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
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
Step-by-Step Example - Create the DLL and Type Library
Open Visual C++ 5.0 and select File|New. On the Projects tab, select
"Win32 Dynamic-Link Library" and name the project "TLBSamp."
Select File|New again. On the Files tab, select "C++ Source File,"
name the file "TLBSamp.c," and press OK.
Repeat step 2 again, and this time choose "Text File" as the file type.
Name the files "TLBSamp.def" and "TLBSamp.odl" respectively.
Next, add the following code to TLBSamp.c:
// 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))
// Rotate nVal left by iNumBits
nRet = (((nVal) << (iNumBits)) |
((nVal) >> (32-(iNumBits))));
// Rotate nVal right by iNumBits
nRet = (((nVal) >> (iNumBits)) |
((nVal) << (32-(iNumBits))));
To make the functions exportable, add the following to TLBSamp.def:
Declare your functions in a type library by adding the following
// 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.
// 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.
// 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
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:
Select File|New. On the Files tab, select "Text File," name the file
"TLBSamp.rc," and press OK.
In the text window that appears add the following line:
1 typelib TLBSamp.tlb
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
To test your DLL and type library, open Visual Basic 5.0 and create a
new standard Project. Form1 is created by default.
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.
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
Add a CommandButton to Form1 and add the following code the button's
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
sMsg = sTest & " | "
sMsg = sMsg & sTest & vbCrLf
nTmp = BitRotate(n1, tlbRotateLeft, 2)
n2 = BitRotate(nTmp, tlbRotateRight, 2)
sMsg = sMsg & Str$(n1) & " : " & Str$(nTmp) & " : " & Str$(n2)
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.