Calling unmanaged C DLL function with char args

Ok,

I have a problem with VB.NET calling an unmanaged VC 6.0 C DLL from a form
that has char * args. The function prototype for the particular C function
in the C DLL is:

long _stdcall srv_binary_dxf_pntbat(char *inputname, char *dxfname, char
*cmdfile)

I have many other functions in this DLL that I call from my form with double
and integer args with NO problems whatsoever. Its the char/string stuff that
is giving me fits. In VB6 I use the following Declare:

'For using my 'C' dll
Public Declare Function srv_binary_dxf_pntbat Lib "srvm20.dll" (ByVal sBCGOfile
_
As String, ByVal sDXFfile As String, ByVal sCMDfile As String) As Long

It works fine in VB6.

I have tried passing the VB.NET args as byte arrays(ByRef and ByVal) with
a terminating NULL char with no luck at all - I have tried marshaling the
args with my declare in a module like this:

Public Declare Ansi Function srv_binary_dxf_pntbat Lib "srvm20.dll" _
(<MarshalAs(UnmanagedType.LPStr)> ByVal sBCGOfile As String, _
<MarshalAs(UnmanagedType.LPStr)> ByVal sDXFfile As String, _
<MarshalAs(UnmanagedType.LPStr)> ByVal sCMDfile As String) As Integer

In my VB.NET form I call this function like so - all args are the correct
type:

lRx = srv_binary_dxf_pntbat(sBDA, sDXF, sCMD)

No matter what I do, it errors with err.number=91 with an error message of
"Object Reference Not set to an instance of an object"

I am just learning .NET and it has been a shall we say a "challenge" for
me. It is so different from VB6 its almost overwhelming - I am determined
to get better at this coding but can one of you wizards help me over this
hump?

Thanks in advance,
Terry

Terry,

> Public Declare Ansi Function srv_binary_dxf_pntbat Lib "srvm20.dll" _
> (<MarshalAs(UnmanagedType.LPStr)> ByVal sBCGOfile As String, _
> <MarshalAs(UnmanagedType.LPStr)> ByVal sDXFfile As String, _
> <MarshalAs(UnmanagedType.LPStr)> ByVal sCMDfile As String) As Integer

Try this:

Public Declare Ansi Function _
srv_binary_dxf_pntbat Lib "srvm20.dll" _
ByVal sBCGOfile As StringBuilder, _
ByVal sDXFfile As StringBuilder, _
ByVal sCMDfile As StringBuilder) As Integer

You will need to include and import for the following:

Import System.Text

You will also need to initialize initialize your StingBuilder objects to the
correct information wether that is a buffer of " " or your data similar to
this:


'Buffer of 1000 spaces
Dim sbBDA As New StringBuilder(1000)

'Actual value
Dim sbBDA As New StringBuilder("C:\somfile.txt")

I can't guarantee it will work but I have found the StringBulider class to
be much easier to work with when working with Strings and C API calls. Like
you, I probably haven't stumbled across the "String" tricks yet. ;-)

> I am just learning .NET and it has been a shall we say a "challenge" for
> me. It is so different from VB6 its almost overwhelming - I am determined
> to get better at this coding but can one of you wizards help me over this
> hump?

Hang in there. I get's much better!!! (and a heck of a lot easier too, even
over VB6 IMHO)

Cal