Need Help passing a string from VB into a C++ DLL??

**dan Burke** · 04-22-2004

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I am a beginner and I cannot figure out how to pass a string from a VB calling program to a C++ DLL. All I get in the C routine is garbage.

The "Declare" argument in VB is:
(ByRef s As String,.....

And the string looks like:

fln = "D:\\bin\\Data.txt"

The VB call is:

err = load_data(fln,...

On the C++ side I have:

int __stdcall load_data(char *s,...

then I use:

if(strlen(s) > 0)
{
strcpy(file_name,s);
data_file = fopen(file_name,"rt");
if(data_file != NULL)
file = true;
}

This fails every time and I am forced to call a find file dialog. I need it not to fail and can not figure out why it does.

A HUGE thank you if you can help.

Dan

**Speedy5** · 04-22-2004

Strings in VB are passed to C/C++ as BSTR. These can be casted into char * directly. But beware that BSTRs have 4 bytes before the string itself which holds the size of the string. The BSTR actually points to the string, though.

Useful VB to C++ site:
http://www.flipcode.com/articles/article_vbdlls.shtml

**anonytmouse** · 04-22-2004

Your code is correct, except that you need to use ByVal for strings rather than ByRef.

Returning a string from C to VB is slightly more complicated. The best way is to allocate the string in vb.

Code:

s = space(260)
some_c_function(s)
MsgBox s

void __stdcall some_c_function(char * s)
{
     strcpy(s, "Hello from C");
}

**dan Burke** · 04-28-2004

Excellant! Thank You

**dan Burke** · 04-28-2004

Excellant! Thank You as well...

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