Access violation when calling ID3D12DescriptorHeap.GetCPUDescriptorHandleForHeapStart

[ghord]

Actual behavior

I'm getting access violation error when calling ID3D12DescriptorHeap.GetCPUDescriptorHandleForHeapStart method. I've destilled my code to short repro. It might be my error, but C++ version looks to be working without crashing.

Expected behavior

Code working correctly

Repro steps

1. NativeMethods.txt content:

D3D12CreateDevice
ID3D12Device
ID3D12DescriptorHeap

2. NativeMethods.json content (if present):

{
  "$schmea": "https://aka.ms/CsWin32.schema.json",
  "allowMarshaling": false,
}

NOTE: I've tried different settings and could not manage to get it to work without access violation. I've also tried version with and without marshalling.

3. Any of your own code that should be shared?

    static unsafe void Main()
    {
        void* result;

        var uuid_ID3D12Device = typeof(ID3D12Device).GUID;
        PInvoke.D3D12CreateDevice(null, Windows.Win32.Graphics.Direct3D11.D3D_FEATURE_LEVEL.D3D_FEATURE_LEVEL_12_0, &uuid_ID3D12Device, &result);

        var device = (ID3D12Device*)result;

        D3D12_DESCRIPTOR_HEAP_DESC desc;

        desc.NumDescriptors = 1;
        desc.Type = D3D12_DESCRIPTOR_HEAP_TYPE.D3D12_DESCRIPTOR_HEAP_TYPE_RTV;
        desc.Flags = D3D12_DESCRIPTOR_HEAP_FLAGS.D3D12_DESCRIPTOR_HEAP_FLAG_NONE;
        desc.NodeMask = 0;

        var uuid_ID3D12DescriptorHeap = typeof(ID3D12DescriptorHeap).GUID;

        device->CreateDescriptorHeap(&desc, &uuid_ID3D12DescriptorHeap, &result);

        var heap = (ID3D12DescriptorHeap*)result;

        //Throws Access Violation
        heap->GetCPUDescriptorHandleForHeapStart();

    }

Here is the version with marshalling enabled:

    static void Main()
    {
        object result;

        var uuid_ID3D12Device = typeof(ID3D12Device).GUID;
        PInvoke.D3D12CreateDevice(null, Windows.Win32.Graphics.Direct3D11.D3D_FEATURE_LEVEL.D3D_FEATURE_LEVEL_12_0, uuid_ID3D12Device, out result);

        var device = (ID3D12Device)result;

        D3D12_DESCRIPTOR_HEAP_DESC desc;

        desc.NumDescriptors = 1;
        desc.Type = D3D12_DESCRIPTOR_HEAP_TYPE.D3D12_DESCRIPTOR_HEAP_TYPE_RTV;
        desc.Flags = D3D12_DESCRIPTOR_HEAP_FLAGS.D3D12_DESCRIPTOR_HEAP_FLAG_NONE;
        desc.NodeMask = 0;

        var uuid_ID3D12DescriptorHeap = typeof(ID3D12DescriptorHeap).GUID;

        device.CreateDescriptorHeap(desc, uuid_ID3D12DescriptorHeap, out result);

        var heap = (ID3D12DescriptorHeap)result;

        //Throws Access Violation
        heap.GetCPUDescriptorHandleForHeapStart();

    }

Compare to correclty working C++ version:

#include <Windows.h>
#include <D3D12.h>
int main(int argc, char** argv)
{

	ID3D12Device *device;
	if(!SUCCEEDED(D3D12CreateDevice(nullptr, D3D_FEATURE_LEVEL::D3D_FEATURE_LEVEL_12_0, __uuidof(ID3D12Device), (void**) & device)))
		return -1;

	D3D12_DESCRIPTOR_HEAP_DESC desc;
	desc.NumDescriptors = 1;
	desc.Type = D3D12_DESCRIPTOR_HEAP_TYPE::D3D12_DESCRIPTOR_HEAP_TYPE_RTV;
	desc.Flags = D3D12_DESCRIPTOR_HEAP_FLAGS::D3D12_DESCRIPTOR_HEAP_FLAG_NONE;
	desc.NodeMask = 0;
		
	
	ID3D12DescriptorHeap* heap;
	if(!SUCCEEDED(device->CreateDescriptorHeap(&desc, __uuidof(ID3D12DescriptorHeap), (void**)&heap)))
		return -1;

	auto handle = heap->GetCPUDescriptorHandleForHeapStart();

	
}

Context

• CsWin32 version: 0.1.588-beta
• Target Framework: net6.0-windows

[ghord]

I guess this is the same issue as in #167 . Is there any recommended workaround for now?

[AArnott]

I don't have a workaround to offer at this point. Thanks for the report and noticing the dupe.

[ghord]

I did some more digging and it appears that support for this calling convention is already implemented in clr in .net 6. Since I'm already using it, I was able to work around this issue with following code (code is proof of concept for now, performance of this is very bad obviously):

    private unsafe D3D12_CPU_DESCRIPTOR_HANDLE GetDescriptorHandle(ID3D12DescriptorHeap* heap)
    {
        var vtableField = typeof(ID3D12DescriptorHeap).GetField("lpVtbl", BindingFlags.NonPublic | BindingFlags.Instance)!;
        var vtableType = typeof(ID3D12DescriptorHeap).GetNestedType("Vtbl", BindingFlags.NonPublic)!;

        var vtable = (nuint*)Pointer.Unbox(vtableField.GetValue(*heap));

        var getDescriptor = (delegate* unmanaged[Stdcall, MemberFunction]<ID3D12DescriptorHeap*, D3D12_CPU_DESCRIPTOR_HANDLE>)(vtable[9]);

        var getDescriptorResult = getDescriptor(heap);

        return getDescriptorResult;
    }

This code requires marshalling to be disabled and manually calls the vtable pointer with newly introduced MemberFunction calling convention annotation instead. I've done some testing and it seems to resolve the issue.

Is there a plan to use this new COM calling convention in generated vtables?

[AArnott]

Is there a plan to use this new COM calling convention in generated vtables?

Not at present. Presumably there should be a way to make this work on .NET Framework as well, and we'd most likely generate the code necessary to make it work there as well as .NET 6.

[AArnott]

Closing as duplicate of #167.