COM interface c# marshalling array of arrays

Question

I have a COM server written in C++ that should interface with a client written in C#. The C++ COM server exports interface functions to a structure that holds an array of structures with an array inside

struct InnerStruct  { int innerArray[ 100 ]; }
struct OuterStruct  { int dummy; InnerStruct outerArray[ 2 ]; }

So the structs have fixed size.

The IDL descriptions are

HRESULT SetStruct( [in] const OuterStruct* par, [out, retval] int code );
HRESULT GetStruct( [out] OuterStruct* par, [out, retval] int code );

The IDL compile fine and I can see the structure in the C# client.

The problem is that when I call the interface functions from the C# client I only get/set the values in the first InnerStruct in the OuterStruct. The second InnerStruct only holds garbage.

The C# debugger shows the right structure for the OuterStruct and knows there are 2 InnerStruct inside the OuterStruct.

In C# the declaration of the interface functions come from a server metafile, so it is not easy to change the description.

I have tried to set a size_is() on the in and out parameters but the MIDL compiler will not accept that.

Can I set up some specific marshaling on the parameters or how do I solve the problem of getting the complete OuterStruct trough COM?

Answer 1

In my IDL file I have tried to write

typedef [transmit_as(OuterStructAliasType)] OuterStruct* HelpType;

and keep the other declarations as

HRESULT SetStruct( [in] const OuterStruct* par, [out, retval] int code );
HRESULT GetStruct( [out] OuterStruct* par, [out, retval] int code );

where in my C++ code I have defined

typedef struct OuterStructAliasType
{
    char    dummy[ sizeof( OuterStruct )];
} OuterStructAliasType;

This seems to work. There maybe are other ways to the trick. Please tell me it they are smarter or better.