OCX return a string value to VB

[awni]

hi
I have a VC++ 6.0 OCX which is used by a VB 6.0 program.
I need to send a string as a argument and have the OCX change
the string's value, how can I do that?

The function looks something like this :

void MyClass::changeStringValue(String_Pointer str1, String_Pointer str2)
{
char *c = "anything 2";
*str1 = "anything";
strcpy(str2, c...
memcpy(str2, c....
//neither copy function worked
}

This method worked for double and long values, it won't work
for strings(i don't know how to declare a String_Pointer in OCX
functions that are visible to VB)

thanks

awni

[Mike Harnad]

Try using a CString as the parameter. CStrings are converted to BSTR (an automation type) in the .odl file of an ActiveX control. Visual Basic should know how to handle a BSTR.

[awni]

no luck

I even tried VARIANT instead of CString , didn't work.
BSTR, LPCSTR didn't either , but LPCSTR accepts strings as
input , BSTR type gives a Type mismatch error.


thanks
awni

[Gnail]

How about do it the simpler but not as efficient way:

1. VB program pass the string to the ocx (LPCTSTR)
2. ocx return a modified the string to VB program (BSTR)
3. VB program use the returned string to overwrite the original string

[awni]

that works real good for single string returns, like

 
BSTR ReturnString()
{
       return a_BSTR
}

but if i need to change 2 or more strings like

void Change2Strings(String_Pointer s1, String_Pointer s2)
{
      *s1 = "something";
      *s2 = "something else";
}

then I'd need something else...

I wonder if allocating an array of type BYTE, with a single
element and allocating a string immediatly after that like

   dim b(1) as byte
   dim s as String
   dim s2 as String

I wonder if that would store the string right after the array
in memory.

I can then pass the array to Visual C++. VB passes the address
of the first element, so if the string follows the array in memory, i can just add 1 to the location of the array, and i've got me the address of the first string, i have no idea how to get to the second string though, maybe add 255? (why am i telling you this?)
I'll try it if i get a chance, and let you know(if i get a chance)

thanks

awni

[Yves M]

Since it's an OCX, you have to use IDL types and you really should use BSTRs, which is the standard string type for COM.

So, in your OCX's idl, declare a function like this:

// idl
[id(10), helpstring("method ChangeStrings")] HRESULT ChangeStrings([in, out] BSTR *pFirst, [in, out] BSTR *pSecond);

Then in your C++ code, write:

HRESULT MyOCX::ChangeStrings(BSTR *pFirst, BSTR *pSecond)
{
  // Change the strings
  BSTR tmp = *pSecond;
  // concatenate both strings into pSecond and set pFirst to an empty string
  *pSecond = SysAllocStringLen(*pFirst, SysStringLen(*pFirst) + SysStringLen(*pSecond));
   wcscpy(*pSecond + SysStringLen(*pFirst), tmp);
   SysFreeString(*pFirst);
   SysFreeString(tmp);
   *pFirst = SysAllocString(0, 0);
   return S_OK;
}

[awni]

thanks everyone
Yves, where's a good source of information about
passing different types between VB and VC?

thanks

awni

[Yves M]

Check in MSDN under:

Component Development > Microsoft Interface Development Language (midl) > SDK documentation > Interface Definitions and Type Libraries > MIDL data types.

There you have all the raw information. In any case, if you are a bit unsure what type you should use, but you know what it should be in VB, make a VB COM dll with functions that take resp. return these types and inspect the resulting dll in the OLEViewer.

[awni]

thank you that was very helpful. I managed to pass/return types
I didn't know how to pass/return before. There are still some that
I'm having problems with, such as user defined types. I did see
them in OLEViewer, but I don't know how to make them visible
in both VB and VC. Any ideas?

thanks

awni