How To Pass a String Between Visual Basic and Your C DLL

MORE INFORMATION

In order to develop your C DLL, it is important to know how Visual Basic handles strings internally in order to know how they will be passed to your DLL functions. Most C functions expect a string to be a null-terminated, ASCII character array. But Visual Basic uses a type-safe Automation string called a BSTR. For the Win32 platform, a BSTR is a long pointer to an allocated memory structure containing a Unicode character string and a 32- bit integer prefix that stores the length of the string.

You can ignored most of this difference, because Visual Basic will automatically convert any Unicode BSTRs to ASCII before passing them to an external function. The problem, however, is with the parameter itself. By default, Visual Basic passes all variables by reference, including strings. Because a BSTR variable is a pointer to a string, when Visual Basic passes it by reference it is actually passing a pointer to a pointer to a string. Most C functions simply expect a pointer to a string (such as LPSTR).

Create Your C DLL: Step-by-Step Example

1. Open Visual C++ 5.0 and click New on the File menu. On the Projects tab, select "Win32 Dynamic-Link Library" and name the project "StrSamp."
2. Once again, click New on the File menu, and select "C++ Source File." On the Files tab, name the file "StrSamp.c," and press OK.
3. Repeat step 2 and this time choose "Text File" as the file type. Name the file "StrSamp.def," and press OK.
4. Next, add the following code to "StrSamp.c:"

         #include <windows.h>
   
         void __stdcall DisplayStringByVal(LPCSTR pszString)
         {
            //pszString is a pointer to a string
            MessageBox(NULL, pszString, "Display String ByVal",
                      MB_OK | MB_ICONINFORMATION);
         }
   
         void __stdcall DisplayStringByRef(LPCSTR* ppszString)
         {
            //ppszSting is a pointer to a pointer to a string
            MessageBox(NULL, *ppszString, "Display String ByRef",
                      MB_OK | MB_ICONINFORMATION);
         }
   
            void __stdcall FillString(LPSTR pszString, LONG cSize)
         {
            // Create a temp buffer with our string
            char buffer[] = "Hello from the C DLL!";
   
            // Copy our temp string to pszString
            // but check the size to make sure we have a buffer
            // big enough to hold the entire string.
            if (cSize > strlen(buffer))
               strcpy(pszString, buffer);
         }
   
         int __stdcall InStrRev(LPCSTR pszString, short iChar)
         {
            // This function is similar to Visual Basic's InStr function
            // except that it searches for the given ASCII character from
            // right to left, returning the character position of the
            // last occurrence (rather than the first) of the character
            // in the string.
            char* pszTmp;
            int nRet = 0;
   
            // Scan for iChar in pszString backwards
            pszTmp = strrchr(pszString, (int)iChar);
            if(pszTmp != NULL)
               nRet = pszTmp - pszString + 1;
   
            return nRet;
         }
   
   					

5. To make the functions exportable, add the following to "StrSamp.def:"

         LIBRARY StrSamp
         DESCRIPTION 'Microsoft KB Sample DLL'
         EXPORTS
            DisplayStringByVal
            DisplayStringByRef
            FillString
            InStrRev
   
   					

6. Compile your DLL from the Build menu. When complete, copy the new DLL to your Visual Basic directory for testing.

The VB Test Application: Step-by-Step Example

1. Create a Standard Project in Visual Basic and name it "StrTest." Form1 is created by default.
2. Add three CommandButtons to Form1.
3. In the code window for Form1, add the following to the General Declarations section:

         Option Explicit
   
         Private Declare Sub DisplayStringByRef Lib "StrSamp.dll" _
            (sMyString As String)
         Private Declare Sub DisplayStringByVal Lib "StrSamp.dll" _
            (ByVal sMyString As String)
         Private Declare Sub FillString Lib "StrSamp.dll" _
            (ByVal sMyString As String, ByVal cBufferSize As Long)
         Private Declare Function InStrRev Lib "StrSamp.dll" _
            (ByVal sMyString As String, ByVal iChar As Integer) _
            As Long
   
   					

   Note that most of the strings are declared ByVal. This does not mean that you are passing these strings by value; rather, you are passing the value of the BSTR variable by value (remember that a BSTR variable is a pointer to a string). So the ByVal keyword has the effect of passing a long pointer to a string (LPSTR), just what the C functions expect.
   
   
4. Add the following code to the click events for each of the CommandButtons:

         Private Sub Command1_Click()
            Dim sTestString1 As String
            Dim sTestString2 As String
   
            sTestString1 = "This is my string passed to the dll by value."
            DisplayStringByVal sTestString1
   
            sTestString2 = "This is my string passed to the dll by reference."
            DisplayStringByRef sTestString2
   
         End Sub
   
         Private Sub Command2_Click()
            Dim sFillTest As String
   
            sFillTest = Space$(260)
            FillString sFillTest, 260
            MsgBox Trim$(sFillTest), vbInformation, "Fill String"
         End Sub
   
         Private Sub Command3_Click()
            Dim sPathString As String
            Dim sMsg As String
            Dim lCharPosition As Long
   
            sPathString = "C:\My Documents\Temp\Item.txt"
            lCharPosition = InStrRev(sPathString, Asc("\"))
   
            If CBool(lCharPosition) Then
               sMsg = "The file '" & Mid$(sPathString, lCharPosition + 1)
               sMsg = sMsg & "' is at this location:" & vbCrLf & vbCrLf
               sMsg = sMsg & Left$(sPathString, lCharPosition - 1)
               MsgBox sMsg, vbInformation, "InStrRev"
            Else
               MsgBox "Cannot find '/' in " & sPathString, vbCritical
           End If
   
         End Sub
   
   					

5. Press the F5 key to run the Visual Basic project in the IDE.
   
   NOTE: If you receive an error message, it may be because Visual Basic cannot find your DLL. Make sure you have copied it to the Visual Basic directory before you run your test application.