C#: in and ref parameters misinterpreted as out parameters #2630

nikitagrgv · 2025-10-11T10:41:39Z

nikitagrgv
Oct 11, 2025

Hi, I'm trying to use pythonnet with a large C# codebase to expose it to python. I'm running into issues with methods that take in parameters.

Methods that take in or ref parameters return tuples, as if they were out parameters. Overloaded operators that use in parameters cause exceptions.

Here's a minimal repro:

using Python.Runtime;

namespace testpy;

public class MyClass(float value)
{
	private float _value = value;

	public static MyClass operator +(in MyClass v0, in MyClass v1)
	{
		return new MyClass(v0._value + v1._value);
	}

	public MyClass MyFunc(in MyClass other)
	{
		return new MyClass(_value + other._value);
	}
}

class Program
{
	static void Main(string[] args)
	{
		Runtime.PythonDLL = "python311.dll";
		PythonEngine.Initialize();

		using (Py.GIL())
		{
			dynamic py = Py.CreateScope();

			py.Set("m1", new MyClass(1).ToPython());
			py.Set("m2", new MyClass(2).ToPython());

			string code = @"
v1 = m1.MyFunc(m2)
print(v1) # prints (<testpy.MyClass object at 0x00000000068FB780>, <testpy.MyClass object at 0x00000000068FB7C0>)
v2 = m1 + m2 # exception
";
			py.Exec(code);
		}
	}
}

MyFunc returns a tuple containing the original return value and its argument.
m1 + m2 causes the following exception:

Python.Runtime.PythonException: <bound method 'op_Addition'> returned a result with an exception set
 ---> Python.Runtime.PythonException: tuple assignment index out of range
   --- End of inner exception stack trace ---
  File "<string>", line 4, in <module>
   at Python.Runtime.PythonException.ThrowLastAsClrException()
   at Python.Runtime.PythonException.ThrowIfIsNull(NewReference& ob)
   at Python.Runtime.PyModule.Exec(String code, BorrowedReference _globals, BorrowedReference _locals)
   at Python.Runtime.PyModule.Exec(String code, PyDict locals)
   at CallSite.Target(Closure, CallSite, Object, String)
   at System.Dynamic.UpdateDelegates.UpdateAndExecuteVoid2[T0,T1](CallSite site, T0 arg0, T1 arg1)
   at testpy.Program.Main(String[] args) in C:\Users\User\RiderProjects\testpy\testpy\Program.cs:line 39

As I understand it, this happens because in and out are C# features rather than dotnet, so pythonnet doesn’t care about it. But what can I do to fix this without rewriting any existing code?

NOTE: I see in debug that this code doesn't care about in, because binding.outs includes in parameters.

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C#: in and ref parameters misinterpreted as out parameters #2630

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C#: in and ref parameters misinterpreted as out parameters #2630

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nikitagrgv Oct 11, 2025

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nikitagrgv
Oct 11, 2025