Calling gcc built DLL from .NET (Visual Basic)

["Thomas X. Hoban" <thoban at verbalogic dot com>]

Does anyone have a good example that shows how to create a DLL using
cygwin/gcc and call it using VB .NET?   I am specifically interested in an
example that shows how to pass a String variable.  In the example that I
show below, I am able to call a C function.  I can successfully pass a long
integer.  But the string that I pass shows garbage in the called function.

My dll code looks like...


(test.c)

#include <stdio.h>
#include <wtypes.h>

define EXPORT __declspec(dllexport)

EXPORT int myTest(char *s1, long i1) {

    printf("Here is the string %s, here is the integer %ld.\n",s1, i1);

}



I then compile as follows:

$ gcc -c test.c
$ gcc -shared -o test.dll test.o

(I have also tried several combinations of linking and compiling with
different options).

In my Visual Basic .NET console application, I create a class that wraps the
"myTest" function and then call it from main as follows:

Public Class LibWrap
    Declare Function myTest Lib
"c:\cygwin\home\Administrator\test_dll\test.dll" (ByRef s1 As String, ByVal
i1 As Long) As Integer
End Class
Module Module1

    Sub Main()
        Dim test As String
        test = "My String"
        LibWrap.myTest(test, 6432)
        LibWrap.myTest(test, 20)
        LibWrap.myTest(test, 3000)
    End Sub

End Module

I also modified the "path" environment variable so vb finds the class path
to cygwin.

As I mentioned, I am able to pass integers successfully, but strings don't
seem to get marshaled to (char *).  I recognize that the .NET String
implementation is different, but thought the .NET marshaling would take care
of conversion.  A simple working example would be extremely helpful.

Thanks


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