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Passing BSTR parameters from VB to a C++ dll

Posted on 2003-03-20
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Last Modified: 2010-04-01
I have a C++ dll that up to now has been working very well with my VB application.  The VB app passed in 2 strings and 2 ints.  I had to change the dll to accept 3 strings and 2 ints.  For some reason, the 3rd string is not coming through properly.  If I step through the code, what I am getting on the C++ end for the 3rd string is totally messed up.  From the VB side if I step into the code, it is passing through a nice string.  The other two strings still function properly.  What is wrong with this third string parameter??  I tried passing constant strings from VB and passing the same string to every parameter and still I have the first two coming through fine and the last one completely messed up!

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Question by:emily_hall
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6 Comments
 
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Author Comment

by:emily_hall
ID: 8175874
Sorry - forgot to add, the VB is sending parameters that are defined like Dim x as String and the C++ code is accepting BSTRs.
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Expert Comment

by:Salte
ID: 8175910
Could you give us the function prototype (parameters, calling convention etc) for both the VB side and the C++ side?

Alf
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Author Comment

by:emily_hall
ID: 8175956
Sure - the c++ side is as follows:

STDMETHODIMP CMyClass::Constructor(BSTR x, BSTR y,
int z, int a, BSTR b)

and the VB calls it with:

Call oMyLib.Constructor(string1, string2, int1, int2, string3)

where string1, string2, string3 are like:
Dim stringX as String
int is: Dim intX as Integer
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Author Comment

by:emily_hall
ID: 8177028
Another part of the mystery:

I just wrote a test program in C++ that would call the same function and the parameter passed in just fine.  Called the function like:

MyLib->Constructor(A2OLE(string1), A2OLE(string2), int1, int2, A2OLE(string3));

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Author Comment

by:emily_hall
ID: 8177213
Problem solved - I recompiled the C++ library, went to the VB code and dropped that library from the references, re compiled, readded the library, recompiled and tried it again and it worked!!

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