Python for .NET "unable to find assembly" error

Question

I'm using CPython and I have a C# dll. I'm trying to use Python for .NET to make them talk. I can't use IronPython because I need to integrate this into an existing CPython system.

I'm completely new to Python for .NET, and I actually have very little experience with Python and no experience with C#. So please forgive me if my question seems very basic.

I'm using Python 2.7.3, and I downloaded pythonnet-2.0-alpha2-clr2.0_131_py27_UCS2 and unzipped it into a folder named pyfornet_test, which also contains the dll I'm trying to use (called DotNet4Class.dll)

Then I run this:

import sys

import os

import clr

sys.path.append(r"C:\pyfornet_test")

clr.AddReference("DotNet4Class.dll")

Which gives me this error:

System.IO.FileNotFoundException: Unable to find assembly 'DotNet4Class.dll'.
   at Python.Runtime.CLRModule.AddReference(String name) in C:\Users\Barton\Documents\Visual Studio 2008\Projects\PyShar
p\trunk\pythonnet\src\runtime\moduleobject.cs:line 375

Any advice would be much appreciated. Thank you!

Answer 1

Is DotNet4Class.dll built against .NET 4? I assume so based on the naming of the dll.

Note the issue here: http://sourceforge.net/tracker/?func=detail&aid=3293169&group_id=162464&atid=823891

clr.AddReference fails when assembly is built with .NET 4.0 - ID: 3293169

I'd read the solution, but essentially, you need to rebuild and recompile the python for .NET project under .NET 4.

I'll also mention that projects like this, that aren't actively developed and used by lots of people, generally have subtle idiosyncrasies that make knowledge of the platform essential to work around problems such as this. It sounds like you're trying to hack this solution in without understanding much about python or .NET which is always going to be fraught with problems.

Answer 2

One reason can be Windows was not enabling it to load from "external sources". To fix this:

• Right-click on the .dll
• "Properties"
• Under "General", click "Unblock"

Answer 3

Try this (without extension .dll):

clr.AddReference(r"C:\pyfornet_test\DotNet4Class")

Answer 4

Did you try clr.FindAssembly?

import clr
import sys
assemblydir = r"C:\pyfornet_test"
assemblypath = r"C:\pyfornet_test\DotNet4Class.dll"
sys.path.append(assemblydir)
clr.FindAssembly(assemblypath)

I don't know why it works, but this code works on my computer (Python 2.7, .NET4)

Answer 5

I have code like this (I copied MyRightClickMenuService.dll to the same directory as my script.py). It is built against .Net 4.0.

# script.py
import clr
import os
import sys
sys.path.append(os.path.dirname(__file__))

clr.AddReference('MyRightClickMenuService')
clr.AddReference('System')
clr.AddReference('System.Security')

from MyRightClickMenuService import (
    AclSecuredNamedPipeBinding,
    MyMenuItem,
    MyContextMenuService,
    etc
)

Answer 6

Checklist

1. The folder(s) containing the DLL(s) is/are added to sys.path before loading. You may append, or sys.path.insert(0, dll_folder) to put it first on the list.
2. You call clr.AddReference('my_dll') without the dll extension (for my_dll.dll), after adding the folder to sys.path
3. The DLL Target Architecture is the same as the CPython version bitness. That is, if Architecture is x64, use 64-bit python, and if Architecture is x86, use 32-bit python. (instructions for this below)

How to check target Architecture for DLL?

• I Used ILSpy (free and open source) -> Open DLL -> Check the output. Below example output.

Answer 7

What worked for me was to Unblock the dll file.

if u download the dll file or took it from different computer it might be blocked. So unblocked solved the issue for me.

To unblock right click on the properties if the dll file and check the Unblock box at the bottom