How to load a dynamic link library (DLL) into a Microsoft Visual C++ 6.0 project

Introduction

In this article, I will summarize the results of my investigation with respect to including a dynamic link library (DLL) into a Microsoft Visual C++ 6.0 project, and using services which are offered by the loaded DLL.

The purpose of this article is to explain in short, but explicitly, what you need to do in order to be able to use services included in a DLL (by services, I refer to any function, a class or a parameter exported into DLL). Three weeks of �DLL hell� I went through can be avoided by doing the steps described below.

Case 1: A DLL you are supplied with is provided by another application and has been generated with the help of OLE (COMM).

Example: I needed to access an external interface of Rose-RT, which has been provided by the developers of Rose-RT as RrtRes.dll in order to control Rose-RT from external application.

Steps to perform:

• Create in Microsoft Visual C++ 6.0 an MFC application (.dll or .exe);
• Go to the menu View, ClassWizard.
• Select an option Add Class�, from a type library.
• Browse to the location of your *.dll.
• A window, which displays the content of your *.dll, will appear. Select all classes you want to include in your project (Ctrl+A � select all content, or Ctrl + mouse click � for selecting a specific set of classes).
• Push the button Open. Header file *.h and *.cpp implementation file will be generated.
• Close the ClassWizard.

Now, all services you selected for use in you project are available!

Case 2: A DLL you are supplied with has been generated as a MFC-shared DLL and it is to be used in a single-threaded code

Example: Suppose you are supplied with LoadMe.dll, services of which should be accessed, but which cannot be exported as mentioned in case 1.

//You need to declare types to point on classes/functions in LoadMe.dll

//Assume, you have a function in your LoadMe.dll with a name 

//EntryPoint, which takes two parameters of types int and const char *, 

//and is of type void. You need to create a new type as a 

//pointer to that function as it is shown below.


typedef void (*EntryPointfuncPtr)(int argc, const char * argv );  
 
//Declare an HINSTANCE and load the library dynamically. Don�t forget 

//to specify a correct path to the location of LoadMe.dll


HINSTANCE LoadME;
LoadMe = LoadLibrary("..\\enter a Path To Your Dll here\\LoadMe.dll");
 
// Check to see if the library was loaded successfully 

if (LoadMe != 0)
    printf("LoadMe library loaded!\n");
else
    printf("LoadMe library failed to load!\n");

//declare a variable of type pointer to EntryPoint function, a name of 

// which you will later use instead of EntryPoint

EntryPointfuncPtr LibMainEntryPoint;            

// GetProcAddress � is a function, which returns the address of the 

// specified exported dynamic-link library (DLL) function. After 

// performing this step you are allowed to use a variable 

// LibMainEntryPoint as an equivalent of the function exported in 

// LoadMe.dll. In other words, if you need to call 

// EntryPoint(int, const char *) function, you call it as 

// LibMainEntryPoint(int, const char *)


LibMainEntryPoint = (EntryPointfuncPtr)GetProcAddress(LoadMe,"entryPoint");

Now, you can access any of the services of LoadMe.dll. Good luck!

Case 3: A DLL you are supplied with has been generated as a MFC-shared DLL and it is to be used in a multithreaded code

Use AfxLoadLibrary() instead of LoadLibrary() mentioned in case 2.

When you are done, don't forget to free the memory and unload the library: FreeLibrary(LoadMe).