.NET gives an easy way to store configuration information in an Application Configuration File. In the simple implementation, you can store information as Key-Value pairs.
For example, consider a case where you have to use a data source in your application. If you hardcore the data source information in your code, you will have a bad time when you have to change this data source. You have to change your source code and re-compile it. This won't work every time you give your product to different customers or when you run your application in different machines!
In earlier days, programmers used to store this information in special files called .ini files or in system registry. The application can read the information from the .ini file or registry and no need to re-compile the code when the values are changed in .ini file or registry.
But this is a pain most of the time. It is not fun opening the registry, locate your entries and make appropriate changes. It is quite possible that you may mess up with some important entries in the registry and make your system not run any more. In fact, in secured systems, administrator may deny access to the registry and users will not have the choice to edit the registry at all.
.NET gives you a simple and easy solution for this problem - the Application Configuration File. Each application can have a configuration file, which is actually an XML file. You can use any text editor (including Notepad) to open the configuration file and change the values. The application will load the values from this configuration file and you do not have to change your source code every time you change your data source or any other information stored in the configuration file.
app.config for Windows applications
Windows applications in VS.NET use the name app.config by default for the configuration file. This will not be automatically created when you create a Windows application. If you need a configuration file for your application, open your project in VS.NET, go to the 'Solution Explorer' and right click on the project name. Choose Add > Add new item from the menu and select 'Application Configuration file' from the list of choices. This will create an app.config file for you in the application root.
By default, the app.config file will have the following content:
To store values in the configuration file, you can create XML elements in the format:
<add key="MyKey" value="MyValue" />
See the sample config entries below:
<add key="DatabasePath" value="c:\\projects\data\spider.mdb" />
<add key="SupportEmail" value="firstname.lastname@example.org" />
And to read from this config file, just use the following code in your application:
string dbPath =
string email =
ConfigurationSettings is the class used to access the contents of the configuration file. Since this class is part of the namespace
System.Configuration, we have to use the fully qualified name
System.Configuration.ConfigurationSettings. As a shortcut, you can use the
using directive on top of the file like below:
If you have the above directive on top of the file, then you can directly use the class
string dbPath = ConfigurationSettings.AppSettings["DatabasePath"];
string email = ConfigurationSettings.AppSettings["SupportEmail"];
In VB.NET, you have to use "( ... )" instead of the "[ ... ]", as shown below:
Dim dbPath as String = ConfigurationSettings.AppSettings("DatabasePath")
Dim email as String = ConfigurationSettings.AppSettings("SupportEmail")
Note: When you compile your application, VS.NET will automatically create a file called <your application name>.exe.config in your bin\debug folder. The contents of the app.config will be automatically copied to this new config file when you compile the application. When you deliver the application to the end user, you have to deliver the exe and this new config file called <your application name>.exe.config and NOT the app.config. Users can modify the data in <your application name>.exe.config file and application will read the data from the config file, when restarted.
web.config for Windows applications
The web applications use the same concept, but they use a config file with the name web.config. There are couple of things to note in this case.
- web.config is created automatically by VS.NET when you create any web project.
- When you compile the web application, web.config is NOT renamed or copied to the BIN folder.
- web.config has several default entries in it to support web/IIS configuration & security.
- You can add the
<appSettings> section in the web.config and add your key/value pairs in that section.
- You can have separate web.config files for each directory in your web application, in addition to the one in the root. For each web page, by default, system will look for a web.config in the same folder as the page and if not found, then looks in the parent folder.