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"?>
    <section name="WeatherClientConfig" type="WeatherClient.CustomConfigLoader, WeatherClient" />
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.7.2" />
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));

Code language: C# (cs)

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

5000Code language: plaintext (plaintext)

Code repository

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

2 thoughts on “C# – Read a custom config section from app.config”

  1. Is there a downloadable working example? This doesn’t work for me. I keep getting an error. I did everything the same except used the work Setting instead of Weather.

    System.Configuration.ConfigurationErrorsException: ‘An error occurred creating the configuration section handler for SettingsClientConfig: Could not load file or assembly ‘SettingsClient’ or one of its dependencies. The system cannot find the file specified. ( line 4)’

    Inner Exception
    FileNotFoundException: Could not load file or assembly ‘SettingsClient’ or one of its dependencies. The system cannot find the file specified.

    Line 4 is the reference to the custom objects

    If there is a downloadable working example I may be able to figure this out, but I don’t see where I’ve done anything different than what you have here.

    • I get this error when I put the wrong assembly name in the config. You have “SettingsClient” as the assembly name in the config. Check if this matches your project’s assembly name.

      My assembly name is “WeatherClient”, so I have that in the config (bolded part):
          <section name="WeatherClientConfig" type="WeatherClient.CustomConfigLoader, WeatherClient" />

      Fixing the configured assembly name should solve the problem for you. Nevertheless, as you requested, I put the working code in a public repo here: https://github.com/makolyte/AppConfigLoaderExample

      Thank you for asking about this – I updated the page to try to better explain the possible errors you might run into with the configuration.


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