InvalidCastException when calling FirstOrDefault on a PythonList in .NET 6 #812

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1. slozier

   added

   .NET Core

   Port to ipy3

   on Mar 10, 2022
2. Simon900225 commented on Dec 13, 2022

   

   Simon900225

   on Dec 13, 2022

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   Hi. I'd like to get this issue resolved in some way. Is there a way for my company to sponsor this? Or could I get some input on how I can troubleshoot, find and fix this issue myself?
3. slozier commented on Dec 14, 2022

   

   slozier

   on Dec 14, 2022

   Last edited by slozier

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   .NET 6 added an overload FirstOrDefault<TSource>(IEnumerable<TSource>, TSource) so, other than being explicit about your type, I'm not sure there's a whole lot you can do for this one:

   import clr
   import System
   clr.AddReference("System.Linq")
   clr.ImportExtensions(System.Linq)
   [0].FirstOrDefault[System.Int32](lambda x: x == 2)
4. Simon900225 commented on Dec 8, 2023

   

   Simon900225

   on Dec 8, 2023

   Last edited by Simon900225

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   I'm back in the project where I had this issue after a years break.
   Being explicit works, but we have a lot of code in different places so I'd have to create a regex that adds [System.Object] after every FirstOrDefault (and similar calls) to be sure that it works for every customer. The thing is I don't fully understand why the new overload should mess this up? If I run the new overload
   range(1,10).FirstOrDefault(lambda x: x == 20, 1) there is no issue.

   I have now downloaded the source of IronPython and managed to compile it so now I'm able to test a bit more. The thing is that I haven't manged to find where the FuncCallInstruction is generated where the return type is set to PythonFunction.

   The difference of the FuncCallInstruction between .NET 5 and .NET 8 looks like this
   {Call(PythonFunction FirstOrDefault[PythonFunction](System.Collections.Generic.IEnumerable1[IronPython.Runtime.PythonFunction], IronPython.Runtime.PythonFunction))}
{Call(System.Object FirstOrDefault[Object](System.Collections.Generic.IEnumerable1[System.Object], System.Func2[System.Object,System.Boolean]))}
5. Simon900225 commented on Dec 13, 2023

   

   Simon900225

   on Dec 13, 2023

   Last edited by Simon900225

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   Another error found both when using the implicit and the explicitly typed call
   List[Object]().FirstOrDefault[System.Object](lambda x: x == 10)
List[Object]().FirstOrDefault(lambda x: x == 10)
   returns
   <function <lambda$29> at 0x0000000000000033> when it should return None

   This also stopped working from .NET 6 and forward.
   I think this is caused by the same error. It seems like it returns the lambda function and not the result of the lambda function. If you add the default value parameter as None it works but it should understand that implicitly.