Well, Charles,
 I don't have yet a solution, but I can give you more information
regarding how the constructor for arrays of structures seems to be
working in C#. I have a similar problem:

[StructLayout( LayoutKind.Sequential, CharSet=CharSet.Ansi )]
public struct MIXERCONTROLDETAILS_LISTTEXT
{
    public uint dwParam1;
    public uint dwParam2;
    [MarshalAs( UnmanagedType.ByValArray, SizeConst=64)]
    public char[] szName;
} ;

 When I use this structure to retrieve data from a COM object
(mixerXXX API in Windows Multimedia SDK), as is (no array), works
fine. The layout is as expected, folowing the sequential declarations
of the members.
  However, if I declare an array mxcd of
MIXERCONTROLDETAILS_LISTTEXT, I've found that the memory mapping of
the structures in the array is something like that:
  0-3: mxcd[0].dwParam1
  4-7: mxcd[0].dwParam2
  8-11: a 4-byte POINTER to another memory area holding
mxcd[0].szName (64 bytes long), not the actual string. <----- Here !
  12-15: mxcd[1].dwParam1
  16-19: mxcd[1].dwParam1
  20-23: a 4-byte POINTER to another memory area holding
mxcd[1].szName (64 bytes long), not the actual string. <----- Here !
   
 Apparently, when you call the constructor for the array, the base
types are left in place, where they were supposed to be, while the
strings (and other arrays) are created by allocating a different
memory area and inserting a pointer to it, where one would expect to
find the actual data.

I tried using explicit layout of the structure, but with no better
results. I even tried using the Size (Size=0x48) attribute for the
structure. The result was that instead of having the actual string at
the expected location, I got a pointer in the first 4 bytes, followed
by 60 bytes of unused space, but the szName string would still reside
somewhere else !

I hope this helps in figuring out the intricate mechanism of C#
memory allocation. I wish I had a solution as much as you do !

Can anyone HELP us ???? Please ???
  Andrei Barborica.

Thomas,

You have a point about allocating the IntPtr and passing it to
GetDataAttr(). However, the method signature does not want an out IntPtr
just and IntPtr. The IDL for this method is as follows:

[id(1), helpstring("method GetDataAttr. Retrieves User Attributes.")]
HRESULT GetDataAttr([in] BSTR bstUserName, [in] BSTR bstPassword, [in] int
iAppID, [in] int iConnType, [in] long lNoEntries, [out,
size_is(,lNoEntries)] DATA_MAP** ppDataMapOut);

This specifies the pointer to array of DATA_MAPs as an out parameter so I do
not no why the signature in the .NET app just wants a IntPtr and not out
IntPtr.

If I do not allocate the IntPtr and set it IntPtr.Zero before passing to
GetDataAttr() I get the following error when executing.
Additional information: The parameter is incorrect.

This is what I also observered.
The strange part is the are occasions when I will get all the data back
except the first string in the [0] structure. It is always garbage, while
all other fields including the first string of structure [1], [2], [n]. This
is strange.

Thanks,
Charles...

Thomas,

The COM object is expecting a pointer to an array of structures. The array
of structures is allocated in the COM object and assigned to the pointer. I
would assume this is why the signature is not out. In the C++ app that uses
the COM object I just pass the pointer to the array of structures the that
COM object allocates and returns. This works as expected. The problems are
the .NET marshalling back the structure.

Thanks,
Charles...