C#•11mo ago

✅ COM + NativeAOT Help

I'm fairly new to COM and I'm trying to implement an RDP Dynamic Virtual Channel client dll which uses the IWTSPlugin COM interface https://learn.microsoft.com/en-us/windows/win32/api/tsvirtualchannels/nn-tsvirtualchannels-iwtsplugin. Can anyone point out what I might be doing wrong here https://gist.github.com/jborean93/394592c89fc8bfd54990d814a29b924f as this just crashes my process and the WTSPlugin-log.txt is never created. I'm not 100% confident on how I'm setting the COM pointer on the VirtualChannelGetInstance implementation but I know for sure that mstsc is calling that particular method it as the logs are showing that it is called and the process crashes because I am doing something dumb. I do have a few other questions that hopefully someone might be able to clarify + Do I need to save the WTSPlugin and StrategyBasedComWrappers instance or can they be discarded once I've passed the pointer to the unmanaged instance + What is the correct way to define another COM interface (pointer to the interface type in COM land) as an argument + Are there any recommendations for PreserveSig or not having it

Gist

IWTSPlugin.cs

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21 Replies

Unknown User•11mo ago

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reflectronic•11mo ago

i would prefer using ComInterfaceMarshaller<IWTSPlugin>.ConvertToUnmanaged over instantiating the StrategyBasedComWrappers yourself

jboreanOP•11mo ago

If it helps I'm compiling it for win-x64 with

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

  <PropertyGroup>
    <TargetFramework>net8.0-windows</TargetFramework>
    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
    <InvariantGlobalization>true</InvariantGlobalization>
    <Nullable>enable</Nullable>
    <PublishAot>true</PublishAot>
  </PropertyGroup>

</Project>

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

  <PropertyGroup>
    <TargetFramework>net8.0-windows</TargetFramework>
    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
    <InvariantGlobalization>true</InvariantGlobalization>
    <Nullable>enable</Nullable>
    <PublishAot>true</PublishAot>
  </PropertyGroup>

</Project>

reflectronic•11mo ago

that doesn't change much, it's just a little cleaner and more efficient i would also skip the PreserveSig, it's just more work

jboreanOP•11mo ago

I'm assuming if I don't have PreserveSig it's just a void return here as none of those functions have an out return value

reflectronic•11mo ago

yea

jboreanOP•11mo ago

Thanks for the hint on ComInterfaceMarshaller, I'll start using that

reflectronic•11mo ago

where exactly do your logs say you get to because at a first glance everything seems OK

jboreanOP•11mo ago

only the VirtualChannelGetInstance is created the WTSPlugin-log.txt is not created which tells me it's failing to call Initialize()

reflectronic•11mo ago

how do you know it's crashing

jboreanOP•11mo ago

the whole mstsc process crashes and I have an WER event log I think I've got it working

    [UnmanagedCallersOnly(EntryPoint = "VirtualChannelGetInstance", CallConvs = [typeof(CallConvStdcall)])]
    private unsafe static int VirtualChannelGetInstance(
        Guid* refiid,
        int* pNumObjs,
        void** ppObjArray
    )
    {
        using StreamWriter writer = new(@"C:\temp\ProcessVirtualChannel\VirtualChannelGetInstance-log.txt", true);
        string now = DateTime.Now.ToString("[HH:mm:ss.fff]");
        writer.WriteLine($"{now} - VirtualChannelGetInstance - {*pNumObjs}");

        if (*refiid != typeof(IWTSPlugin).GUID)
        {
            return unchecked((int)0x80004002);  // E_NOINTERFACE
        }

        *pNumObjs = 1;
        if (ppObjArray != null)
        {
            void* unmanagedPtr = ComInterfaceMarshaller<IWTSPlugin>.ConvertToUnmanaged(_plugin);
            *ppObjArray = unmanagedPtr;

            now = DateTime.Now.ToString("[HH:mm:ss.fff]");
            writer.WriteLine($"{now} - Created pointer - {*pNumObjs}");
        }
        else
        {
            now = DateTime.Now.ToString("[HH:mm:ss.fff]");
            writer.WriteLine($"{now} - setting pNumObjs");
        }

        return 0;
    }

    [UnmanagedCallersOnly(EntryPoint = "VirtualChannelGetInstance", CallConvs = [typeof(CallConvStdcall)])]
    private unsafe static int VirtualChannelGetInstance(
        Guid* refiid,
        int* pNumObjs,
        void** ppObjArray
    )
    {
        using StreamWriter writer = new(@"C:\temp\ProcessVirtualChannel\VirtualChannelGetInstance-log.txt", true);
        string now = DateTime.Now.ToString("[HH:mm:ss.fff]");
        writer.WriteLine($"{now} - VirtualChannelGetInstance - {*pNumObjs}");

        if (*refiid != typeof(IWTSPlugin).GUID)
        {
            return unchecked((int)0x80004002);  // E_NOINTERFACE
        }

        *pNumObjs = 1;
        if (ppObjArray != null)
        {
            void* unmanagedPtr = ComInterfaceMarshaller<IWTSPlugin>.ConvertToUnmanaged(_plugin);
            *ppObjArray = unmanagedPtr;

            now = DateTime.Now.ToString("[HH:mm:ss.fff]");
            writer.WriteLine($"{now} - Created pointer - {*pNumObjs}");
        }
        else
        {
            now = DateTime.Now.ToString("[HH:mm:ss.fff]");
            writer.WriteLine($"{now} - setting pNumObjs");
        }

        return 0;
    }

Used ConvertToUnmanaged and played around with the ppObjArray signature

reflectronic•11mo ago

i mean that looks the same, but if it works it works

jboreanOP•11mo ago

you're telling me 🙂

reflectronic•11mo ago

idk i couldn't tell that anything was wrong

jboreanOP•11mo ago

I'm assuming I messed up my ppObjArray signature and setting that value somehow but as you said if it works it works Do I need to keep the WTSPlugin instance alive as a static field or can it be dropped after providing the pointer to it?

reflectronic•11mo ago

it can be dropped

jboreanOP•11mo ago

Thanks for the advice!

Unknown User•11mo ago

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MODiX•11mo ago

Use the /close command to mark a forum thread as answered

Unknown User•11mo ago

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jboreanOP•11mo ago

As sorry I wasn’t aware that was a thing here 🙂

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