Returning an Array of C# Structures

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hpw - 14 Jun 2007 14:41 GMT

Hi all,

i'm trying to use COM Interop to return an array of c# structures to
Native code.
I tried to marshall the Array as SAFEARRAY.

The structure:

    [StructLayout(LayoutKind.Sequential)]
    [ComVisible(true)]
    public struct teststruct{
        [MarshalAs(UnmanagedType.LPStr)]
        public string string1;

        [MarshalAs(UnmanagedType.LPStr)]
        public string string2;

        [MarshalAs(UnmanagedType.LPStr)]
        public string string3;

        [MarshalAs(UnmanagedType.SafeArray)]
        public string[] parameters;

    };

is given back via a function

        [return: MarshalAs(UnmanagedType.SafeArray)]
        public teststruct[]
GetTestStruct([MarshalAs(UnmanagedType.LPStr)]string strParam, int
iParam) ;

I've generated the TLB via tlbexp.exe and used the Objects in a C++
Program.

Other calls with string-arrays and int-arrays work find. But for this
particular case i get the following _com_error:
0x80028019 - Old format or invalid type library

I also tried to specifiy the SafeArraySubType.
No Effect.

Any Help would be appreciated

HPW

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herc - 14 Jun 2007 16:53 GMT

> i'm trying to use COM Interop to return an array of c# structures to
> Native code. I tried to marshall the Array as SAFEARRAY.

Question: Does it HAVE to be an array or can you return a
collection?

Personally, I HATE passing arrays (SAFEARRAYS) around in COM, they are
a real pain. If a collection is an option, look into the IEnumVARIANT
interface. I would susspect that a C# ICollection interface is a
IEnumVARIANT. Google it and I think you will find much easier ways of
dealing with this issue;)

Cartoper

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hpw - 14 Jun 2007 17:14 GMT

> > i'm trying to use COM Interop to return an array of c# structures to
> > Native code. I tried to marshall the Array as SAFEARRAY.
[quoted text clipped - 7 lines]
> IEnumVARIANT. Google it and I think you will find much easier ways of
> dealing with this issue;)

Hi Cartoper,

thanks for your comments.

At the moment i did a similar workaround even though i do not like it:
i implemented an object with a dual-interface that allows me to read
the entires of the array.
I can pass back a reference to this Helper-Object that i can use to
read the structs.

If i pass back those things as Array (wich implements the ICollection
and IList interface) i get
an mscorlib::_Array that i can use. Unfortunately reading the items
using the IList interface always gives
me an Error "invalid argument type" if i try to read the arrays as
VARIANTs.

I havent tried using an IEnumVARIANT yet but i will also try this.

br
HPW

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SteveR - 09 Jul 2007 07:52 GMT

This is exactly the same problem I'm having. Have you got any code samples
you could post or url's I can refer to?

Signature

Steve

> > i'm trying to use COM Interop to return an array of c# structures to
> > Native code. I tried to marshall the Array as SAFEARRAY.
[quoted text clipped - 9 lines]
>
> Cartoper

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hpw - 25 Jul 2007 12:50 GMT

Hi Steve,

sorry for the late answer.

I couldn't solve the problem - so i found a workaround for this.
I've used a wrapper for the list and returned this instead of the
array.

Alternatively you can also use one of the list-Classes provided by
System.Collections. ...

here is what i did:

    [Guid("xxxxxxxxxxxxxxx")]
    [ComVisible(true)]
    [InterfaceType(ComInterfaceType.InterfaceIsDual)]
    public interface ITeststructCollection {
        void add(teststruct c);
        int count();
        teststruct get([MarshalAs(UnmanagedType.I4)] int i);
        void Dispose();
    };

and the implementation for this (TeststructCollection).
the call which returns the Interface looks like

TeststructCollection
GetCallEntries([MarshalAs(UnmanagedType.LPStr)]string smthing, int i);

Native code looks like this

...
        _CallEntryCollectionPtr entries=engine-

>GetCallEntries(pszSmthing,i);

        CoTaskMemFree(pszSmthing);

        ICallEntryCollection *coll;
        hResult=entries->QueryInterface(&coll);

        if (FAILED(hResult)) {
            char ch[255];
            ::FormatMessage(FORMAT_MESSAGE_FROM_SYSTEM,NULL,hResult,NULL,ch,
255,NULL);
            char pszErr[1024];
            sprintf(pszErr,"Reading COM-Entries Error: 0x%08x - %s
\n",hResult,ch);
            fprintf(stderr,pszErr);
            return ;
        }

        int count=coll->count();
...

hope this helps.

br
HPW

> This is exactly the same problem I'm having. Have you got any code samples
> you could post or url's I can refer to?
[quoted text clipped - 14 lines]
>
> > Cartoper

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Returning an Array of C# Structures

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