DYNAMIC LINK LIBRARIES

Ø DLL is a binary file that provides a library of functions, objects and resources.

Ø All the API functions are contained in DLL.

Ø Functions present in DLLs can be linked during execution.

Ø Several applications can share these function.

Ø Since linking is done dynamically the functions do not become a part of the executable file and thus have no effect on the size of the file.

Ø It is possible to create user defined DLLs.

Ø Features:

a.     Sharing of common code between different executable files.

b.     Breaking an application into component parts to provide a way to easily upgrade application’s components.

c.      Keeping resource data out of an application’s executable, but still readily accessible to an application.

Ø Types Of DLL:

MFC Extension DLL:

·  Supports C++ interface.

·  The DLL can export whole classes and the client can construct objects of those classes or derive classes from them.

·  Dynamically links to the code in the DLL version of the MFC library.

·    Extension DLLs are quite small in size.(simple extension DLL are 10KB in size, which loads quickly)

MFC Regular DLL:

·  Can export only C-style functions.

·  It can’t export C++ classes, member functions, or overloaded functions because every C++ compiler has its own method of decoding names.

·     One can choose to statically link or dynamically link to the MFC library.

·     Statically linked regular DLL is about 144KB in size.

·     In case of dynamic linking, the size is about 17KB .

Ø Steps to create DLL:

For Creating own DLL file we have, the steps to be followed are:

1. Start Visual C++, Goto File->New->Select the Win32

Dynamic Link-Library->Specify the Project Name (Like SanDLL). Click on OK button.                                            

2. Choose A simple DLL project->Click on Finish button.

 

3. Go to FileView then select SanDLL.CPP file from source file. Write the following function just like as follows:

int _stdcall Decrease(int f)

{

            return(f-1);

}

Save the Current CPP file.

4. Go to File->New and select Text File, give the file name as the project name (like SanDLL.def).

 

Write the following code in the opening source editor.

LIBRARY SanDLL

EXPORTS

Decrease @1

After that we save and build the project.

5: Copy the .dll file that you create (Like SanDLL) from Debug folder to the Window’s System directory.

 6: Start the Visual Basic Application and design the application according to your DLL requirement.

 

For Example. 

For Creating the DLL file we have to follow the following steps:

7:  Write the following code in coding section and execute your application.

Private Declare Function Decrease Lib "SanDLL" (ByVal a As Integer) As Integer

Private Sub Command1_Click()

Dim a As Integer

a = CInt(Text1.Text)

a = Decrease(a)

Label2.Caption = "Decremented Value is:" & a

End Sub