What is wrong in this source generated COM code?

[DrkWzrd]

I'm trying to get a grasp of source generated COM and I'm just writing this as an small test.

After 2 hours looking for information and trying different things (everything found in the web is in the "old" COM style, surprinsingly) I don't understand why the error in the array of filters in SetFileTypes.

What's wrong? Is sometype of incompatibility? I have to give in and just put an IntPtr there?

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
internal struct COMDLG_FILTERSPEC
{
    [MarshalAs(UnmanagedType.LPWStr)]
    public string pszName;
    [MarshalAs(UnmanagedType.LPWStr)]
    public string pszSpec;
}

[GeneratedComInterface(StringMarshalling = StringMarshalling.Utf16)]
[Guid("43826d1e-e718-42ee-bc55-a1e261c37bfe")]
internal partial interface IShellItem
{
    void BindToHandler(IntPtr pbc, ref Guid rbhid, ref Guid riid, out IntPtr ppv);
    void GetParent(out IShellItem ppsi);
    void GetDisplayName(uint sigdnName, out string ppszName);
    void GetAttributes(uint sfgaoMask, out uint psfgaoAttribs);
    void Compare(IShellItem psi, uint hint, out int piOrder);
}

[GeneratedComInterface(StringMarshalling = StringMarshalling.Utf16)]
[Guid("42f85136-db7e-439c-85f1-e4075d135fc8")]
internal partial interface IFileDialog
{
    [PreserveSig] int Show(IntPtr parent);
    void SetFileTypes(uint cElements, COMDLG_FILTERSPEC[] rgFilterSpec);//It complains about the array
    void SetFileTypeIndex(uint iFileType);
    uint GetFileTypeIndex();
    void Advise(IntPtr pfde, out uint pdwCookie);
    void Unadvise(uint dwCookie);
    void SetOptions(uint fos);
    uint GetOptions();
    void SetDefaultExtension(string pszDefaultExtension);
    void GetResult(out IShellItem ppsi);
    void SetFileName(string pszName);
    void GetFileName(out string pszName);
    void SetTitle(string pszTitle);
    void SetOkButtonLabel(string pszText);
    void SetFileNameLabel(string pszText);
}

[huoyaoyuan]

The problem is the string member in the struct. It requires marshalling which is not supported by the source generator (and P/Invoke source generator). You must define them as char* or IntPtr and marshal manually.

[DrkWzrd]

Thank you! Do you know if it will be supported?

[huoyaoyuan]

The marshalling logic is complex and needs to be customized for each type. I'm not aware of any plan of it.

[DrkWzrd]

So... this should be fine, am I right? (As I said before, this is just investigation and playing with this new thing, but I want to learn what is "the right way" to do it)

[CustomMarshaller(typeof(ComDialogFilterSpecs), MarshalMode.Default, typeof(COMDLG_FILTERSPEC_Marshaller))]
[CustomMarshaller(typeof(ComDialogFilterSpecs), MarshalMode.ManagedToUnmanagedIn, typeof(COMDLG_FILTERSPEC_Marshaller))]
internal static class COMDLG_FILTERSPEC_Marshaller
{
    [StructLayout(LayoutKind.Sequential)]
    internal struct COMDLG_FILTERSPEC
    {
        public nint pszName;
        public nint pszSpec;
    }

    public static COMDLG_FILTERSPEC ConvertToUnmanaged(ComDialogFilterSpecs managed)
    {
        return new COMDLG_FILTERSPEC
        {
            pszName = Marshal.StringToHGlobalUni(managed.pszName),
            pszSpec = Marshal.StringToHGlobalUni(managed.pszSpec),
        };
    }

    internal static void Free(COMDLG_FILTERSPEC unmanaged)
    {
        if (unmanaged.pszName != nint.Zero)
            Marshal.FreeHGlobal(unmanaged.pszName);

        if (unmanaged.pszSpec != nint.Zero)
            Marshal.FreeHGlobal(unmanaged.pszSpec);
    }

    public static ComDialogFilterSpecs ConvertToManaged(COMDLG_FILTERSPEC unmanaged)
    {
        return new ComDialogFilterSpecs
        {
            pszName = Marshal.PtrToStringUni(unmanaged.pszName) ?? throw new InvalidOperationException(),
            pszSpec = Marshal.PtrToStringUni(unmanaged.pszSpec) ?? throw new InvalidOperationException(),
        };
    }
}

//-------------------------------
//...

void SetFileTypes(uint cElements, [In, MarshalUsing(CountElementName = nameof(cElements))] ComDialogFilterSpecs[] rgFilterSpec);

[huoyaoyuan]

Yeah it's basically the concept, with minor tweaks for best practices etc.