What is the CONFIG file format?
.config files are commonly used within a system as configuration files. Using configuration files, various options, parameters and preferences within specific applications or the whole system are defined. In most .config files, all properties are stored as plain text - most often as XML, which is the format Microsoft .NET uses for App.config (desktop applications) and Web.config (ASP.NET web projects). Some programs instead use an INI-style key=value format or YAML.
Computer software and operating systems often rely on many settings and parameters necessary for controlling ongoing processes. .config files serve this purpose - among other things, they specify file directories, connection strings, and application behaviour switches. In some programs, more than one such file may be used, but this is usually seen in more complex applications.
- Since these are text-based files, they can be edited with any plain-text editor, including Notepad on Windows. Before editing, you should familiarise yourself with the file's content, as incorrect changes can negatively affect the functioning of programs or the operating system.
.configfiles define how a program initialises or operates at runtime - most applications read them during startup. In .NET, a project'sApp.configis copied and renamed to<AssemblyName>.exe.configin the output folder.- The .NET runtime also reads a machine-wide
machine.configand merges its settings with the application configuration; app-level settings take precedence over machine defaults. Related formats include .cfg files used by many non-.NET applications.
Security & safety
RISK: MEDIUMA text .config is safe to open and read. The risk is in EDITING: because a .config often controls whether its program starts (and, for .NET, how it connects to databases), a bad value, typo or broken XML tag can stop the application working. Always copy the original to a backup before changing anything, edit with a plain- text/code editor (never a word processor), and change one thing at a time. Also note that .config files can hold sensitive data (database connection strings, passwords, API keys in Web.config) - treat them as confidential, and be wary of downloaded 'optimized' config files from untrusted sources.
Format details
in a nutshellFULL NAMEConfiguration File
DEVELOPERN/A (generic extension; the dominant case is Microsoft .NET App.config / Web.config)since No single origin; .NET App.config/Web.config since .NET Framework 1.0 (2002)
CATEGORYDeveloper Files
MIME TYPEtext/xml
TYPEUsually a plain-text settings file - most often XML (.NET App.config/Web.config), sometimes key=value/INI or YAML; occasionally app-specific
Programs that open CONFIG files
Windows4 apps
Microsoft Visual Studio
Freemium Open the project's App.config/Web.config with full schema awareness - the native editor for .NET configuration files. 
Windows Notepad
Built-in Right-click → Open with → Notepad for quick edits of small text configs; lacks XML help, so take care with structure. 
Notepad
Open-source Right-click → Edit with Notepad++; shows XML structure and line numbers for safe editing of text configs. 
Visual Studio Code
Free Open the .config to read/edit with XML/INI highlighting and validation. Best free choice, especially for .NET App.config/Web.config. 
macOS2 apps
TextEdit
Built-in Open the .config, then Format → Make Plain Text before saving so it stays a plain-text config (don't save as RTF).
Visual Studio Code
Free Open to edit the config with highlighting/validation; safest cross-platform choice. 
Linux3 apps
GNOME Text Editor / gedit
Open-source Open the .config to read/edit; the default GNOME editor handles config files fine.
nano / vim
Open-source Edit in the terminal: 'nano file.config' - standard for server/framework config files. Back up first. Technical details
deep spec| Primary format | XML - a `<configuration>` root element wrapping `<appSettings>`, `<connectionStrings>`, and `<system.*>` sections in the standard .NET layout |
| File encoding | UTF-8 (standard for .NET XML config files; encoding declared in the XML prolog) |
| MIME type | `text/xml` |
| File signature | None universal; the .NET XML variant starts with `<?xml ...?>` at offset 0; INI-style key=value variants have no magic bytes |
| Root element (.NET) | `<configuration>` - required outermost element; children such as `<appSettings>` and `<connectionStrings>` follow a schema enforced by the runtime |
| Runtime read mechanism | Read at application startup by the .NET `ConfigurationManager` (or `IConfiguration` in .NET Core / .NET 5+); changes require an app restart unless a configuration reload is explicitly configured |
| Machine-level defaults | `machine.config` in the .NET Framework installation folder provides system-wide defaults; application `.config` settings always override machine defaults |
| Build-time transformation | XML Document Transform (`.xdt`) files such as `App.Debug.config` and `App.Release.config` are merged by MSBuild to produce environment-specific output files |
| Deployed file name | For .NET desktop apps the output is renamed to `<AssemblyName>.exe.config`; for ASP.NET the file stays as `Web.config` in the web root |
| Connection strings section | `<connectionStrings>` child element stores named database connection strings; the `providerName` attribute specifies the ADO.NET provider (e.g. `System.Data.SqlClient`) |
| Alternate text formats | Some applications use `.config` for INI-style `key=value` text, YAML, or TOML instead of XML - the internal format is entirely application-defined |
| Security handling | IIS blocks direct HTTP access to `.config` files by default; sensitive values such as passwords can be encrypted in place using `aspnet_regiis -pe` on Windows |
| Line endings | Typically CRLF on Windows, LF on Linux/macOS; the XML parser normalises both |
| Related extensions | `.cfg`, `.ini`, `.conf`, `.xml`, `.json`, `.yaml` |
| Released | No single origin; .NET App.config/Web.config since .NET Framework 1.0 (2002) |
| Specification | learn.microsoft.com |
CONFIG conversions
Possible
Not possible
Community Q&A
asked by usersNo questions yet - be the first to ask about CONFIG files.
Frequently asked questions
How do I open a .config file?
Most are plain text - open them with VS Code, Notepad++ or Notepad on Windows, TextEdit (plain-text mode) on Mac, or gedit/nano on Linux. .NET App.config/Web.config files are XML and edit nicely in VS Code or Visual Studio. If the contents look like gibberish, it's a binary config only its program can read.
What is a Web.config or App.config file?
They're the standard .NET configuration files: Web.config for ASP.NET web apps, App.config for desktop apps. They're XML holding settings like connection strings and app keys, read by the application at startup.
Can I edit a .config file safely?
Yes, but back it up first. Use a plain-text/code editor (XML-aware for .NET configs), change one setting at a time, and keep the original so you can restore it if the program stops working. Never use a word processor.
Why does my .config file look like random characters?
Either it's a binary/proprietary config that only its program can read, or you opened an XML config in a tool that doesn't display it well. Try VS Code; if it's still gibberish, change those settings through the program itself.
What's the difference between .config, .cfg, .ini and .conf?
Practically none in purpose - all are common names for configuration files. .config is most associated with .NET XML configs; .cfg/.ini/.conf are usually key=value/INI text. The extension is just the developer's choice.
I edited a .config and now the program won't start - what do I do?
Restore the backup you made, or delete the .config so the program regenerates a default one (you'll lose custom settings). This is exactly why copying the file before editing matters; broken XML is a common cause for .NET apps.
References
1Microsoft Learn - Configuring apps (App.config/Web.config)learn.microsoft.com
2Microsoft Learn - INI file format (key=value/[section])learn.microsoft.com
