How To Make C DLL More Accessible to VB with a Type Library

Article translations Article translations
Article ID: 189133 - View products that this article applies to.
This article was previously published under Q189133
Expand all | Collapse all

On This Page


Since its first release, Visual Basic has provided the Declare statement as a means for you to take advantage of DLL functions written in other languages, such as C. But Declare statements are less than perfect and often require you to know as much about the DLL as you do about Visual Basic code. A type library creates a more Visual Basic-friendly way of calling exported C functions.

This article demonstrates how to create a type library when you build your DLL, and how to reference that library from Visual Basic.


Type libraries are compound document files (.tlb files) used in Automation. They contain important information about the types, objects, modules, and interfaces exposed by an Automation server to its clients. Fortunately, a server doesn't need to be Automation-aware to take advantage of a type library. In fact, most C DLLs are not Automation servers. All that is required is that the C DLL declare its functions as members of a module in a type library. An Automation client, such as Visual Basic, can read this information and bind to it as it would any object. No need for Declare statements or hard to remember constants because Visual Basic does all the work.

There are several advantages in creating a type library for your DLL. The most important of these is better type safety. But you also get the advantage of better performance, because Visual Basic automatically binds to your functions using early-binding. In contrast, all Declare statements are late-bound. Furthermore, you gain greater control over the way your DLL is presented to Visual Basic programmers. The type library allows you to provide Visual Basic-friendly names for functions and parameters, along with helpful extras like enumerations and User Defined Types (UDTs).

Currently, type libraries are created using scripts written in either the Interface Definition Language (IDL) or the Object Description Language (ODL). These scripts are then compiled using MkTypLib.EXE or MIDL.EXE that come with Visual Studio. Visual C++ takes some of the work out of creating type libraries, because any ODL files that you associate with your DLL project will automatically be compiled with MIDL when you compile your project.

Step-by-Step Example - Create the DLL and Type Library

  1. Open Visual C++ 5.0 and select File|New. On the Projects tab, select "Win32 Dynamic-Link Library" and name the project "TLBSamp."
  2. Select File|New again. On the Files tab, select "C++ Source File," name the file "TLBSamp.c," and press OK.
  3. Repeat step 2 again, and this time choose "Text File" as the file type. Name the files "TLBSamp.def" and "TLBSamp.odl" respectively.
  4. Next, add the following code to TLBSamp.c:
          #include <windows.h>
          // MyDll_ReverseString -- Reverses the characters of a given string
          void __stdcall MyDll_ReverseString(LPSTR lpString)
          // MyDLL_Rotate -- Returns bit rotation of 32-bit integer value
          int __stdcall MyDll_Rotate(int nVal, int nDirect, short iNumBits)
             int nRet = 0;
             if((iNumBits < 1) || (iNumBits > 31))
                return nRet;
             case 0:
                // Rotate nVal left by iNumBits
                nRet = (((nVal) << (iNumBits)) |
                        ((nVal) >> (32-(iNumBits))));
             case 1:
                // Rotate nVal right by iNumBits
                nRet = (((nVal) >> (iNumBits)) |
                        ((nVal) << (32-(iNumBits))));
             return nRet;
  5. To make the functions exportable, add the following to TLBSamp.def:
          LIBRARY TLBSamp
          DESCRIPTION 'Microsoft KB Sample DLL'
  6. Declare your functions in a type library by adding the following to TLBSamp.odl:
          // This is the type library for TLBSamp.dll
          // Use GUIDGEN.EXE to create the UUID that uniquely identifies
          // this library on the user's system. NOTE: This must be done!!
          // This helpstring defines how the library will appear in the
          // References dialog of VB.
             helpstring("KB Sample: Make your C DLL More Accessible"),
          // Assume standard English locale.
          // Assign a version number to keep track of changes.
          library TLBSample
          // Define an Enumeration to use in one of our functions.
          typedef enum tagRotateDirection
          // Now define the module that will "declare" your C functions.
             helpstring("Sample functions exported by TLibSamp.dll"),
          // Give the name of your DLL here.
          module MyDllFunctions
             // Add a description for your function that the developer can
             // read in the VB Object Browser.
                helpstring("Returns the reverse of a given string."),
             // Specify the actual DLL entry point for the function. Notice
             // the entry field is like the Alias keyword in a VB Declare
             // statement -- it allows you to specify a more friendly name
             // for your exported functions.
             // The [in], [out], and [in, out] keywords tell the Automation
             // client which direction parameters need to be passed. Some
             // calls can be optimized if a function only needs a parameter
             // to be passed one-way.
             void __stdcall ReverseString([in, out] LPSTR sMyString);
                helpstring("Rotates a Long value in the given direction."),
             // Besides specifying more friendly names, you can specify a more
             // friendly type for a parameter. Notice the Direction parameter
             // has been declared with our enumeration. This gives the VB
             // developer easy access to our constant values.
             int __stdcall BitRotate([in] int Value,
                                     [in] RotateDirection Direction,
                                     [in] short Bits);
          } // End of Module
          }; // End of Library
  7. Compile your DLL and type library by choosing "Rebuild All" from the Build menu. When complete, copy the new DLL (TLBSamp.dll) to your Visual Basic directory for testing.
NOTE: As a matter of convenience, you may wish to include your type library in your DLL as a resource. This would free you from having to distribute a separate TLB file to your Visual Basic developers.

To add the library as a resource, complete the following steps:
  1. Select File|New. On the Files tab, select "Text File," name the file "TLBSamp.rc," and press OK.
  2. In the text window that appears add the following line:

    1 typelib TLBSamp.tlb
  3. Save the file and recompile your DLL. When complete, copy the new DLL (TLBSamp.dll) to your Visual Basic directory for testing; overwrite the previous file if prompted.

Step-by-Step Example - The Visual Basic Test App

  1. To test your DLL and type library, open Visual Basic 5.0 and create a new standard Project. Form1 is created by default.
  2. From the Project menu, select References to call up the References dialog box, and then click Browse to find your new type library (or your DLL if you added the library as a resource). Once you have located it, press OK. Visual Basic will automatically register the library for you the first time you reference it. Make sure that your library ("KB Sample: Make your C DLL More Accessible") has been checked in the references List, and then close the dialog box.
  3. Press the F2 key to bring up the Object Browser. Note that your library (TLBSamp) has been added to the Visual Basic project, and that your functions can now be called just as if they were native Visual Basic functions. Visual Basic will even drop down your enumeration list when the developer is typing in the Direction parameter to the BitRotate function.
  4. Add a CommandButton to Form1 and add the following code the button's click event:
          Private Sub Command1_Click()
             Dim n1 As Long, n2 As Long, nTmp As Long
             Dim sTest As String, sMsg As String
             sTest = "Hello World!"
             n1 = 100
             ReverseString sTest
               sMsg = sTest & " | "
             ReverseString sTest
               sMsg = sMsg & sTest & vbCrLf
             nTmp = BitRotate(n1, tlbRotateLeft, 2)
             n2 = BitRotate(nTmp, tlbRotateRight, 2)
               sMsg = sMsg & Str$(n1) & " : " & Str$(nTmp) & " : " & Str$(n2)
             MsgBox sMsg
          End Sub
  5. Now press the F5 key to run the vb5allB 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 or your system path before you run your test app.


For additional information on the structure of ODL or IDL, please see the following articles in the Microsoft Developer Network (MSDN) Library:
TITLE : Type Libraries and the Object Description Language
TITLE : Interface Definitions and Type Libraries

For additional information, please see the following articles in the Microsoft Knowledge Base:
143258 : How to Create Constants and DLL Declarations in a Type Library

122285 : How To Add Type Libraries as Resources to .dll and .exe Files

142840 : Visual Basic Requirements for Exported DLL Functions

(c) Microsoft Corporation 1998, All Rights Reserved. Contributions by Richard R. Taylor, Microsoft Corporation


Article ID: 189133 - Last Review: July 1, 2004 - Revision: 4.1
  • Microsoft Visual Basic 5.0 Learning Edition
  • Microsoft Visual Basic 6.0 Learning Edition
  • Microsoft Visual Basic 5.0 Professional Edition
  • Microsoft Visual Basic 6.0 Professional Edition
  • Microsoft Visual C++ 5.0 Professional Edition
  • Microsoft Visual Basic 5.0 Enterprise Edition
kbhowto KB189133

Give Feedback


Contact us for more help

Contact us for more help
Connect with Answer Desk for expert help.
Get more support from