How to Pass User-Defined Structure Containing Strings to DLL

• Microsoft Visual Basic Professional Edition for Windows, version 3.0

SUMMARY

This articles shows by example how to pass a user-defined structure that contains strings to a DLL. The example enables a DLL to read and write the strings in a user-defined structure.

MORE INFORMATION

The following step-by-step example passes a user-defined structure that contains strings to a DLL to manipulate.

1. Start a new project in Visual Basic.



2. From the File menu, choose New Module (ALT F M). MODULE1.BAS will be created by default. Add the following code to the .BAS module:

   
      ' Fixed-length string elements of a structure are packed in memory
      ' as are other values in Visual Basic. The following structure takes up
      ' 16 bytes of memory:
      '
      Type MYSTRINGSTRUCT
         str1 As String * 8
         str2 As String * 8
      End Type
      ' Enter the following Declare statement as one, single line
   
      Declare Sub MyStructProc Lib "Name of DLL your create"
         (lpStringStruct As MYSTRINGSTRUCT)
    




3. Add a command button (Command1) to Form1.



4. Add the following code to the Command1_Click event of Form1:

   
      Sub Command1_Click ()
      Dim StringStruct As MYSTRINGSTRUCT
         StringStruct.str1 = "str1"
         StringStruct.str2 = "str2"
         MyStructProc StringStruct
         TEXT1.Text = StringStruct.str1
         TEXT2.Text = StringStruct.str2
      End Sub
    




5. Add two text controls (Text1 and Text2) to Form1.



6. Create the C code needed to make the DLL. In the .h file of the DLL a user-defined type will create a mirror image of the type you defined in the Visual Basic .BAS file. Char str[8] is equivalent to Visual Basic declaration of str1 as String * 8. This structure definition takes up 16 bytes in memory as does the Visual Basic structure definition.

   
      typedef struct STRINGSTRUCT{
   
      char str1[8] ;
      char str2[8] ;
      } FAR * LPSTRINGSTRUCT ;
   
      /* Declaration of the function */ 
      void FAR PASCAL MyStructProc(LPSTRINGSTRUCT) ;
    




7. Add the following code to your .c file:

   
      #include "The .h file where you added the code above"
   
      void FAR PASCAL MyStructProc(LPSTRINGSTRUCT lpStringStruct)
      {
      /* You need to use lstrcpyn because the structure from Visual
      Basic is packed, and the strings are not Null terminated. The way
      structures are passed from Visual Basic to a DLL is fully described
      beginning on page 566 in the Visual Basic version 3.0 for Windows
      "Programmers Guide," Chapter 24, "Calling Procedures in DLLs," in
      "User-Defined Types" under "Calling DLL Procedures with Specific Data
      Types." */ 
   
         lstrcpyn(lpStringStruct->str1, "change11", 8) ;
         lstrcpyn(lpStringStruct->str2, "change22", 8) ;
      } 

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