RuntimeBinderException - Cannot implicitly convert type 'Python.Runtime.PyObject' to 'System.Collections.Generic.List<string>'

Environment

Pythonnet version: 2.3.0
Python version: 3.6 (Anaconda 32-bit)
Operating System: Windows 10

Details

I have the following code:

Interface1.cs

using System.Collections.Generic;

public interface Interface1
{
    string Test();

    List<string> GetList();

    List<MyType> GetListOfMyType();
}

public class MyType
{
}

Class1.cs

using System.Collections.Generic;

namespace Python.EmbeddingTest
{
    public class Class1 : Interface1
    {
        public string Test()
        {
            return "test";
        }

        public List<string> GetList()
        {
            return new List<string>() { "testing" };
        }

        public List<MyType> GetListOfMyType()
        {
            return new List<MyType>() { new MyType(), new MyType() };
        }
    }
}

Module1.py

import clr

clr.AddReference("Python.EmbeddingTest")
from Python.EmbeddingTest import *

class Class2(Class1):

    def Test(self):
        return "Test from Class2"
    
    def Method1(self):
        return "Method1"

Test

I am trying to do something like what the following test is trying to do:

[Test]
public void TestGenericListMarshalling()
{
    var scope = Py.Import("module1");
    var attr = scope.GetAttr("Class2");
    dynamic c2 = attr.Invoke();
    List<string> ls = c2.GetList();
}

Exception

However, I get the following exception:

Microsoft.CSharp.RuntimeBinder.RuntimeBinderException occurred
  HResult=0x80131500
  Message=Cannot implicitly convert type 'Python.Runtime.PyObject' to 'System.Collections.Generic.List<string>'
  Source=Python.EmbeddingTest
  StackTrace:
   at Python.EmbeddingTest.PyImportTest.TestGenericListMarshalling() in D:\Users\Tom\Dropbox\Algo Trading\pythonnet\src\embed_tests\pyimport.cs:line 90

Is it possible to marshall generic .NET Lists?

@tomhunter-gh

@tomhunter-gh can you try from the master?

pip install git+https://github.com/pythonnet/pythonnet -U --force

This should have been fixed recently:
#427

Hi @denfromufa

I pulled latest from master but I'm afraid I still get the problem. It looks like casting to a string[] array is successful, but the C# base class (and interface) are defined with a List<string> so I was expecting to be able to cast the dynamic back to a List<string>..

Were you able to reproduce the issue? What I did was I added the Interface1.cs, Class1.cs, Module.py to the Python.EmbeddingTest project. (By the way, in that project, any reason the 'fixtures' folder is not included in the project file?)

Thanks

@tomhunter-gh

@tomhunter-gh can you share your full project in a zip file? just paste it here to this issue.

fixtures are not included because it is python code, but could be included as a resource. we have a separate pyproj in the directory with python tests.

https://www.dropbox.com/s/0u4rbu5s2lr24b0/pythonnet.zip?dl=0

By the way I'm using Python 3.6 32-bit, so you may need to change settings in the Python.Runtime project file.

My test is in: pythonnet\src\embed_tests\pyimport.cs

Thanks

@tomhunter-gh

@tomhunter-gh actually that pull request which I linked above #427 leads to this issue :(

Here is the code that I commented out to pass your the test:

            /*if (value is IList && value.GetType().IsGenericType)
            {
                using (var resultlist = new PyList())
                {
                    foreach (object o in (IEnumerable)value)
                    {
                        using (var p = new PyObject(ToPython(o, o?.GetType())))
                        {
                            resultlist.Append(p);
                        }
                    }
                    Runtime.XIncref(resultlist.Handle);
                    return resultlist.Handle;
                }
            }*/

The reason is because before the list is passed back to C#, it gets converted to Python list.

So quick workaround would be to provide equivalent conversion from Python to C#: list -> List.

But this is of course a lot of conversion back and forth and maybe a performance issue.

This is rather hackish workaround, but passes all tests! Use at your own risk :)

pythonnet_hackish.zip

Thanks @denfromufa - that has fixed the issue!

Here's the change in case anyone else needs it:

(You also need to add a reference to Microsoft.CSharp.)

Thanks

related issue:

#514

I believe this should already be fixed in 3.0, but we need a test.

added a regression test for pythonnet#451

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RuntimeBinderException - Cannot implicitly convert type 'Python.Runtime.PyObject' to 'System.Collections.Generic.List<string>' #451

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Interface1.cs

Class1.cs

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RuntimeBinderException - Cannot implicitly convert type 'Python.Runtime.PyObject' to 'System.Collections.Generic.List<string>' #451

Description

Environment

Details

Interface1.cs

Class1.cs

Module1.py

Test

Exception

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den-run-ai commented on Apr 5, 2017

tomhunter-gh commented on Apr 6, 2017

den-run-ai commented on Apr 6, 2017

tomhunter-gh commented on Apr 6, 2017

tomhunter-gh commented on Apr 6, 2017

den-run-ai commented on Apr 7, 2017

den-run-ai commented on Apr 7, 2017

tomhunter-gh commented on Apr 7, 2017

den-run-ai commented on Jul 20, 2017

lostmsu commented on Sep 23, 2021

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