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This is a bit of an interop question, so I thought I'd ask those wiser than me (so, that's most of you...)

I have a bit of C# code, wrapped up in COM:

namespace Blah
  public interface Blether
    int IainTest(ref byte[] Hello);

I am setting up a variant as a parameter to IDispatch::Invoke. If "Hello" were string then I'd have:

vtParam [n].vt = VT_BSTR;
vtParam [n].bstr = ...;

If it was ref string, I'd have:

vtParam [n].vt = VT_BSTR | VT_BYREF;
vtParam [n].bstr = ...;

If it was an array of bytes, I have:
vtParam [n].vt = VT_UI1 | VT_ARRAY;
vtParam [n].parray = pMySafeArrayOfBytes;

All of the above works.

But I'm failing on the top example: An in/out array parameter. I've tried:
vtParam [n].vt = VT_UI1 | VT_ARRAY | VT_BYREF;
vtParam [n].parray = pMySafeArrayOfBytes;

But I get a crash in the COM marshalling code. Looking at the assembly, it appears to be checking for VT_ARRAY, masking out VT_ARRAY, then looking at what's left - and that's not a simple type.

Any tips?


if that works
vtParam [n].vt = VT_UI1 | VT_ARRAY;
vtParam [n].parray = pMySafeArrayOfBytes;

dont use the VT_BYREF flag. This will transport a pointer, which is invalid/inaccessable in another process.

Consider marshalling as a "package service" which want to deliver packets od data not references...
Well, I can do VT_BYREF | VT_I4 to "connect" to a (..., ref int, ...) function, so the marshaller is clever enough there. Just VT_ARRAY | x means the marshaller has to package up a safearray with its contents to pass to the other end too.

My problem is that the marshaller does not "bring back" the array afterwards...

I have done some reading into MarshalAs which started to look hopeful, but that might be just for C# to c++ (ie, PInvoke), not COM to C#.

In VARIANT structure, you will see member named pparray without any document in MSDN. I try to use it to pass an SafeArray to SAFEARRAY** (in idl) parameter, like this:
vtParam[n].vt = VT_I4 | VT_ARRAY | VT_BYREF;
vtParam[n].pparray = &mySafeArrayPointer; // use address of your safeArrayPointer

Remember to free it~

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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