Interacting with a C++ DLL

[Saya]

Unsolved problem ...

I'm new to C# and having a problem with interaction between a (legacy) C++ dll

and functions in my C# module. I'm using a third party C++ DLL, no source

available (so, not possible to rebuilt it with Managed extensions)
At a certain point in my C#-app I have to return the address of a (managed-)

read-buffer into one of the arguments of a callback function, needed by that

DLL

The legacy C++ DLL callback interface looks like this
static MC_STATUS MediaToFile( char* Afilename
void* AuserInfo
int* AdataSize
void** AdataBuffer,
int AisFirst
int* AisLast

The fourth callback argument: <void** AdataBuffer> needs to be returned with

the address of the managed buffer used in my C# function
I've implemented the above in C# as follows

public delegate int MediaToFile
[MarshalAs(UnmanagedType.LPStr)
string filename
IntPtr info
ref int size
ref IntPtr bufferPtr
int first
ref int last)

..

//Data read in my C#-app are put into CBinfo.buffer
[StructLayout(LayoutKind.Sequential)
public class CBinf

public IntPtr stream
public int messageID
[MarshalAs(UnmanagedType.ByValArray, SizeConst=8192)
public byte[] buffer

..
// Instantiating it
public CBinfo cbi
..

//In the MediaToFile implementation I wrote the following
..
FileStream fs=fi.OpenRead()
nBytesRead=fs.Read(cbi.buffer, 0, nBytes)
..

Next step, the problem: I have to deliver the "cbi.buffer" location (its

address) to the callback argument: "ref IntPtr bufferPtr"
The DLL apparently needs to 'look back' into the buffer data
How to do this
(In C++ I used to write it like this: *AdataBuffer = CBInfo->buffer;

Doing Marshal.nnn / Unsafe / danger of GarbColl moving the buffer....? Please

advise - Thank you for your time

[key]

" function, needed by that

DLL:

The legacy C++ DLL callback interface looks like this:
static MC_STATUS MediaToFile( char* Afilename,
void* AuserInfo,
int* AdataSize,
void** AdataBuffer,
int AisFirst,
int* AisLast)

The fourth callback argument: <void** AdataBuffer> needs to be returned with
the address of the managed buffer used in my C# function.
I've implemented the above in C# as follows:

public delegate int MediaToFile(
[MarshalAs(UnmanagedType.LPStr)]
string filename,
IntPtr info,
ref int size,
ref IntPtr bufferPtr,
int first,
ref int last);

...

//Data read in my C#-app are put into CBinfo.buffer:
[StructLayout(LayoutKind.Sequential)]
public class CBinfo
{
public IntPtr stream;
public int messageID;
[MarshalAs(UnmanagedType.ByValArray, SizeConst=8192)]
public byte[] buffer;
}
...
// Instantiating it:
public CBinfo cbi;
...

//In the MediaToFile implementation I wrote the following:
...
FileStream fs=fi.OpenRead();
nBytesRead=fs.Read(cbi.buffer, 0, nBytes);
...
Next step, the problem: I have to deliver the "cbi.buffer" location (its

address) to the callback argument: "ref IntPtr bufferPtr".
The DLL apparently needs to 'look back' into the buffer data.
How to do this?
(In C++ I used to write it like this: *AdataBuffer = CBInfo->buffer;)

Doing Marshal.nnn / Unsafe / danger of GarbColl moving the buffer....? Please
advise - Thank you for your time!