.DLL File Extension
Dynamic Link Library
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What is a DLL file?
A DLL file is a Dynamic Link Library, which contains compiled functions, drivers, or other data that multiple Windows programs can use. Many Windows programs use DLL files to perform common functions. Multiple programs can even use the same DLL files at the same time.
Windows comes with many default DLL files, which software developers can reference when creating programs that perform standard functions, such as opening dialog windows, communicating with external devices, and reading and writing files. Referencing DLL files saves programmers time (since they don't need to manually code common functions) and reduces their programs' file sizes (since the programs don't need to contain the resources stored in the DLL file).
When a program needs to use the data stored in a DLL file, it dynamically loads that DLL file into system memory. The program can then access and use the functions (and other data) the DLL file includes as though it were a part of the program.
Third-party programs may install additional DLL files on your computer. These DLL files function the same way as Windows' default DLL files, but are likely shared among only programs created by that developer.
What to do if you are missing a DLL file
If a program requires a DLL file that you do not have, it will likely show a "missing DLL" or "DLL not found" error message. In these instances, there are several actions you can take to attempt to resolve the DLL error, including:
- Reinstalling the program that is trying to use the DLL file
- Updating any related device drivers
- Checking whether you accidentally deleted the DLL file
- Checking for and applying Windows Updates
- Restarting your computer
- Performing a System Restore
In most cases, you should not attempt to manually find and download the DLL file you are missing from the internet. DLL files hosted on non-Microsoft DLL download sites may be vehicles for malware.
Common DLL Filenames
user32.dll - Contains instructions for graphical elements such as dialog boxes and windows.
shell32.dll - Contains Windows shell API functions.
msvcrt.dll - Contains standard C library functions. If you're missing msvcrt.dll, you can install it via the Microsoft Visual C++ 2015 Redistributable Package.
mfc42u.dll - A Microsoft Foundation Class (MFC) library required by some Windows programs.
mfc80u.dll - An MFC library required by some Windows programs created with Visual C++ 2005. If you're missing mfc80u.dll, you can install it via the Microsoft Visual C++ 2005 Redistributable Package.
mfc90u.dll - An MFC library required by some Windows programs created with Visual C++ 2008. If you're missing mfc90u.dll, you can install it via the Microsoft Visual C++ 2008 Redistributable Package.
mfc100u.dll - An MFC library required by some Windows programs created with Visual C++ 2010. If you're missing mfc100u.dll, you can install it via the Microsoft Visual C++ 2010 Redistributable Package.
How to open a DLL file
Most users will never need to and never should open a DLL file. In fact, opening or modifying a DLL file may cause errors within the program(s) that reference that file, and prevent it from functioning correctly.
However, Windows developers and other advanced users can decompile DLL files to view and modify their contents. Tools that can decompile DLL files include Microsoft Visual Studio, JetBrains dotPeek, Redgate .NET Reflector, and Resource Hacker.
To protect against malware attacks, never install a DLL file you downloaded from the internet unless you trust the file's source.
Programs that open or reference DLL files
A DLL file may also be a webpage generated dynamically when accessed by a user. It may incorporate server-side scripts, such as Perl or VB scripts that generate the webpage's HTML.
Webpages with DLL file extensions are typically hosted on Microsoft IIS web servers. The DLL extension must be mapped to the correct scripting engine on the server for the page to be processed correctly.