Creating and consuming MFC DLLs for Beginners

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Introduction

1. Simplify project management. Logical units of work can be split up among different groups during development process to help simplify project management. 
2. Conserve memory. If two or more applications use the same DLL, the DLL is loaded into the RAM and this can be shared by all of the applications.
3. Extend the features of an application. An application can determine the availability of DLL's dynamically load them & extend/limit the functionality of the application.
4. Flexibility of programming languages. You can choose the language best suited for a particular module. Perhaps use Visual Basic for the UI & C++ for the business logic. 

Writing the application using a DLL

1. Writing the DLL whose functions we are going to call
2. Writing a test client for the DLL

Creating the DLL

1. Fire up Visual C++ & choose the Project as MFC AppWizard(Dll) and type in Project name as MyDll
2. Let the default selection for DLL type remain, i.e "Regular DLL using Shared MFC DLL"
3. Click Finish and then Ok to get Visual Studio to generate the necessary files.
4. Go to the class view, right click on “MyDll classes” and choose “New Class”.
5. Now choose Class type as "Generic Class" .
6. Type in the Class name CMyClass, the wizard automatically fills in the rest of the names.
7. Go to the class view again
8. Right click on the CMyClass and choose “Add Member Function”
9. Type in the name of the function type as CString and fill in the function declaration as SayHello (CString strName). Choose Access type of the function as public. Note : CString is a MFC class that makes manipulation of strings very easy.
10. Now go to file view and type in the following code into CMyClass.cpp as shown below in Code Snippet 2

11. To be able to call the functions from an external application we have to prefix all function signatures with __declspec(dllexport) . This change is made in the CMyClass.h file as shown in Code Snippet 1

Compile the application and your DLL is ready

// CMyClass.h 
//{-----------     Code Snippet 1 --------------------------
class CMyClass  
{
public:
	__declspec(dllexport)  CString SayHello (CString strName);
	__declspec(dllexport)  CMyClass();
	__declspec(dllexport)  virtual ~CMyClass();

};

//{-----------     Code Snippet 1 --------------------------

// CMyClass.cpp
//{-----------     Code Snippet 2 --------------------------

CString CMyClass::SayHello(CString strName)
{
	//Return the input string with a Hello prefixed 
	 return "Hello " + strName; 
}

//{-----------     Code Snippet 2 --------------------------

Creating the DLL Client

1. Select New MFC AppWizard(exe) from the project menu and type in the project name TestDLL
2. Choose Dialog based application and click Finish
3. You would now be looking at the application in the resource view. Add an edit box to the application by selecting the edit box, next click on the dialog box and drag.
4. Also create a CString (value) variable associated with it, call this variable m_edit i.e Click on the edit box and press CTRL and W to bring up the class wizard. Choose the member variables tab and choose IDC_EDIT1 and click on “Add Variable”, of type "Value" and type m_edit
5. Now a file called TestDLLDlg.cpp would have been generated .
6. Double click the “Ok” button on the dialog, the wizard pops up a box asking a name for the function, choose the default name “OnOk” to go to the TestDLLDlg.cpp file
7. Modify TestDLLdlg.cpp to look like the Code Snippet 1, the code entered has no effect and the project at this point will not compile. (Basically we are calling a method of a class object, this object has not been defined as yet.)
8. Goto the file TestDLLDlg.h and include the header file for your class i.e MyClass.h .
9. In the file TestDLLDlg.h declare an object of your class objMyClass present in the DLL
10. After steps 8 & 9 the code looks as in Code Snippet 2
11. We need to modify the project settings to compile the project, Click on "Project->Settings->Link" and in the "Object/Library Modules" enter the complete or relative path to the DLL's library file. i.e. A .lib file is generated in the same folder as your DLL, I have entered "..\MyDll\Debug\MyDll.lib" here.
12. Compile the application and if it has compiled successfully. Run it. 
    
    Hey, Why are we getting this stupid box saying "Unable to located DLL" ? 
    Solution: Copy the MyDll.dll to the same folder as the TestDll.exe (See my comment http://www.codeproject.com/Articles/2516/Creating-and-consuming-MFC-DLLs-for-Beginners?msg=228387#xx228387xx)

Run the application, enter a name in the text box and click the okay button, this will show a message box containing the same text prefixed with a hello.

//TestDLLdlg.cpp
// ----------------- Code Snippet 1 ----------------------------
void CTestDLLDlg::OnOK() 
{
		
	UpdateData(true);
	CString strResult = objMyClass.SayHello(m_edit);
	AfxMessageBox (strResult);
	//CDialog::OnOK();
	
}
// ----------------- Code Snippet 1 ----------------------------

// ----------------- Code Snippet 2 ----------------------------
//TestDLLDlg.h
/////////////////////////////////////////////////////////////////////////////
// CTestDLLDlg dialog


 #include "..\MyDll\MyClass.h" 

class CTestDLLDlg : public CDialog
{
// Construction
public:
	CTestDLLDlg(CWnd* pParent = NULL);	// standard constructor
	 CMyClass objMyClass; 

// Dialog Data

// ----------------- Code Snippet 2 ----------------------------
 

After 10 years (of publishing this article) along came Ed to provide some comments which I feel may be useful - http://www.codeproject.com/Articles/2516/Creating-and-consuming-MFC-DLLs-for-Beginners?msg=4338620#xx4338620xx