Cannot convert an object to a string-array in vb.net but I can in C#.net (core)

Question

I cannot cast an object to a string-array (or collection) in VB.net, I am able to do it in C#.

VB.net code:

Runtime.PythonDLL = "/usr/lib/python3.9/config-3.9-x86_64-linux-gnu/libpython3.9.so"
Using Py.GIL
    Dim APT As Object = Py.Import("apt")
    Dim Cache As Object = APT.Cache()
    Dim PyObj As Object = Cache.keys()
    Dim PKGs As String() = CType(PyObj, String())
End Using

I get the exception Unable to cast object of type 'Python.Runtime.PyObject' to type 'System.String[]'.

C#.net code which work:

Runtime.PythonDLL = "/usr/lib/python3.9/config-3.9-x86_64-linux-gnu/libpython3.9.so";
using (Py.GIL())
{
    dynamic APT = Py.Import("apt");   // Type: dynamic {Python.Runtime.PyModule}, <module 'apt' from '/usr/lib/python3/dist-packages/apt/__init__.py'>
    dynamic Cache = APT.Cache();      // Type: dynamic {Python.Runtime.PyModule}, <apt.cache.Cache object at 0x7fa701ec6760>
    dynamic PyObj = Cache.keys();     // Type: dynamic {Python.Runtime.PyModule}, ['0ad', '0ad-data',...]
    String[] PKGs = (String[])PyObj;
}

I see the same in the debugger for vb.net and C#.net.

Debugger in vb.net:

Debugger in C#.net:

I'm aware I use the dynamic type in C# and Object in VB. I do use option strict=Off in VB. As fare I know VB doesn't have dynamic but I assume you can use Object instead if strict=Off. Maybe that's the reason and it's just not possible?

I also tried to convert to:

arraylist
List(of String)
HasetSet(of String)
Dictionary(Of String, Object)

Tried both TryCast, DirectCast and CType.

Ahh....nice, look like it could be the solution :-) But I see it was introduced in .core 3.0 and I still haven't upgrade from 2.2 yet so I can't confirm yet. I did though find a method in the Python.net library which do the conversion. — MrCalvin
– MrCalvin, Commented Oct 16, 2021 at 18:26
Glad you found a way to convert to the needed type. FYI: dynamic creates a DLR object. As you discovered, this is not the same as a non-strict VB object. Hence the newer CTypeDynamic as per Jeroen's comment. But DynamicObject class has been around since .core 1.1, so I suspect there was some way to do it in VB, even without CTypeDynamic. — ToolmakerSteve
– ToolmakerSteve, Commented Jan 7, 2022 at 2:32

MrCalvin · Accepted Answer · 2021-10-16 18:33:27Z

1

Beside the solution from @Jeroen Mostert, which I prefer as it's more universal , I did find a method in the Python.net library that do the job: pyObject.As<T>()

Runtime.PythonDLL = "/usr/lib/python3.9/config-3.9-x86_64-linux-gnu/libpython3.9.so"
Using Py.GIL
   Dim APT As Object = Py.Import("apt")
   Dim Cache As Object = APT.Cache()
   Dim PyObj As PyObject = Cache.keys()
   Dim PKGs As String() = PyObj.As(Of String())() '<==========
   Console.WriteLine(PKGs(0))
End Using
PythonEngine.BeginAllowThreads() 'Must be called to release the Py.GIL thread!

answered Oct 16, 2021 at 18:33

MrCalvin

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