Marshal an allocated struct from COM C++ to C#

Question

Hi,

I have a native com component and I want to access it from C#.

Here is a part of the idl :

typedef struct { 
	BOOL bStuff; 
	BSTR sAString;
} MyInfos;
interface IMyInterf : IUnknow
{
HRESULT iGetMyInfos([out] MyInfos **ppInfos);
}

In the iGetMyInfos method, the struct is allocated with CoTaskMemAlloc, filled and returned in the ppInfos parameter.

I want to use this in C#. tlbimp doesn't look strong with struct, so I tried to write the marshal by myself :

namespace Mynamespace
{
    using System;
    using System.Runtime.CompilerServices;
    using System.Runtime.InteropServices;

    [StructLayout(LayoutKind.Sequential)]
    public struct MyInfos {
        [MarshalAs(UnmanagedType.Bool)]
        public bool bStuff;
        [MarshalAs(UnmanagedType.BStr)]
        public string sAString;
    };

    [ComImport, InterfaceType((short)1), Guid("xxxx")]
    public interface IMyInterf 
    {
       [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType=MethodCodeType.Runtime)]
        void iGetMyInfos([Out, MarshalAs(UnmanagedType.Struct)] out MyInfos infos);
    }

    [ComImport, Guid("xxxx")]
    public class CoMyObj { }
}

I have other methods wich return strings as out parameters, and these work well.

But I never succeeded to retrieve the content of My MyInfos struct. I try many MarshalAs types, without success.

The string in the struct is empty and the Boolean is always true (?).

Does anybody can tell me how to solve this ?

I saw many samples with struct, but none with a struct allocated by the com component.

Thanks a lot !

J.

Answer 1

Try change below.  the C code has it shown as **ppInfos which is a pointer to a pointer which indicates it is a pointer to an array.

From :     void iGetMyInfos([Out, MarshalAs(UnmanagedType.Struct)] out MyInfos infos);
To   :     void iGetMyInfos([Out, MarshalAs(UnmanagedType.Struct)] out MyInfos[] infos);

See webpage below

http://stackoverflow.com/questions/11131676/how-do-i-marshall-a-pointer-to-a-pointer-of-an-array-of-structures

---

jdweng

Answer 2

Thanks.

I got an exception :

Cannot marshal 'parameter #1': Invalid managed/unmanaged type combination (Arrays can only be marshaled as LPArray, ByValArray, or SafeArray)." [thrown by System.StubHelpers.StubHelpers.ThrowInteropParamException]

---

For information, the native COM code looks like this :

(no error management for clarity)

STDMETHODIMP CMyObj::iGetMyInfos(MyInfos **ppInfos)
{
    MyInfos *pInfos=NULL;
    pInfos=(MyInfos *)CoTaskMemAlloc(sizeof(*pInfos)));
    memset(pInfos, 0, sizeof(*pInfos));
    pInfos->bStuff=FALSE;
    pInfos->sAString=SysAllocString(L"tototo");
    *ppInfos=pInfos;
	return S_OK;
}

Answer 3

Try doing it in two steps like in the link I provided.  The error is occuring because you are getting a long pointer array back instead of a MarshalAs(UnmanagedType.Struct)] .

Or try this

 void iGetMyInfos([Out, MarshalAs(UnmanagedType.LPArray)] out MyInfos[] infos);

---

jdweng

Answer 4

Thanks a lot.

It works with LPArray.

It also works if I use IntPtr and Marshal.PtrToStructure.