Visual Basic Programmer

This examples builds a standard DLL that you can call by using the normal DLL calling conventions. For full details, see the article Creating a Windows DLL with Visual Basic.

Thanks to Luke Emmet for pointing this article out. The basic staps are: Hack the linking process. Make an executable program to call the linker. Reomve the default Form1 and create the following Sub Main.

Public Sub Main() Dim SpecialLink As Boolean, fCPL As Boolean, fResource _ As Boolean Dim intPos As Integer Dim strCmd As String Dim strPath As String Dim strFileContents As String Dim strDefFile As String, strResFile As String Dim oFS As New Scripting.FileSystemObject Dim fld As Folder Dim fil As File Dim ts As TextStream, tsDef As TextStream strCmd = Command Set ts = oFS.CreateTextFile(App.Path & "\lnklog.txt") ts.WriteLine "Beginning execution at " & Date & " " & _ Time() ts.WriteBlankLines 1 ts.WriteLine "Command line arguments to LINK call:" ts.WriteBlankLines 1 ts.WriteLine " " & strCmd ts.WriteBlankLines 2 ' Determine if .DEF file exists ' ' Extract path from first .obj argument intPos = InStr(1, strCmd, ".OBJ", vbTextCompare) strPath = Mid(strCmd, 2, intPos + 2) intPos = InStrRev(strPath, "\") strPath = Left(strPath, intPos - 1) ' Open folder Set fld = oFS.GetFolder(strPath) ' Get files in folder For Each fil In fld.Files If UCase(oFS.GetExtensionName(fil)) = "DEF" Then strDefFile = fil SpecialLink = True End If If UCase(oFS.GetExtensionName(fil)) = "RES" Then strResFile = fil fResource = True End If If SpecialLink And fResource Then Exit For Next ' Change command line arguments if flag set If SpecialLink Then ' Determine contents of .DEF file Set tsDef = oFS.OpenTextFile(strDefFile) strFileContents = tsDef.ReadAll If InStr(1, strFileContents, "CplApplet", _ vbTextCompare) > 0 Then fCPL = True End If ' Add module definition before /DLL switch intPos = InStr(1, strCmd, "/DLL", vbTextCompare) If intPos > 0 Then strCmd = Left(strCmd, intPos - 1) & _ " /DEF:" & Chr(34) & strDefFile & Chr(34) & _ " " & _ Mid(strCmd, intPos) End If ' Include .RES file if one exists If fResource Then intPos = InStr(1, strCmd, "/ENTRY", vbTextCompare) strCmd = Left(strCmd, intPos - 1) & Chr(34) & _ strResFile & _ Chr(34) & " " & Mid(strCmd, intPos) End If ' If Control Panel applet, change "DLL" extension to ' "CPL" If fCPL Then strCmd = Replace(strCmd, ".dll", ".cpl", 1, , _ vbTextCompare) End If ' Write linker options to output file ts.WriteLine "Command line arguments after " & _ "modification:" ts.WriteBlankLines 1 ts.WriteLine " " & strCmd ts.WriteBlankLines 2 End If ts.WriteLine "Calling LINK.EXE linker" Shell "linklnk.exe " & strCmd If Err.Number <> 0 Then ts.WriteLine "Error in calling linker..." Err.Clear End If ts.WriteBlankLines 1 ts.WriteLine "Returned from linker call" ts.Close End Sub

This program does roughly the same thing that Visual Basic does when it creates a DLL except it adds the /DEF flag to the command. Compile the executable. Rename the normal Visual Basic linker from Link.exe to LinkLnk.exe. On my system, it's at C:\Program Files\Microsoft Visual Studio\VB98. Copy the executable program that you compiled into this directory and name it Link.exe. When Visual Basic links the DLL, it calls this program, which calls the renamed LinkLnk.exe program, adding the new /DEF parameter. Export the DLL's routines. Create a file named .def where is the name of the DLL. In this example, the DLL is named Fibonacci.dll so this file is called Fibonacci.def. Add code to this file similar to the following:

NAME MathLib LIBRARY MathMod DESCRIPTION "Add-on Library of Mathematical Routines" EXPORTS DllMain @1 Fibo @2

This tells the linker about the main entry point DllMain and this example's function Fibo, both of which are created shortly. Build the DLL. Create a new ActiveX DLL project. Leave the default Class1 class alone. You will not use it but Visual Basic needs it to it has something to compile into the ActiveX DLL. Add a code module and insert this code:

Public Const DLL_PROCESS_DETACH = 0 Public Const DLL_PROCESS_ATTACH = 1 Public Const DLL_THREAD_ATTACH = 2 Public Const DLL_THREAD_DETACH = 3 Public Function DllMain(hInst As Long, fdwReason As Long, _ lpvReserved As Long) As Boolean Select Case fdwReason Case DLL_PROCESS_DETACH ' No per-process cleanup needed Case DLL_PROCESS_ATTACH DllMain = True Case DLL_THREAD_ATTACH ' No per-thread initialization needed Case DLL_THREAD_DETACH ' No per-thread cleanup needed End Select End Function ' Return a Fibonacci number. Public Function Fibo(ByVal N As Integer) As Long If N <= 1 Then Fibo = 1 Else Fibo = Fibo(N - 1) + Fibo(N - 2) End If End Function

DllMain is the DLL entry point. Fibo is a function that the DLL is exporting. Compile the DLL. This should invoke the new Link.exe you built. If you look in that program's directory, you should see the log file it generates. Build a test program to call the DLL. Make a standard Visual Basic EXE. Declare the routine exported by the DLL as in the following code:

Private Declare Function Fibo Lib _ "C:\WebSite\HowToSrc\a2\Fibonacci.dll" (ByVal N As _ Integer) As Long

Insert the path to the DLL on your computer. Run the program.That should do it. Watch out for typos. If the .DEF file doesn't spell the function's name correctly, the DLL won't compile and the error messages are not very good.