What is App.config in C#.NET? How to use it?

Solution 1:

At its simplest, the app.config is an XML file with many predefined configuration sections available and support for custom configuration sections. A "configuration section" is a snippet of XML with a schema meant to store some type of information.

Overview (MSDN)
Connection String Configuration (MSDN)

Settings can be configured using built-in configuration sections such as connectionStrings or appSettings. You can add your own custom configuration sections; this is an advanced topic, but very powerful for building strongly-typed configuration files.

Web applications typically have a web.config, while Windows GUI/service applications have an app.config file.

Application-level config files inherit settings from global configuration files like machine.config. Web also applications inherit settings from applicationHost.config.

Reading from the App.Config

Connection strings have a predefined schema that you can use. Note that this small snippet is actually a valid app.config (or web.config) file:

<?xml version="1.0"?>
<configuration>
    <connectionStrings>   
        <add name="MyKey" 
             connectionString="Data Source=localhost;Initial Catalog=ABC;"
             providerName="System.Data.SqlClient"/>
    </connectionStrings>
</configuration>

Once you have defined your app.config, you can read it in code using the ConfigurationManager class. Don't be intimidated by the verbose MSDN examples; it's actually quite simple.

string connectionString = ConfigurationManager.ConnectionStrings["MyKey"].ConnectionString;

Writing to the App.Config

Frequently changing the *.config files is usually not a good idea, but it sounds like you only want to perform one-time setup.

See: Change connection string & reload app.config at run time which describes how to update the connectionStrings section of the *.config file at runtime.

Note that ideally you would perform such configuration changes from a simple installer.

Location of the App.Config at Runtime

Q: Suppose I manually change some <value> in app.config, save it and then close it. Now when I go to my bin folder and launch the .exe file from here, why doesn't it reflect the applied changes?

A: When you compile an application, its app.config is copied to the bin directory¹ with a name that matches your exe. For example, if your exe was named "test.exe", there should be a "text.exe.config" in your bin directory. You can change the configuration without a recompile, but you will need to edit the config file that was created at compile time, not the original app.config.

_{1: Note that web.config files are not moved, but instead stay in the same location at compile and deployment time. One exception to this is when a web.config is transformed.}

.NET Core

New configuration options were introduced with .NET Core and continue with the unified .NET (version 5+). The way that *.config files works hasn't fundamentally changed, but developers are free to choose new, more flexible configuration paradigms.

As with .NET Framework configuration .NET Core can get quite complex, but implementation can be as simple as a few lines of configuration with a few lines of c# to read it.

Configuration in ASP.NET Core
Configuration in .NET Core

Solution 2:

Simply, App.config is an XML based file format that holds the Application Level Configurations.

Example:

<?xml version="1.0"?>
<configuration>
  <appSettings>
    <add key="key" value="test" />
  </appSettings>
</configuration>

You can access the configurations by using ConfigurationManager as shown in the piece of code snippet below:

var value = System.Configuration.ConfigurationManager.AppSettings["key"];
// value is now "test"

Note: ConfigurationSettings is obsolete method to retrieve configuration information.

var value = System.Configuration.ConfigurationSettings.AppSettings["key"];

Solution 3:

Just to add something I was missing from all the answers - even if it seems to be silly and obvious as soon as you know:

The file has to be named "App.config" or "app.config" and can be located in your project at the same level as e.g. Program.cs.

I do not know if other locations are possible, other names (like application.conf, as suggested in the ODP.net documentation) did not work for me.

PS. I started with Visual Studio Code and created a new project with "dotnet new". No configuration file is created in this case, I am sure there are other cases. PPS. You may need to add a nuget package to be able to read the config file, in case of .NET CORE it would be "dotnet add package System.Configuration.ConfigurationManager --version 4.5.0"