C# Return Type Issue

Question

I have a C# Class Library DLL that I call from Python. No matter what I do, Python thinks the return type is (int) I am using RGiesecke.DllExport to export the static functions in my DLL, here is an example of a function in my C# DLL:

[DllExport("Test", CallingConvention = CallingConvention.Cdecl)]   
public static float Test()
{

    return (float)1.234;
}

[DllExport("Test1", CallingConvention = CallingConvention.Cdecl)]   
public static string Test1()
{

    return "123456789";
}

If I return an (int) it works very reliably in Python. Does anybody know what's going on? This is my Python code:

    import ctypes
    import sys
    from ctypes import *

    self.driver = ctypes.cdll.LoadLibrary(self.DLL)
    a= self.driver.Test()

Eoin

Answer 1

Yes, that is correct; as explained in the ctypes documentation, it is assumed that all functions return ints. You can override this assumption by setting the restype attribute on the foreign function. Here is an example using libc (linux):

>>> import ctypes
>>> libc = ctypes.cdll.LoadLibrary("libc.so.6")
>>> libc.strtof
<_FuncPtr object at 0x7fe20504dd50>
>>> libc.strtof('1.234', None)
1962934
>>> libc.strtof.restype = ctypes.c_float
>>> libc.strtof('1.234', None)
1.2339999675750732
>>> type(libc.strtof('1.234', None))
<type 'float'>

Or a for a function that returns a C string:

>>> libc.strfry
<_FuncPtr object at 0x7f2a66f68050>
>>> libc.strfry('hello')
1727819364
>>> libc.strfry.restype = ctypes.c_char_p
>>> libc.strfry('hello')
'llheo'
>>> libc.strfry('hello')
'leohl'
>>> libc.strfry('hello')
'olehl'

This method should also work in your Windows environment.

Answer 2

Just because the datatypes are given the same name doesn't mean that they are actually the same. It sounds as it python is expecting a different structure for what it calls a "float".

This previous answer may be of use, and this answer states that a python float is a c# double; so my advice would be to try returning a double from your c# code.