`Marshal.PtrToStructure<>()` Doesn't work for inherited fields

[ThaDaVos]

Description

Taking below declarations, when using Marshal.PtrToStructure<>(), the fields from the base record aren't filled, adding to that, the StructLayoutAttribute is not inherited (no problem, just stating), throwing an error that the size could not be determined (giving a hint it doesn't see the fields) - I also tried setting the fields to protected and public/internal:

[StructLayout(LayoutKind.Sequential, Pack = 1, CharSet = CharSet.Ansi)]
public record A<T> where T : struct
{
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
    private string name;
    public string Name => name;

    private T value;
    public T Value => value;
}

public struct BValues
{
    public int x;
    public int y;
    public int z;
}

[StructLayout(LayoutKind.Sequential, Pack = 1, CharSet = CharSet.Ansi)]
public record B : A<BValues>
{

}

Reproduction Steps

Please see the given code snippet above

Expected behavior

I expected the fields from the base class to be marshalled/filled first, or at least, filled at all

Actual behavior

They stay empty

Regression?

No response

Known Workarounds

Changing the fields to be private, making the properties virtual and re-implementing the logic - defeating the purpose of the base class except methods - so moving the field/properties to each subrecord

Configuration

• Which version of .NET is the code running on?
  Dotnet Version: 9.0.104
• What OS and version, and what distro if applicable?
  Windows 11 24h2
• What is the architecture (x64, x86, ARM, ARM64)?
  x86-windows
• Do you know whether it is specific to that configuration?
  Not sure, it's a interop project for Clarion with NativeAoT
• If you're using Blazor, which web browser(s) do you see this issue in?
  No

Other information

No response

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[AaronRobinsonMSFT]

@ThaDaVos Thanks for letting us know about this. Can you try marking the record as a record struct instead?

@jkoritzinsky Could this be an issue with marshalling generic types when T is blittable? I thought we had originally blocked marshalling any generic type, but changed that at some point.

I expected the fields from the base class to be marshalled/filled first, or at least, filled at all

The interop team no longer advocates for marshalling through class inheritance. It is still an option and something we continue to support, but it is definitely a place where issues still lurk. It is unlikely to be fixed as opposed to blocked since we know there are bugs we can't easily fix. For the current scenario, removing the record, can you construct an example of this scenario that works on .NET Framework? If so, then we would fix it, otherwise we are likely to down the "block it" path.

[AaronRobinsonMSFT]

Oh shoot, record B can be defined as is if it is a record struct. Hmmmm. Yeah I think this is something we should get to the bottom of and then decide the cost to support it if it is new behavior compared to .NET Framework.

[AaronRobinsonMSFT]

Looks like this is the same behavior in both .NET 9 and .NET Framework. I switch from class to record so the same code would compile on both - see repro below.

I think we should explicitly block the scenario that should not work, which is using inheritance to obscure the failure about the generic type.

Output

...\Repro>dotnet run --framework net472
System.ArgumentException: The specified Type must not be a generic type definition.
Parameter name: structureType
   at System.Runtime.InteropServices.Marshal.PtrToStructure(IntPtr ptr, Type structureType)
   at System.Runtime.InteropServices.Marshal.PtrToStructure[T](IntPtr ptr)
   at Program.Main(String[] args) in ...\Repro\Program.cs:line 37
B structure successfully marshaled.

...\Repro>dotnet run --framework net9.0
System.ArgumentException: The specified Type must not be a generic type. (Parameter 'T')
   at System.Runtime.InteropServices.Marshal.PtrToStructure[T](IntPtr ptr)
   at Program.Main(String[] args) in ...\Repro\Program.cs:line 37
B structure successfully marshaled.

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFrameworks>net9.0;net472</TargetFrameworks>
  </PropertyGroup>

</Project>

using System;
using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Sequential, Pack = 1, CharSet = CharSet.Ansi)]
class A<T> where T : struct
{
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
    private string name;
    public string Name => name;

    private T value;
    public T Value => value;
}

struct BValues
{
    public int x;
    public int y;
    public int z;
}

[StructLayout(LayoutKind.Sequential, Pack = 1, CharSet = CharSet.Ansi)]
class B : A<BValues>
{

}

public class Program
{
    public static void Main(string[] args)
    {
        // Allocate memory for the structure
        IntPtr mem = Marshal.AllocCoTaskMem(0x1000);

        try
        {
            var result = Marshal.PtrToStructure<A<BValues>>(mem);
            Console.WriteLine("A<BValues> structure successfully marshaled.");
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex);
        }
        try
        {
            var result = Marshal.PtrToStructure<B>(mem);
            Console.WriteLine("B structure successfully marshaled.");
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex);
        }

        // Free the allocated memory
        Marshal.FreeCoTaskMem(mem);
    }
}

[ThaDaVos]

So it's the Generic causing the issue?
If so, if I remove the generic and just add the field itself to B with the type I would use as Generic, then it should work as expected?

The reason is, I am - in the actual code - trying to deduplicate code, in this case, I've got an Account struct with Parameters per platform which is supported by the library - the parameters are unique per platform, so I want to prevent redeclaring the common fields like Id, Name and Platform each time - I also looked at using something instead of the Parameters struct which allows having keys/values of different kinds, but couldn't find something which works with P/Invoke and Marshalling - which also exists on the side of Clarion (which is pretty limited in regards to types)