C# - Read a custom config section from app.config

In this this article, I’ll show the simplest to get a custom config section from app.config and load it into your own config class. You can implement IConfigurationSectionHandler and use ConfigurationManager.GetSection() to do this. I’ll show the steps below.

1 – Add a custom config class

The first step is to create a class that’ll hold whatever configuration values you want to put in app.config. Here’s a simple config class:

public class WeatherClientConfig
{
	public bool IsEnabled { get; set; }
	public string WeatherAPIUrl { get; set; }
	public string Timeout { get; set; }
}
Code language: C# (cs)

2 – Implement IConfigurationSectionHandler to load the section

Implement IConfigurationSectionHandler.Create() to load the section into an object:

Use reflection to match the target type with the section name.
Use XmlSerializer to deserialize the section XML to the target type.

This is called by ConfigurationManager.GetSection() (which you’ll use in the final step).

using System;
using System.Configuration;
using System.Linq;
using System.Xml;
using System.Xml.Serialization;

public class CustomConfigLoader : IConfigurationSectionHandler
{
	public object Create(object parent, object configContext, XmlNode section)
	{
		if (section == null)
		{
			throw new ArgumentNullException($"XMLNode passed in is null.");
		}

		var type = AppDomain.CurrentDomain.GetAssemblies()
		.SelectMany(a => a.GetTypes())
		.FirstOrDefault(t => t.Name == section.Name);

		if (type == null)
		{
			throw new ArgumentException($"Type with name {section.Name} couldn't be found.");
		}

		var serializer = new XmlSerializer(type, new XmlRootAttribute(section.Name));

		using (XmlReader reader = new XmlNodeReader(section))
		{
			return serializer.Deserialize(reader);
		}
	}

}

Code language: C# (cs)

Notice that this works on any type, not just the config class that was added in step 1.

3 – Add the custom config section in app.config

You need to add a section in configSections. Then you can add an XML representation of your config class in app.config. Take a look at the highlighted sections in the app.config below:

Note: If configSections is missing, add the whole configSections node. If it’s already in there, just add the section node within the configSections node.

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <configSections>
    <section name="WeatherClientConfig" type="WeatherClient.CustomConfigLoader, WeatherClient" />
  </configSections>
  <startup>
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.7.2" />
  </startup>
  <WeatherClientConfig>
    <IsEnabled>true</IsEnabled>
    <WeatherAPIUrl>https://localhost:12345</WeatherAPIUrl>
    <Timeout>5000</Timeout>
  </WeatherClientConfig>
</configuration>
Code language: HTML, XML (xml)

The <WeatherClientConfig> node matches the config class from step 1.

Explaining the <section> node in app.config

The <section> node basically means: “I want to use this config loader to load this custom XML section.” Take a look at the <section> node again:

<section name="WeatherClientConfig" type="WeatherClient.CustomConfigLoader, WeatherClient" />
Code language: HTML, XML (xml)

This means: “I want to use the WeatherClient.CustomConfigLoader class from assembly WeatherClient to load custom section WeatherClientConfig.”

name = The name of the custom section XML node (i.e. <WeatherClientConfig>).
type = The type info for the class that implements IConfigurationSectionHandler. Format: Namespace.Class, Assembly.

If the name or type doesn’t match, it throws a ConfigurationErrorsException.

4 – Use ConfigurationManager.GetSection()

Now that all the pieces are in place, load the config section by calling ConfigurationManager.GetSection():

static void Main(string[] args)
{
	var config = (WeatherClientConfig)ConfigurationManager.GetSection(nameof(WeatherClientConfig));

	Console.WriteLine(config.WeatherAPIUrl);
	Console.WriteLine(config.IsEnabled);
	Console.WriteLine(config.Timeout);
}
Code language: C# (cs)

This outputs the following, exactly what you would expect, since it’s what’s in the app.config:

https://localhost:12345
True
5000Code language: plaintext (plaintext)

Code repository

The code shown here is available in this public repository: https://github.com/makolyte/AppConfigLoaderExample.

C# – Read a custom config section from app.config