A DLL is a library that contains code and data that can be used by more than one program at the same time. For example, in Windows operating systems, the Comdlg32 DLL performs common dialog box related functions. Therefore, each program can use the functionality that is contained in this DLL to implement anOpen dialog box. This helps promote code reuse and efficient memory usage.
By using a DLL, a program can be modularized into separate components. For example, an accounting program may be sold by module. Each module can be loaded into the main program at run time if that module is installed. Because the modules are separate, the load time of the program is faster, and a module is only loaded when that functionality is requested.
Additionally, updates are easier to apply to each module without affecting other parts of the program. For example, you may have a payroll program, and the tax rates change each year. When these changes are isolated to a DLL, you can apply an update without needing to build or install the whole program again.
The following list describes some of the files that are implemented as DLLs in Windows operating systems:
The following list describes the changes that have been introduced in Microsoft Windows 2000 and in later Windows operating systems to help minimize dependency issues:
By using a DLL, a program can be modularized into separate components. For example, an accounting program may be sold by module. Each module can be loaded into the main program at run time if that module is installed. Because the modules are separate, the load time of the program is faster, and a module is only loaded when that functionality is requested.
Additionally, updates are easier to apply to each module without affecting other parts of the program. For example, you may have a payroll program, and the tax rates change each year. When these changes are isolated to a DLL, you can apply an update without needing to build or install the whole program again.
The following list describes some of the files that are implemented as DLLs in Windows operating systems:
- ActiveX Controls (.ocx) files
An example of an ActiveX control is a calendar control that lets you select a date from a calendar. - Control Panel (.cpl) files
An example of a .cpl file is an item that is located in Control Panel. Each item is a specialized DLL. - Device driver (.drv) files
An example of a device driver is a printer driver that controls the printing to a printer.
DLL advantages
The following list describes some of the advantages that are provided when a program uses a DLL:- Uses fewer resources
When multiple programs use the same library of functions, a DLL can reduce the duplication of code that is loaded on the disk and in physical memory. This can greatly influence the performance of not just the program that is running in the foreground, but also other programs that are running on the Windows operating system. - Promotes modular architecture
A DLL helps promote developing modular programs. This helps you develop large programs that require multiple language versions or a program that requires modular architecture. An example of a modular program is an accounting program that has many modules that can be dynamically loaded at run time. - Eases deployment and installation
When a function within a DLL needs an update or a fix, the deployment and installation of the DLL does not require the program to be relinked with the DLL. Additionally, if multiple programs use the same DLL, the multiple programs will all benefit from the update or the fix. This issue may more frequently occur when you use a third-party DLL that is regularly updated or fixed.
DLL dependencies
When a program or a DLL uses a DLL function in another DLL, a dependency is created. Therefore, the program is no longer self-contained, and the program may experience problems if the dependency is broken. For example, the program may not run if one of the following actions occurs:- A dependent DLL is upgraded to a new version.
- A dependent DLL is fixed.
- A dependent DLL is overwritten with an earlier version.
- A dependent DLL is removed from the computer.
The following list describes the changes that have been introduced in Microsoft Windows 2000 and in later Windows operating systems to help minimize dependency issues:
- Windows File Protection
In Windows File Protection, the operating system prevents system DLLs from being updated or deleted by an unauthorized agent. Therefore, when a program installation tries to remove or update a DLL that is defined as a system DLL, Windows File Protection will look for a valid digital signature. - Private DLLs
Private DLLs let you isolate a program from changes that are made to shared DLLs. Private DLLs use version-specific information or an empty .local file to enforce the version of the DLL that is used by the program. To use private DLLs, locate your DLLs in the program root folder. Then, for new programs, add version-specific information to the DLL. For old programs, use an empty .local file. Each method tells the operating system to use the private DLLs that are located in the program root folder.