Downgrade arity error to warning in some cases for ContiguousCollectionMarshaller

[ThadHouse]

I wanted to write a custom string marshaller that was generic across multiple of our libraries. They all have their own free() functions, so I wanted to inject a type to allow me to pass extra data to the marshaller.

[CustomMarshaller(typeof(string), MarshalMode.ManagedToUnmanagedOut, typeof(LengthStringMarshaller<,>))]
[ContiguousCollectionMarshaller]
public static unsafe class LengthStringMarshaller<TFree, TUnmanagedElement> where TUnmanagedElement : unmanaged
                                                                            where TFree : ILengthStringFree<TUnmanagedElement>
{
    public static string AllocateContainerForManagedElements(TUnmanagedElement* unmanaged, int numElements)
    {
        if (unmanaged is null)
            return "";

        if (typeof(TUnmanagedElement) != typeof(byte))
        {
            throw new InvalidOperationException("TUnmanagedElement must be byte");
        }

        string ret = Encoding.UTF8.GetString((byte*)unmanaged, numElements);
        TFree.FreeString(unmanaged, numElements);
        return ret;
    }
   ...
}

I don't control either managed (string) or unmanaged (byte), so I can't attach the free function to either of those types using static interfaces. But this doesn't work, because the marshaller expects the generic arity to be N + 1 exactly. It expects that because NativeMarshallingAttribute requires N + 1

Talking to @jkoritzinsky, this could be a case where we could downgrade that error to a warning assuming the managed type is non generic, and again downgrade from warning to nothing if the managed type is defined in another assembly.

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Issue Details

---

I wanted to write a custom string marshaller that was generic across multiple of our libraries. They all have their own free() functions, so I wanted to inject a type to allow me to pass extra data to the marshaller.

[CustomMarshaller(typeof(string), MarshalMode.ManagedToUnmanagedOut, typeof(LengthStringMarshaller<,>))]
[ContiguousCollectionMarshaller]
public static unsafe class LengthStringMarshaller<TFree, TUnmanagedElement> where TUnmanagedElement : unmanaged
                                                                            where TFree : ILengthStringFree<TUnmanagedElement>
{
    public static string AllocateContainerForManagedElements(TUnmanagedElement* unmanaged, int numElements)
    {
        if (unmanaged is null)
            return "";

        if (typeof(TUnmanagedElement) != typeof(byte))
        {
            throw new InvalidOperationException("TUnmanagedElement must be byte");
        }

        string ret = Encoding.UTF8.GetString((byte*)unmanaged, numElements);
        TFree.FreeString(unmanaged, numElements);
        return ret;
    }
   ...
}

I don't control either managed (string) or unmanaged (byte), so I can't attach the free function to either of those types using static interfaces. But this doesn't work, because the marshaller expects the generic arity to be N + 1 exactly. It expects that because NativeMarshallingAttribute requires N + 1

Talking to @jkoritzinsky, this could be a case where we could downgrade that error to a warning assuming the managed type is non generic, and again downgrade from warning to nothing if the managed type is defined in another assembly.

Author:	ThadHouse
Assignees:	-
Labels:	area-System.Runtime.InteropServices, untriaged
Milestone:	-