Problem marshalling with Unmanaged.LPUTF8Str

[msedi]

I didn't want to file this as a bug because I think I'm causing the problem but I don't know why.

I'm updating our Vulkan library P/Invokes and interop structures to .NET5 where we can use the Unmanaged.LPUTF8Str for marshalling UTF8 (nul-terminated) strings. So decided to go with this instead of marshalling as byte and then converting it to a C# string which makes our code more readable.

The problem is though, that the method

[DllImport(VulkanLibrary.Name)] static extern void vkGetPhysicalDeviceProperties(IntPtr physicalDevice, out VkPhysicalDeviceProperties properties);

throws some memory exception with the struct given below. The docs explicitly say that it is a nul-terminated UTF8 string so I thought it should work. What does work is using UnmanagedType.ByValTStr and I have no clue why.

  [StructLayout(LayoutKind.Sequential)]
    public readonly struct VkPhysicalDeviceProperties
    {
        public readonly VulkanVersion ApiVersion;
        public readonly VulkanVersion DriverVersion;
        public readonly int VendorId;
        public readonly int DeviceId;
        public readonly VkPhysicalDeviceType DeviceType;

        // The docs say that it is an UTF8 (nul-terminated) string, the question is why LPUTF8Str doesn't work.
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = MaxPhysicalDeviceNameSize)]
        public readonly string DeviceName;       

        public readonly Guid PipelineCacheUuid;
        public readonly VkPhysicalDeviceLimits Limits;
        public readonly VkPhysicalDeviceSparseProperties SparseProperties;
    }

[jkoritzinsky]

The difference between ByValTStr and LPUTF8Str is where the string is placed in memory.

For the ByValTStr case, the native representation of the VkPhysicalDeviceProperties struct would look like the following:

    struct VkPhysicalDeviceProperties
    {
        VulkanVersion ApiVersion;
        VulkanVersion DriverVersion;
        int VendorId;
        int DeviceId;
        VkPhysicalDeviceType DeviceType;
        char DeviceName[MaxPhysicalDeviceNameSize]; // Marshalled in-place in the structure
        GUID PipelineCacheUuid;
        VkPhysicalDeviceLimits Limits;
        VkPhysicalDeviceSparseProperties SparseProperties;
    };

For the LPUTF8Str case, the string is marshalled as a pointer to the allocated string, so the struct would look like the following:

    struct VkPhysicalDeviceProperties
    {
        VulkanVersion ApiVersion;
        VulkanVersion DriverVersion;
        int VendorId;
        int DeviceId;
        VkPhysicalDeviceType DeviceType;
        char* DeviceName; // Marshalled as a pointer
        GUID PipelineCacheUuid;
        VkPhysicalDeviceLimits Limits;
        VkPhysicalDeviceSparseProperties SparseProperties;
    };

From what it sounds like, the ByValTStr case has the matching layout to the native code, which would be why it works and why the LPUTF8Str case does not.

[msedi]

@jkoritzinsky: Thanks a lot for explaining this. For me it was not obvious from the docs.