Setting up parameter for IDispatch::Invoke when calling "ref byte []"

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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:

C#

namespace Blah
{
  public interface Blether
  {
...
    [DispId(19)]
    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:

C++

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

If it was ref string, I'd have:

C++

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

If it was an array of bytes, I have:

C++

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?

Iain.

Posted 30-May-13 21:22pm

Iain Clarke, Warrior Programmer

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KarstenK · Answer 1 · 2013-05-31T01:41:00

Solution 1

if that works

C#

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...

Posted 31-May-13 1:41am

KarstenK

Comments

Iain Clarke, Warrior Programmer 31-May-13 7:45am

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#.

Iain.

Member 11780753 · Answer 2 · 2015-07-14T05:49:00

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:

C++

vtParam[n].vt = VT_I4 | VT_ARRAY | VT_BYREF;
vtParam[n].pparray = &mySafeArrayPointer; // use address of your safeArrayPointer

Remember to free it~