Working with C DLLs in Visual Basic

Some tips

• As working with C functions from Visual Basic often leads to GPF's while still developing, remember to save your project before running it.
  
• As a GPF takes down the IDE, remember to compile and test using the EXE instead of runnign your code in the IDE...
• By default, Basic passes all parameters by reference, ie. it passes the address of the variable and not its actual value. The routine will thus work on the original data, which is the reason why changes made inside the routine remain when this routine returns. Before working with C functions, it's something that we take for granted.
  C, however, passes numeric data by value, and strings by reference as its default behavior. "Passing by value" means that the routine will work on a copy of the value held by the data that is passed to the routine, ie. the changes made by the routine to this data will only be local, and the original data will remain untouched.
  
  For example,
  
  Option Explicit
  
  Dim strGlobalString As String
  
  Sub ValRoutine(ByVal strDummy As String)
  strDummy = "ABCD"
  MsgBox "strGlobalString=" + strGlobalString + Chr$(13) + "strDummy=" + strDummy
  End Sub
  
  Sub RefRoutine(strDummy As String)
  strDummy = "ABCD"
  MsgBox "strGlobalString=" + strGlobalString + Chr$(13) + "strDummy=" + strDummy
  
  End Sub
  
  Private Sub Form_Click()
  strGlobalString = "1234"
  
  Call ValRoutine(strGlobalString)
  Call RefRoutine(strGlobalString)
  End Sub
  

• The magical words are ByVal and ByRef. ByRef is implicit, since by default Basic passes parameters by reference.
• Input strings: If a string will only be read by a routine, all you have to do is pass it by value.
  Output strings: If a routine will write into a string you passed it, you must first set it to a fixed size, long enough to fit.
  
  There are three ways to do this:
  
  Dim strMyString * 128
  
  Dim strMyString
  strMyString = Space (128)
  
  Dim strMyString
  strMyString = String (128, vbNullChar)
  
  Then, remember to pass this string by value, so that Visual Basic turns it from a regular BSTR Basic string (The variable holds 2 information: The address of the string, and its length) to a C-type string (The variable only contains the address of the string, and the string ends with a chr$(0) .)
• If you need to retrieve the address of either a variable or routine, use the no-longer-documented-but-still-operational function VarPtr (). When used with a string, use VarPtr (strMyString(0)) to get the address of the first character.
• If you need to declare a variable to hold the address of a variable, use a Long. As of now, a pointer is 4-byte wide, just like a Long in VB. if you need to pass the address of a pointer, ie. a pointer to a pointer (void** pMyPointer), pass this Long by reference.
• If the C function returned a pointer to a data block, you can copy it to a VB string by using the following function:
  
  Public Declare Sub CopyMem Lib "Kernel32" Alias "RtlMoveMemory" (pTo As Any, pFrom As Any, ByVal lCount As Long)
  

• If you need to work with C functions often, you might want to buy THE classic: Dan Appleman's "Visual Basic 5.0 Programmer's Guide to the Win32 API" (ZD Press, $59.99, ISBN 1-56276-446-2.)
• Another great book : "Hardcore VB"
• Getting GPFs ? Check that you got your ByRef and ByVal right in the DLL declaration; If the DLL will write to a variable, make sure you either set enough bytes (strings), or that the variable indicating the number of bytes available for output is right (eg. if you pass it a 4-byte buffer, and tell it that this buffer is 8-byte long, the DLL may happily write 8 bytes instead of the 4 your buffer can hold.
• If you get a 'Bad DLL Calling convention (Error 49)' when calling an API, check that it was compiled with _stdcall instead of _default.
  

Checking loaded DLL's

There are different utilities available to check which DLLs are currently loaded:

• Use the "Microsoft System Information" utility (MSINFO32.EXE on your disk. It is probably in some location like: \Program Files\Common Files\Microsoft Shared\MSInfo. Alternatively, you can run it via the "System Info..." button in the About box of some Microsoft Office apps (Word, Excel, Power Point, etc.).)
  
  In the tree view on the left, select "Active Modules" (or "Loaded modules", depending on the version you have) to get a complete list of all loaded DLLs, including the full path of the DLL's filename
• The freeware utility HandleEx
• Display Loaded Modules v1.5

Resources

• Notes for Developing DLLs for use with Microsoft (R) Visual Basic (R)
• Dynamic Link Libraries (creation, etc.)
• Visual Basic Secrets
• VB <---> C++ DLL tutorial project
• Using Windows API in VB Tutorial
• Using DLLs and The Windows API
• (MS Support) How To Write C DLLs and Call Them from Visual Basic
• Passing structure from C++ to VB
• Creating a Windows DLL with Visual Basic by Ron Petrusha
• Using C++ DLL's in VB by Mike D Sutton

Temp stuff

AddressOf

StrConv

StrPtr

VarPtr

BSTR

__cdecl, __stdcall

FnPtr(AddressOf DLLPrnt)