Using ctypes in python to acces a C# dll's methods

I would like to implement C# code in a critical part of my python program to make it faster. It says (on Python documentation and this site) that you can load a Dynamic Link Library (and so say the PyDocs) as follows:

cdll.LoadLibrary("your-dll-goes-here.dll")


This is the part of my code that takes care of this feature:

from ctypes import *
z = [0.0,0.0]
c = [LEFT+x*(RIGHT-LEFT)/self.size, UP+y*(DOWN-UP)/self.size]
M = 2.0

iterator = cdll.LoadLibrary("RECERCATOOLS.dll")

array_result = iterator.Program.ITERATE(z[0],z[1],c[0],c[1],self.iterations,M)

z = complex(array_result[0],array_result[1])
c = complex(array_result[2],array_result[3])
last_iteration = int(round(array_result[4]))


And the RECERCATOOLS.dll that I use is this (C# code, not C or C++):

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using KarlsTools;

public class Program
{
public static Array ITERATE(double z_r,double z_i,double c_r,
double c_i, int iterations,
double limit)
{
Complex z = new Complex(z_r, z_i);
Complex c = new Complex(c_r, c_i);

for (double i = 1; Math.Round(i) <= iterations; i++)
{
z = Complex.Pow(z, 2) + c;
if (Complex.Abs(z) < limit)
{
double[] numbers = new double[] { Complex.Real(z),
Complex.Imag(z),
Complex.Real(c),
Complex.Imag(c),
i};
return numbers;
}
}
double iter = iterations;
double[] result = new double[] { Complex.Real(z),
Complex.Imag(z),
Complex.Real(c),
Complex.Imag(c),
iter};
return result;
}
}


To build this DLL I use "Build" command over the Visual Studio 2010 project, which only contains this file and a reference to "Karlstools", a module that allows me to use complex numbers.

I don't know why but when I try to run my Python code, it just throws an exception:

[...]
array_result = iterator.Program.ITERATE(z[0],z[1],c[0],c[1],self.iterations,M)
File "C:\Python32\lib\ctypes\__init__.py", line 353, in __getattr__
func = self.__getitem__(name)
File "C:\Python32\lib\ctypes\__init__.py", line 358, in __getitem__
func = self._FuncPtr((name_or_ordinal, self))
AttributeError: function 'Program' not found


I need help with this, since it keeps throwing me exceptions even with everything is set to
public
and the function as
static
, or even when if I try to access the function directly without specifying the "Program" class... I have no clue where the problem could be.

Answer

The tips you'll find regarding calling DLL from Python using ctypes rely most of the time to the DLL being written in C or C++, not C#. With C# you pull in the whole machinery of the CLR, and the symbols are most likely mangled and not what ctypes expect, and you'll get all sorts of troubles from the garbage collection of your output array.

I've had very good success when interfacing python and C# code by using python for dot net (http://pythonnet.sf.net), you may want to try this.

On the other hand if you are in for pure performance, consider rewriting your code as a native C extension for Python using the Python/C API (http://docs.python.org/c-api/).

Comments